Owner’s Manual M -W M I N I
Multi-Watt
N
I
I
M
TWENTY~FIVE
Owner’s Manual
Hello from the Home of Tone
Discerning player and all around intuitive human that you are, you’ve
put your trust in us to be your amplifier company. This is something we don’t
take lightly. By choosing this instrument to be a part of your musical voice,
you have become much more than a customer to us, you’re now a lifetime
member of the MESA worldwide family.
WELCOME! Our goal is to never let you down.
Your reward is that you are now the owner of an amplifier bred of a 40 year
heritage in high performance and it benefits from the many pioneering and
patented MESA features and circuits contained in your model. Feel confident, as we do, that this amp will inspire many hours of musical satisfaction
and years of lasting enjoyment.
With you in mind, this amp was designed and built by players who know
the value of a fine musical instrument and the commitment it takes to make
great music. The same commitment to performance, quality, value and
support we make to you...our new family member and friend.
Important Safety Instructions
•
Read these instructions.
•
Follow all instructions.
•
Keep these instructions.
•
Heed all warnings.
•
Do not use this apparatus near water.
•
Clean only with dry cloth.
•
Do not block any ventilation openings. Install in accordance with the manufacturer’s instructions.
•
Do not install near any heat sources such as radiators, heat registers, stoves, or other apparatus (including amplifiers) that produce heat.
•
Do not defeat the safety purpose of the polarized or grounding-type plug. A polarized plug has two blades with one wider than the other.
A grounding type plug has two blades and a third grounding prong. The wide blade or the third prong are provided for your safety.
If the provided plug does not fit into your outlet, consult an electrician for replacement of the obsolete outlet.
•
Protect the power cord from being walked on or pinched particularly at plugs, convenience receptacles, and the point where they exit
from the apparatus.
•
Only use attachments/accessories specified by the manufacturer.
•
Unplug this apparatus during lightning storms or when unused for long periods of time.
•
Refer all servicing to qualified service personnel. Servicing is required when the apparatus has been damaged in any way, such as powersupply cord or plug is damaged, liquid has been spilled or objects have fallen into the apparatus, the apparatus has been exposed to rain
or moisture, does not operate normally, or has been dropped.
•
To insure proper ventilation always make sure there is at minimum four inches (101.6mm) of space behind the rear of the apparatus.
The ventilation should not be impeded by covering the ventilation openings with items, such as newspapers, tablecloths, curtains, etc.
Do not impede ventilation by placing objects on top of the apparatus which extend past the rear edge of its cabinet.
•
No naked flame sources, such as lighted candles, should be placed on the apparatus.
•
The apparatus shall not be exposed to dripping or splashing and no objects filled with liquids, such as vases, shall be placed on
the apparatus.
•
WARNING: To reduce the risk of fire or electric shock, do not expose this apparatus to rain or moisture.
•
The AC plug is the mains disconnect. The plug should remain accessible after installation.
•
WARNING: EU: permission from the Supply Authority is needed before connection.
•
WARNING: Always make sure proper load is connected before operating the amplifier. Failure to do so could pose a shock hazard
and may result in damage to the amplifier.
•
Do not expose amplifier to direct sunlight or extremely high temperatures.
•
Always insure the amplifier is properly grounded. Always unplug AC power cord before changing fuse, tubes or removing chassis.
Use only same type and rating when replacing fuse.
•
Avoid direct contact with heated tubes. Keep amplifier away from children.
•
To avoid damaging your speakers and other playback equipment, turn off the power of all related equipment before making the
connections.
•
Do not use excessive force when handling buttons, switches and controls. Do not use solvents such as benzene or paint thinner
to clean the unit.
•
Always connect to an AC power supply that meets the power supply specifications listed on the rear of the unit.
Export models: Always insure unit is wired for proper voltage. Make certain grounding conforms with local standards.
YOUR AMPLIFIER IS LOUD! EXPOSURE TO HIGH SOUND VOLUMES MAY CAUSE PERMANENT HEARING DAMAGE.
Your MESA/Boogie Amplifier is a professional instrument. Please treat it with respect and operate it properly.
READ AND FOLLOW PROPER USAGE INSTRUCTIONS
Multi-Watt
Mini Rectifier Twenty-Five
TABLE OF CONTENTS
OVERVIEW_______________________________________________________________ 5-6
FRONT PANEL CONTROLS
Getting Started & Helpful Hints__________________________________________________ 7
CHANNEL MODES
Channel 1: Clean / Pushed ____________________________________________________ 7-8
Channel 2: Vintage / Modern ____________________________________________________ 8
Multi-Watt™ Power ____________________________________________________________ 8
THE CONTROLS
Gain, Treble________________________________________________________________ 8-9
Mid, Bass and Presence_____________________________________________________ 10-11
Master_____________________________________________________________________ 11
Standby and Power On ________________________________________________________ 11
REAR PANEL CONTROLS, SWITCHES & JACKS
Speakers____________________________________________________________________ 13
FX Loop ___________________________________________________________________ 13
AC Socket __________________________________________________________________ 13
Fuse____________________________________________________________________ 13-14
Factory Sample Settings (2 Channels) __________________________________________ 15-17
Personal Setting Sheets______________________________________________________ 18-20
Tube Description & Task Charts_________________________________________________ 21
Tube Noise & Microphonics____________________________________________________ 22
Diagnosing Pre-Amp Tube Problems______________________________________________ 23
Speaker Impedance & Possible Hook-Up Schemes - Amplifier to Speaker Cabs___________ 24-28
Bias Adjustment by Randall Smith ____________________________________________ 29-31
Triodes, Pentodes & Irishmen by Randall Smith__________________________________ 32-34
Parts Chart__________________________________________________________________ 35
Multi-Watt
Mini Rectifier Twenty-Five
Owner’s Guide & Operating Manual
CONGRATULATIONS on your choice of the Mini Rectifier Twenty-Five and Welcome to the MESA Family. The instrument you
have chosen rides atop a 20-year legacy of world-class high gain performance and hit-making sounds that have been at the
core of – and even a catalyst for – some of the very best of Modern Rock. You will quickly find that its Mini moniker and physical size bear no resemblance to its stature, power and command over blistering-tight overdrive in the realm of big gain Tone.
Don’t think for a moment this is a trendy down-line toy or marketing-derived imitation of our mighty Recto. This is the real
deal… in every way a high-end instrument. Lurking within this expensive metal chassis, lies one of our most expressive and
nuance-enhancing circuits to date and it creates an exciting, adrenaline-producing Tone machine… one of the most fun to play
in the entire MESA collection.
Whether you’re rockin’ hard or putting the immense versatility of the Mini’s four Modes to bear on a variety of styles, you’ll get
years of inspiration and enjoyment from this little gas-in-glass-powered jewel. We always strive to build classics, icons that
you’ll treasure amongst your most valuable musical instruments. From the feedback we’ve received thus far… it seems the
Mini Recto is on its way toward achieving that status.
Overview
Like its original two channel forefathers, this Recto keeps things straight ahead and easy to navigate. This
generation features two footswitchable Channels that each contain two Modes and while it looks basic and drives easy, a world
of stylistic versatility lives within these two simple rows of controls.
CHANNEL 1 (Green indicator / top channel), focuses on rhythm sounds, both clean and overdriven. The CLEAN Mode delivers
sparkling, big-headroom rhythm sounds that breathe with rich, warm air on the bottom end. When driven to clip and combined
with the 10 watt Power setting, this Mode misbehaves with real attitude and shines for urgent, yet soulful, Blues solo sounds
and furry Rock rhythm. PUSHED aptly describes what to do with the second Mode in Channel one and here you’ll find the next
region of gain. With a more stripped EQ and just enough added drive, PUSHED invites aggressive Rock rhythm and Crunch
styles to the party and even taunts your mid-gain solo work, especially in the 10 Watt Power setting.
FRONT PANEL: MINI RECTIFIER Twenty-Five Head
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
MI
NI
ON
CH 1
FT
SW
25 W
POWER
STANDBY
CH 2
MODERN
10 WATTS
ON
TWENTY~FIVE
REAR PANEL: MINI RECTIFIER Twenty-Five Head
CAUTION:
Unplug power
before replacing
fuse
TUBE GUIDE
5 x 12AX7
2 x EL 84
WARNING!
ALL TUBE AMPLIFIER
RECTIFIER
M I N I -TW E N T Y-FI V E
D Y N A - W AT T
HANDBUILT IN PETALUMA, CA
CAUTION: CHASSIS SURFACE MAY BE HOT!
PAGE 5
8 OHM
RETURN
4 OHM
SPKRS
1A
SLO
BLO
CAUTION!
SEND
FX LOOP
120 V~
60 Hz
1A
PATENTED
6,621,907
7,602,927
CHANNEL 2 (Red indicator / bottom channel), features the iconic Recto high gain sounds in both their faces, the iconic liquid
wall of VINTAGE and the aggressive fury of MODERN.
VINTAGE excels at thick, high gain chording and single note soloing with an elastic feel and a broad harmonic spread. It has
a looser, more organic sound and the overdrive drapes itself around the notes creating a huge, three-dimensional image. VINTAGE solo work is further enhanced by the 10 Watt Power position. Here, the attack envelope changes and becomes rounder
and more voice-like, while upper harmonics recede slightly to create single note sounds of true beauty. VINTAGE and 10 Watt
compliment each other in ways that will open new doors for the Recto that were previously open only to more “well behaved”
or “cultured” amps. It may also open wide the eyes of current big-power Recto owners who experience for the first time, what
this power range/wiring style does for a sound they had typecast. This is a new and exciting face of Recto and it will surely
expand its stylistic footprint.
MODERN is the opposite in every way. This aggressive Mode comes right at you - maybe even for you - with stunning attack
and a hi-mid bump that keeps things percussive and tracking with hyper-accuracy. It is almost hard to believe that this amount
of gain can be infused and yet still deliver a response – especially in bass frequencies – with this much speed and definition.
But this is pure Recto and MODERN contributes an equal part to achieving its status as the other most recorded sound in Rock.
Needless to say, MODERN is all about all things Heavy and Metal. And while this sound is best demonstrated using the 25 Watt
Power setting, be sure not to typecast MODERN too quickly. Using the 10 Watt Power setting to sweeten the mids and soften
the attack here, shows another side to MODERN as well that’s more forgiving and vocal, creating a great alternate solo voice
with a little more punch and dynamic content.
We keep saying it, but it’s hard to overstate the value, effectiveness and stylistic “power” of the Multi-Watt™ Channel Assignable Power switches. These dedicated wattage range choices unveil different power characteristics and colors – so they are
really power “voicing” switches as well. In this Recto, the choice of EL84 power tubes with their amazing clip characteristics,
prompted us to explore the different wiring schemes possible. This experimentation led to the 10 Watt Power Mode being wired
for Triode operation. This “more vintage” wiring scheme sweetens and rounds out the sound, turning what might otherwise be
a lower volume version of the same voice, into more “soulful” Recto experience. Use the 25 Watt setting for its bold attack,
tight tracking low end, increased definition and all out headroom. This setting works great with all the sounds and certainly
showcases the mightiness in the Mini Rec, shocking unsuspecting players and bystanders alike with power and sonic size. But
any time you need to tame this little beast and put a vintage patina of warmth and silkiness around things… kick down to 10
Watts and enjoy a whole new and different world of Recto expression.
Jumping around to the Rear Panel, you are met with a lesson in elegant simplicity. Nothing here but the AC Power Socket, the
Loop and the Speaker Outputs. While this stands in stark contrast to many other MESA models, including the Mini’s big power
Recto brothers, it was a decision based on stylistic, genre-appropriate traditions and not a cost cutting road to a price point. In
fact, many fans have requested simpler feature sets from us over the years and the “lunchbox” format is the perfect arena for
this conceptual departure.
The Effects Loop on the Mini is wired in Series with the dry signal. The sonic integrity of this circuit is exceptional so you should
have no worry of degraded Tone when adding processing to your signal chain (other than changes caused by your processors).
The SEND and RETURN jacks are switching type and remove the Loop from the signal path when nothing is plugged into the
SEND and RETURN jacks.
Well that wraps up the fly-by overview of the Mini. Now it’s time to get into the Modes and Controls of this little dynamo.
PAGE 6
FRONT PANEL : Controls & Features
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
MI
NI
FT
SW
25 W
CH 2
MODERN
10 WATTS
ON
CH 1
TWENTY~FIVE
POWER
STANDBY
ON
HELPFUL HINTS
1) When powering up (especially from a cold start), always allow the power tubes time to warm up before hitting them with
high voltage (STANDBY ON and playing). Turn POWER to ON, wait at least 30 seconds, then turn STANDBY to ON.
2) The center (3 position) mini toggle just to the left of the STANDBY and POWER labeled FT SW provides manual access to
the Channels when the Footswitch is not connected. The top position calls up Channel 1, the bottom position calls up Channel
2. To use the Footswitch to access the Channels, select the FT SW (center position) and connect the Footswitch to the jack on
the left bottom of the Faceplate labeled FT SW.
3) A good general rule to follow; As the GAIN goes up, the BASS should come down. This will help prevent low-end flub and
tubbiness and keep the attack tracking tighter. This is especially true in the CLEAN and PUSHED Modes of Channel 1 and also
the VINTAGE Mode of Channel 2 when searching for clipped sounds that lie somewhere between clean and dirty and maxed
GAIN settings.
4) You will experience more footswitching noise (pop) when switching between Channels when only one is set to the 10W Power
setting. This is due to the more radical voltage changes that occur when knocking the power section down to the 10W level
from the higher 25 watt power mode. (Using both Channels in 10W will not create excess noise as the voltage doesn’t change
so radically). May we suggest when configuring your Channels for live performance footswitching applications, try using the
25W power mode whenever possible as there will be no voltage swing. This will provide the quietest footswitching performance.
CHANNEL MODES
Each of the Recto’s two channels contain a choice of modes - two in Channel 1 and two in Channel 2, so that each channel
may be configured for an array of sounds. These Mode Select mini toggles radically alter the voicing of the Channel and many
internal changes occur as you switch through the Modes. How you choose to use these different modes is up to you, but we
think you will agree that this scheme provides some of the most versatile footswitching available anywhere.
CHANNEL 1: CLEAN / PUSHED CLEAN: This is the lowest gain circuit of the Mini Recto’s four modes and is optimized
for producing balanced pristine clean sounds. For the best understanding of how to achieve a great sound in this mode, please
refer to the GAIN Control section of this manual. However, a great place to start is 12:00 or so on
the GAIN Control with more sparkle available below this and more warmth apparent above...from
there, adjust according to your guitar’s individual response.
CLEAN
INPUT
PUSHED
GAIN
Because of its more traditional architecture this mode also works extremely well for vintage style
drive sounds. As mentioned earlier in the Overview, these sounds are enhanced by coupling them
with the 10 Watt Power selection where you can combine vintage preamp response with authentic
power overdrive. By turning the gain all the way up, a beautiful vintage solo sound is possible, especially with neck single coil pick-ups. The TREBLE and MIDDLE Controls can also add gain and sustain to this sound (reduce
Presence to blend highs), but you will probably want to run the BASS Control below 10:30 to avoid flubbiness and preserve a
focused attack.
FT SW
VINTAGE
MODERN
PAGE 7
PUSHED: This mode is a radical departure from the sweet, shimmering blend of the CLEAN modes low gain character. Huge
increases in gain in the first stages of the pre-amp produce one of the biggest differences between all four modes and transforms
what you thought to be a tame and gentle clean channel into a raging crunch machine. This incredible amount of gain (for a
low gain circuit) also creates one of the most expressive solo modes in your Recto. Because there are less stages of gain for
the signal to travel through and the tone control network is tuned for the brighter nature of clean sounds, this mode responds
more quickly to your pick attack and has a more urgent, snappy feel. Don’t overlook this mode for some of the Mini Recto’s
coolest overdrive solo sounds.
NOTE: When using PUSHED with the GAIN Control maxed (5:30), avoid setting the TREBLE Control higher than (2:30). Settings
above (2:30) may generate microphonic oscillation in certain preamp tubes in the V1 position. This can be avoided altogether
by setting the TREBLE Control sensibly when the GAIN Control is maxed. Use the PRESENCE Control for additional brightness.
CHANNEL 2: VINTAGE / MODERN VINTAGE: This high gain mode is the famous liquid Recto voice. Its lush harmonic
content and fat creamy feel is featured in so many recordings and is now a staple for so many headed to the studio for an album
project. Combining this super juicy, expressive preamp with the Rectos’ black magic power section
creates colors in gain that most players find truly addictive. Single note solo work is effortless as
the strings become easy to play with the VINTAGE modes musical and natural tube compression.
INPUT
Spend time learning the lower regions of the VINTAGE mode as this is a place where many beautiful
GAIN
FT SW
sounds live.
CLEAN
PUSHED
VINTAGE
MODERN: Aggressive. This is the word that best describes the menacing power of this Recto’s
most rebellious of all modes. A take-no-prisoners crushing assault of top end cut and lightning fast
response creates a sound of unparalleled aggression that has set a new standard for hard core sounds. The added tightness
of the low end response combined with the radically more present top end keeps the MODERN mode tracking accurately even
at extreme gain settings.
MODERN
MULTI-WATT™ POWER
Each Channel contains our patented MULTI-WATT™ power switch that allows you to choose
either 25 Watts of Pentode wired power or 10 watts of Triode power enabling you to match the power output and feel to each
of your foot switchable preamp sounds. The 10 WATT position (switch down) rewires the output tubes to
Triode style wiring for a vintage-voiced and oh-so-clippable 10 watts of power. This produces a smoother,
rounder voice that clips with a warmer attack characteristic and a creamier feel.
25 WATTS
MASTER
10 W
The 25 WATT setting (switch up) brings on line our patented Dyna-Watt™Pentode based wiring for the
full power and headroom. This setting is preferred for pristine clean sounds and tight-tracking high gain
sounds as it delivers a burst of power and headroom far beyond the expectation of its rated power. This
ingenious and unique MESA circuit stores up voltage in the power section and releases it at the instant of
attack – when you need it – then sags down to a more “normal” voltage at a time-specific rate. All this translates into a bouncy,
dynamic sound and an amazing, precise, yet elastic feel on the strings. Pure tube magic! This circuit has been appreciated by
players around the world for decades and has furthered the original Boogie “little amp that could” legend since its introduction
in the now-classic Studio 22 back in 1983.
25 W
10 WATTS
THE CONTROLS
GAIN This control adjusts the predominant gain stage in each channels’ circuit with the function and taper being optimized for each
individual channel. Remember that your Recto is really two separate multi-mode amplifiers built onto one chassis, so though each channel
looks identical, the GAIN Control for each channel comes on in a different place and adjusts a different point in that channel’s circuit.
In most guitar amplifiers, and especially in all-tube circuits, the GAIN Control is the most powerful control in the preamp. It shapes the
overall style and character of the sound and is responsible for whether the sound is clean, overdriven or anywhere in between. In your
Recto, the GAIN Control is even more powerful. It not only determines the amount of drive, but also acts as an integral part of the tone
control string as well.
PAGE 8
GAIN REGIONS
Brightest
Most Balanced
Warmer & Saturated
1) GAIN Control has three tonal regions: LOW (7:00 - 11:00) provides the cleanest,
least saturated sounds and in this region the sound will be brighter and contain more
upper harmonics lending a three dimensional character to the sound.
CLEAN
PUSHED
VINTAGE
MODERN
To simplify the GAIN Controls role in shaping the overall tone of the sound we will look
at it in two ways: 1) alone and 2) in conjunction with the tone controls.
GAIN
MIDDLE (11:15 - 2:00) enhances the saturation and replaces some of the upper harmonics with a richer, warmer quality and a fuller bottom end response. Not yet fully saturated,
this region is the easiest place to get a great sound in both channels. This region contains
many of the Recto’s best sounds...especially for soloing due to the crucial blend of an
expressive attack combined with ample sustain.
HIGH (2:15 - 5:00) saturates the signal and enhances low and low mid frequencies. While
this region provides the maximum saturation and sustain, it also compresses and softens the attack characteristics. For this
reason we suggest using this higher region of the GAIN Control sparingly and only when maximum sustain is needed.
NOTE: Due to the Recto’s extreme gain potential, the highest regions of the GAIN Control may possibly push the pre-amp tubes
past what they can handle, producing microphonic squealing. While we screen and test the tubes your amplifier was shipped
with and the tubes in your amp passed our rigourous test, we can’t predict how the tubes will respond over time exposed to
extreme gain settings. Your tubes are warranted for a period of 6 months under normal use, but you can save yourself the
present and future inconvenience of having to deal with annoying microphonic tube problems by simply using a little common
sense...Don’t turn the Gain all the way up!
If you must for a specific part or at very low volumes, back down the TREBLE and PRESENCE Controls. Your Recto was designed to provide amazing gain and tone at less than extreme settings - removing the need for you to crank everything all the
way up. If you are not able to achieve the sound you want at sensible settings on any or all of the controls, your problem may
lie elsewhere in the signal chain, i.e. pick-ups, cabinetry, processing etc. Keep in mind you can always call on one of our product specialist Monday through Thursday and seek some advice should you find yourself struggling to get the sound you want.
2) GAIN - In conjunction with the Tone Controls - Basically, a simple rule applies...as the Gain is increased the Tone control
string has less and less effect on the signal until at 5:00, the signal is so saturated that you are getting mostly Gain and very
little Tone. Again, this is the reason we suggest using the GAIN Control in its middle region. Here, the Tone control string is very
active and provides maximum shaping power - allowing you to dial virtually any sound you desire.
TREBLE As in most tube guitar amplifiers, the TREBLE Control (in both channels of your Recto ) is the most powerful of
the tone controls and is next in line only to the GAIN Control as a shaping tool. Because it is first in the signal path of the tone
controls - and from here the Middle and Bass receive their signal - it is by far the dominant tone control. For this reason the setting of the TREBLE Control is very important for equal representation of the three frequency regions to appear at their respective
controls. Like most of the controls on your Recto, there is an optimum region of the TREBLE Control where
ample top end is mixed in and yet enough signal is still passed on to the MIDDLE and BASS Controls.
TREBLE
As you might surmise, 11:00 - 1:30 is the sweet spot. There are definitely great sounds above and below this
middle region, but the balance between the TREBLE Control and the other two tone controls is compromised.
The one place you may want to throw caution to the wind and set the TREBLE Control above this median
zone presents itself in Channel 1 of your new Recto. In both modes (Clean & Pushed), the TREBLE Control
can be used to dump extra gain into the mix. This is especially effective in the PUSHED mode for crunch
SWEET SPOT
sounds. When doing so use the PRESENCE Control to roll off some of the more than ample top end for a
more compressed feel and fatter voice. The BASS Controls’ effectiveness will be reduced, so you may have to run a much
higher setting than you are used to seeing to achieve a balance. This said, keep in mind that the TREBLE Control in Channel 1
Pushed should not be set much above 3:00 to avoid unwanted microphonic tube problems.
PAGE 9
MID The MID Control is responsible for the blend of midrange frequencies in the mix and though its effect is not as dramatic
as that of the TREBLE Control, it plays an integral part in achieving any sound in your Recto. It is capable of changing the feel
dramatically as it blends in a group of frequencies that tend to soften or stiffen the way a sound feels to play.
Most players tend to lean in the direction of lower MIDDLE Control settings (7:00 - 11:00) where a scoop in this region produces girth (by letting the Bass become a little more dominant ) and a lack of punch lends a more compressed, even feel to
the strings and therefore less apparent resistance to the pick. As the MIDDLE Control is increased, (11:30 - 1:30) the sound
is rounded-out and filled-in with a focused midrange attack appearing rather quickly. As you
MID
would guess, the feel starts to change – becoming slightly more resistant. Above this region
the MIDDLE Control could be used to compensate for either weaker pick-ups or for times
when a specific deficiency is created either by an extremely high setting of other tone controls,
or a physical anomaly in the room. While these MIDDLE Control settings (2:00 - 5:00) can
ADDS PUNCH TO
WEAKER PICK-UPS
introduce added gain and create enhanced focus, the trade-off will be a stiffer, more forward,
SCOOPED AND
EASY TO PLAY
less compressed feel.
Channel 1 utilizes a different MIDDLE Control than that of the Channel 2 with a custom-designed taper and value. In its low
range (below 12:00) it functions as a normal midrange control with a taper suited to blending fine increments of these frequencies. Most players lean toward a fairly radical scoop (7:00 - 10:30) for clean playing, preferring to let Treble and Bass remain
dominant, thus producing the signature sparkle and breath essential for a pristine clean sound.
As Channel 1’s MIDDLE Control is swept past 1:00, it starts to add gain in these midrange frequencies adding cut and punch.
As the top end of the control is reached, (3:00 - 5:00) it becomes an additional gain control capable of taking both CLEAN and
PUSHED modes to extremes. Experiment with this cranked region in conjunction with conservative settings of the other tone
controls to balance both sound and feel. While this added flexibility may make Channel 1’s MIDDLE Control a little more tricky
to learn at first, it will become quite valuable as you start to realize the power of this super versatile channel.
BASS Last, but not least in the string of tone controls, we come to the BASS. This control works similarly in both channels
in that it determines the amount of low frequencies present in a sound. However, the actual frequencies and style of lows it
mixes in changes with the channels. Like the MIDDLE Control, it falls in line signal-wise after the TREBLE Control and the same
scheme applies. When the TREBLE Control is set high, the effectiveness of the BASS and MIDDLE Controls is reduced. If the
TREBLE Control is set low, these two controls become dominant.
BASS
For the most balanced sound and a balance of power between the three rotary tone controls, try to
use the TREBLE Control in its middle ranges. This scenario produces nearly equal representation of
all the frequencies on the tone controls and provides a great neutral starting point for further tweaking.
While the Mini Recto’s EL84 power section is not capable of delivering the massive amount of bass
frequencies that the big Recto’s do with their 6L6 power tubes, it is still worth mentioning to use the
BASS control with finesse. Too high a BASS setting can cause a bloated, flubby attack and create a
sound that is unbalanced and even downright bad. A good rule to use as a guideline is this; as the
GAIN is increased, the BASS should be decreased. The higher gain settings add fullness and warmth
VARIABLE ACCORDING
- and bass - allowing you to get away with lower BASS settings. Another thing that high settings of
TO TREBLE & MID CONTROL
the BASS does is to eat up headroom. Low frequencies take more power to deliver at a given volume
than high frequencies due to their longer wave length. Your amp will clip sooner if there is a large amount of bass it has to amplify. This is most noticeable in Channel 1 in the CLEAN Mode when looking for clean rhythm sounds. The highest headroom is
attained for clean chording by keeping the GAIN and BASS not far above 12:00 - somewhere between 11:00 and 12:30. This
will ensure that you are not “loading up” the front end with too much gain or low end and any clipping you experience will most
likely come from the tubes reaching their limit of power.
PRESENCE
This Control handles high frequency attenuation that is above that of the TREBLE control and its location in
the circuit moves depending on the Channel and Mode you have called up. In some Modes it’s a roll-off control at the end of
the preamp, in others it controls the amount of negative feedback at a given frequency in the power section. It acts indepenPAGE 10
BRIGHT &
AGGRESSIVE
WARM
& VOCAL
PRESENCE
dently of the other rotary tone controls and is crucial in voicing the Modes. It is a powerful global
tone control. Lower PRESENCE Control settings darken and, in fact compress the signal, which
works well to fatten single note solo sounds, giving them girth and focus. Some of the best lead
sounds in your Recto will find the PRESENCE Control in its lower regions where a balanced, vocal
response is achieved.
One example of this would be when dialing up Bluesy solo sounds in the CLEAN Mode. Here, it
is helpful to add gain early in the preamp with the GAIN and TREBLE set higher (5:00 & 2:00) than normal and then roll back
the PRESENCE (9:00 – 10:00) to get the sound to warm up again. This scheme can be repeated throughout all the modes and
also in reverse - where the PRESENCE is higher and the TREBLE is reduced. That said, many of the best sounds are achieved
with both TREBLE and PRESENCE straying above 12:00 noon only occasionally, as they both pack powerful frequencies that
can be dangerous at the top of their range.
Higher settings unleash the mighty roar of your Mini Recto and produce brighter, more pointed sounds that will cut through
a mix. This can be great for sparkling clean sounds in Channel 1 and more aggressive crunch rhythm sounds in PUSHED,
VINTAGE and MODERN.
MASTER
This control is the feed from the end of the pre-amp to the driver stage and the Effects Loop. As you can see,
each Channel is fitted with its own MASTER Control, enabling the two channels relative volumes to be matched regardless of
their extremely different sound styles and gain signatures. The MASTER Control makes
MOST SENSIBLE RANGE
possible a wide range of sounds through its ability to use very low GAIN settings at high
EASIEST TO BALANCE CHANNELS
REASONABLE FX LOOP SIGNAL LEVELS
volumes and conversely, high GAIN settings at low volumes and everywhere in between.
25 WATTS
10 W
MASTER
Again, we suggest using the MASTER Control in its sensible ranges (9:00 - 2:00). Here,
the channels will be easier to match with each other and the Effects Loop will see more
reasonable signal levels.
NOTE: Because the MASTER Control determines the send level of the Effects Loop, extreme
settings may cause possible overloading of FX processor’s or pedal’s Input stage, but also
may make balancing the two channels’ Effect Send level difficult.
25 W
10 WATTS
STANDBY
Perfect for set breaks...this toggle switch also serves an even more important purpose. From cold-start, the
STANDBY position allows you to warm up the tubes - especially power tubes - before applying the high voltage by switching the
Standby to the “ON” position. Before POWER is switched to “ON” - make sure the STANDBY switch is in the “STANDBY” position.
Wait at least 30 seconds and then flip the STANDBY switch to the ON position. This prevents the shock of high voltage hitting
cold tubes and reduces the likelihood of tube problems and increases their toneful life substantially.
POWER ON This switch delivers the A.C. power to the Mini Recto. Make sure the unit is grounded (all three terminals of
the A.C. power cord must be connected to avoid injury to the user as well as to the unit) and that the proper voltage is present.
ON
CH 1
FT
SW
CH 2
POWER
STANDBY
ON
Always follow the cold start procedure described in the STANDBY section above when powering up your
amplifier. This will reduce the likelihood of tube problems and increase their musical life.
That covers the Channels, Modes and Controls found on the Front Panel of your Mini Recto, now let’s
swing around to the Rear Panel and take a look at the Loop and Speaker Outputs.
PAGE 11
REST AREA
PAGE 12
REAR PANEL
CAUTION:
Unplug power
before replacing
fuse
TUBE GUIDE
5 x 12AX7
2 x EL 84
WARNING!
ALL TUBE AMPLIFIER
RECTIFIER
M I N I -TW E N T Y-FI V E
D Y N A - W AT T
HANDBUILT IN PETALUMA, CA
CAUTION: CHASSIS SURFACE MAY BE HOT!
8 OHM
RETURN
4 OHM
SPKRS
1A
SLO
BLO
CAUTION!
SEND
FX LOOP
120 V~
60 Hz
1A
PATENTED
6,621,907
7,602,927
SPEAKERS One 4 Ohm and one 8 Ohm jack is provided for speaker interfacing. MESA/Boogie cabinets are built wired to 8
Ohms unless custom ordered or modified by an outside party (i.e. a previously owned cabinet). One 8 Ohm cabinet should be
connected to the 8 Ohm output. When using two 8 Ohm MESA cabinets, connect one cabinet to the 4 Ohm output and “daisy
chain” the second cabinet by coming out of the first cabinet’s Paralleled Out jack into the second cabinet’s input jack (MESA
cabinets provide a Parallel jack for this purpose).
8 OHM
RETURN
4 OHM
SPKRS
FX LOOP
SEND
If you are using non-MESA cabinets with your Mini Recto, it is very important to know your cabinet’s
impedance and make sure you are connecting speakers correctly and safely to the amp. Failure to connect speakers correctly can cause expensive damage to tubes, transformers and the amp in general.
ALWAYS make sure to connect your cabinet correctly to your amp. Check out the information later
in this manual regarding speaker impedance and speaker hook-up schemes. No matter how unusual
your speaker setup, it is generally possible to get good performance.
FX LOOP
Your Mini Recto benefits from 40 years of high performance amp design and the Effects Loop featured here is
a result of that experience and expertise.
The Effects Loop is wired in Series with the dry signal and as long as you are using processors of good quality, you should
experience seamless interfacing with no tonal degradation. That said, anything you insert in this extremely sensitive junction of
an amplifier (between preamp and power section) may have the potential to slightly alter the sound. Also, every company that
builds processors has their own way of doing things and there are controls and features that empower, and some that make
things more complicated while adding questionable sonic benefit. We suggest taking your amp and trying it with the processor
you are auditioning to make sure you will have great performance when they are coupled.
While most pedal-type effects will work in the Loop, it is always best to connect overdrive, distortion, boost, compression and
wah or envelope filter effects between the instrument and the Front End of the amplifier. These effects are designed to alter the
sound of the instrument before it is amplified, and you will get better results not using the Loop for these applications.
To interface your outboard processors with the Effects Loop, connect them like this:
1.Connect the FX SEND to the INPUT of your first processor.
2.Connect (daisy-chain) subsequent processors in series. OUTPUT feeding INPUT.
3.Connect the OUTPUT of the last processor in line to the amp’s FX RETURN.
4.Adjust INPUT and/or OUTPUT levels on processors to achieve “unity gain”.
If you want to check on how much (if any) signal loss and/or tonal degradation is occurring due to your outboard effects and/
or cable length, simply A/B the sound with your effects connected and then pull the cables out of both your SEND and RETURN
jacks quickly and compare the sound. If the signal jumps up in volume you may want to increase either the input levels, output
levels or both on some or all of your devices. Ideally you want to hear no change in volume as you insert and remove the
cables from the FX SEND and RETURN jacks (a unity gain scenario). A best case scenerio is signal sent from the FX SEND is
arriving back at the RETURN with no overall change in gain.
PAGE 13
If you experience no change in signal level (and therefor have no loss in gain) but hear a loss of top end sparkle or clarity,
check the cable lengths in your FX LOOP signal path. It is always advisable to use the shortest length cables of the highest
quality and preferably “Low Capacitance” type cable. If you are forced to use longer runs of cable to incorporate processors
on your pedal board you can try incorporating a Buffer to better match the impedances and reduce the losses due to extra cable
capacitance. With a Buffer in the signal chain you will be able to run longer lengths of FX LOOP cabling without virtually any
top end roll-off due to capacitance.
You can also use a Buffer or Line Driver on the front end’s signal path (between your guitar, your front end pedal board effects
and the INPUT of your amplifier) to reduce losses and tonal compromises resulting from impedance differences and cable
lengths. A buffer may change the relationship between your guitar’s pickups and the first tube stage, but this is often preferable
to the losses that occur from long cable lengths and/or impedance mismatches.
AC SOCKET (Quick Disconnect Style)
This is the AC MAINS Power Cord Socket. The standardized removable power
cable supplied with your amp can only be plugged in one way. Always connect the male end to a grounded wall socket with
the proper voltage present (117 Volts on U.S.A. Models). To Avoid The Risk Of Shock, Never Alter The Power Cable in any
way. Altering the Power Cable will void your warranty and put you at risk while leaving your
amplifier open to the possibility of damage.
120 V~
60 Hz
1A
1A
SLO
BLO
MAIN FUSE
This is the A.C. (Alternating Current) main fuse and provides amplifier circuit
protection
from
power
and/or rectifier tube failure as well as from A.C. voltage fluctuations. ALWAYS
CAUTION:
Unplug power
replace a blown fuse with one of the specific rating and type as (conveniently) printed on the back of
before replacing
fuse
your amplifier right next to the fuse holder. NEVER INSTALL A FUSE OF A HIGHER RATING than the
value printed on the back of your amplifier. Doing so will very likely produce damage well beyond
the original reason your fuse blew in the first place.
A power tube short or failure is often the cause of a blown fuse. Position yourself behind the amplifier and follow the cold start procedure
mentioned in the STANDBY switch section of this manual. Watch the power tubes as you flip the STANDBY to the ON position. If a power
tube is going bad or is arcing you will often see it as you power up! Flip the STANDBY switch to Standby immediately and replace the
faulty power tube and the fuse as necessary.
If you see nothing abnormal as you move the STANDBY switch to ON, it is possible that a power tube shorted ‘temporarily’ and caused
the blown Fuse. Occasionally, whether from just luck or from cooling down, a short in a power tube ‘reconnects’ itself temporarily and
can operate normally again… but this tube should be considered faulty and replaced as soon as possible to ensure uninterrupted performance. If you can identify a tube that is arcing or shorting by the method mentioned above and the tubes have minimal use, consider
replacing the obviously faulty or failed tube and leaving the others installed.
If you haven’t changed tubes for a while after heavier use, this failure may be telling you it’s time to change all your power tubes. Save
any working but used tubes as your new spares.
Spare fuses (of the proper type and rating) are a must for the fabled cord bag along with spare tubes. Always have both on hand – at
the gig or at home – since tubes decide when it’s time to stop working. Spare tubes and fuses can be worth their weight in gold should
you ever experience a tube failure.
PAGE 14
FACTORY SAMPLE SETTINGS
Sample Settings #1:
TITLE: Sweet Clean (CH1) / Soulful Solo (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
MODERN
25 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
10 WATTS
Sample Settings #2
TITLE: Fat Clean (CH1) / Classic Rock (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
MODERN
25 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
10 WATTS
Sample Settings #3
TITLE: Edge Clean (CH1) / Fat Crunch (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
MODERN
25 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
10 WATTS
Sample Settings #4
TITLE: Old School Cranked (CH1) / Stinging Blues (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
MODERN
PRESENCE
MASTER
25 W
10 WATTS
PAGE 15
ON
CH 1
FT
SW
CH 2
POWER
STANDBY
ON
FACTORY SAMPLE SETTINGS
Sample Settings #5:
TITLE: Rock Clean (CH1) / Liquid Gain Solo (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
MODERN
25 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
10 WATTS
Sample Settings #6
TITLE: Tight Crunch (CH1) / Blistering Lead (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
MODERN
25 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
10 WATTS
Sample Settings #7
TITLE: Punk Rhythm (CH1) / Fast Bright Lead (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
MODERN
25 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
10 WATTS
Sample Settings #8
TITLE: Little Amp Cranked (CH1) / (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
MODERN
PRESENCE
MASTER
25 W
10 WATTS
PAGE 16
ON
CH 1
FT
SW
CH 2
POWER
STANDBY
ON
FACTORY SAMPLE SETTINGS
Sample Settings #9:
TITLE: Rock Rhythm (CH1) / Ridiculous Gain Lead (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
MODERN
25 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
10 WATTS
Sample Settings #10:
TITLE: Tight Gain Rhythm (CH1) / Lo Power Lead (CH2)
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
MODERN
PRESENCE
MASTER
25 W
10 WATTS
PAGE 17
ON
CH 1
FT
SW
CH 2
POWER
STANDBY
ON
PERSONAL
SAMPLESETTINGS
SETTINGS
FACTORY SAMPLE
TITLE:
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
25 W
MODERN
10 WATTS
CLEAN
25 WATTS
PUSHED
10 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
TITLE:
INPUT
FT SW
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
ON
CH 1
FT
SW
25 W
POWER
STANDBY
CH 2
MODERN
10 WATTS
CLEAN
25 WATTS
PUSHED
10 W
ON
TITLE:
INPUT
FT SW
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
ON
CH 1
FT
SW
25 W
MODERN
10 WATTS
CLEAN
25 WATTS
PUSHED
10 W
POWER
STANDBY
CH 2
ON
TITLE:
INPUT
FT SW
VINTAGE
GAIN
TREBLE
MID
BASS
MODERN
PRESENCE
MASTER
25 W
10 WATTS
PAGE 18
ON
CH 1
FT
SW
CH 2
POWER
STANDBY
ON
PERSONAL
SAMPLESETTINGS
SETTINGS
FACTORY SAMPLE
TITLE:
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
25 W
MODERN
10 WATTS
CLEAN
25 WATTS
PUSHED
10 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
TITLE:
INPUT
FT SW
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
ON
CH 1
FT
SW
25 W
POWER
STANDBY
CH 2
MODERN
10 WATTS
CLEAN
25 WATTS
PUSHED
10 W
ON
TITLE:
INPUT
FT SW
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
ON
CH 1
FT
SW
25 W
MODERN
10 WATTS
CLEAN
25 WATTS
PUSHED
10 W
POWER
STANDBY
CH 2
ON
TITLE:
INPUT
FT SW
VINTAGE
GAIN
TREBLE
MID
BASS
MODERN
PRESENCE
MASTER
25 W
10 WATTS
PAGE 19
ON
CH 1
FT
SW
CH 2
POWER
STANDBY
ON
PERSONAL
SAMPLESETTINGS
SETTINGS
FACTORY SAMPLE
TITLE:
INPUT
FT SW
CLEAN
25 WATTS
PUSHED
10 W
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
25 W
MODERN
10 WATTS
CLEAN
25 WATTS
PUSHED
10 W
ON
CH 1
FT
SW
POWER
STANDBY
CH 2
ON
TITLE:
INPUT
FT SW
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
ON
CH 1
FT
SW
25 W
POWER
STANDBY
CH 2
MODERN
10 WATTS
CLEAN
25 WATTS
PUSHED
10 W
ON
TITLE:
INPUT
FT SW
VINTAGE
GAIN
TREBLE
MID
BASS
PRESENCE
MASTER
ON
CH 1
FT
SW
25 W
MODERN
10 WATTS
CLEAN
25 WATTS
PUSHED
10 W
POWER
STANDBY
CH 2
ON
TITLE:
INPUT
FT SW
VINTAGE
GAIN
TREBLE
MID
BASS
MODERN
PRESENCE
MASTER
25 W
10 WATTS
PAGE 20
ON
CH 1
FT
SW
CH 2
POWER
STANDBY
ON
PAGE 21
2x
EL 84
MINI RECTIFIER® TWENTY FIVE
TUBE TASK CHART
A
FRONT OF CHASSIS
TUBE NOISE & MICROPHONICS
You may occasionally experience some form of tube noise or microphonics. Certainly no cause for alarm, this quirky behavior comes with the territory and the Tone. Much like changing a light bulb, you don’t
need a technician to cure these types of minor user serviceable annoyances and in fact, you’ll be amazed at how easy it is to
cure tube problems...by simply swapping out a pre-amp or power tube!
First may we suggest that you set the amplifier up on something so that you can get to the tubes comfortably without having to
bend down. It also helps to have adequate lighting as you will need to see the tube sockets clearly to swap tubes. Use caution
and common sense when touching the tubes after the amplifier has been on as they may be extremely hot! If they are hot and
you don’t want to wait for them to cool off, try grasping them with a rag and also note that the glass down around the bulbous
silvery tip is considerably less hot which makes it easier to handle. Gently rock the tube back and forth as you pull it away from
its socket.
DIAGNOSING POWER TUBE FAILURE
There are two main types of tube faults: shorts and noise. Both large and
small tubes may fall prey to either of these problems but diagnosis and remedy is usually simple.
If a fuse blows, the problem is most likely a shorted power tube and shorts can either be mild or severe. In a mildly shorted tube
the electron flow has overcome the control grid and excess current flows to the plate. You will usually hear the amp become
distorted and begin to hum slightly. If this occurs, quickly look at the power tubes as you switch the amp to STANDBY and try
to identify one as glowing red hot. It is likely that two of a pair will be glowing since the “shorted” tube will pull down the bias
for its adjacent mates, but one tube may be glowing hotter — and that one is the culprit. The other two are often fine — unless
they’ve been glowing bright red for several minutes.
Because there is no physical short inside the tube (just electrons rioting out of control) merely switching to STANDBY for a few
moments then back to ON will usually cure the problem...at least temporarily. Watch the tubes carefully now. Should the problem
recur, the intermittent tube will visibly start to over heat before the others and thus it can be identified. It should be replaced with
one from the same color batch, shown on its label. Call us and we will send one out to you.
The severe short is not nearly so benign. In the worst cases, a major arcing short occurs between the plate and the cathode
with visible lightning inside the glass and a major noise through the speaker. If this is seen to happen, IMMEDIATELY turn the
amp to STANDBY. By this time the fuse probably will have blown. Such a short is usually caused by a physical breakdown inside
the tube including contaminate coming loose or physical contact (or near contact) between the elements. Replace it and the
fuse with the proper slo-blo type and power up the amp using the power up procedure as we described earlier in this manual.
TUBE NOISE
Often caused by contamination within in a tube, the culprit can usually be identified, and by lightly tapping
on the glass, you will probably hear the noise change. Hearing some noise through the speakers while tapping on the 12AX7’s
is normal however. And the one nearer the INPUT will always sound louder because its output is being further amplified by the
second 12AX7.
The power tubes should be all but quiet when they are tapped. If crackling or hissing changes with the tapping, you have probably found the problem. To confirm a noisy power tube, merely put the Recto on Standby, remove it from its socket and turn
it back on. It will cause no damage to run the Recto briefly with one power tube missing. You may notice a slight background
hum, however, as the push-pull becomes unbalanced. Whenever you are trying to diagnose a suspect tube, keep your other
hand on the POWER and STANDBY switches ready to shut them off instantly in the unlikely case you provoke a major short.
If you think you’ve located a problem tube but aren’t sure, we recommend substituting the suspect with a new one just to be sure
of your diagnoses. You will be doing yourself and us a big favor by just following the simple guidelines previously mentioned
regarding tube replacement. You’ll probably be successful with much less effort than is required to disconnect everything and
haul the unit to a technician who will basically perform the same simple tests. If the tubes are still within their six-month warranty period, we will happily send you a replacement. Just note the color designation on the tube label so that we can send you
the appropriate match.
PAGE 22
DIAGNOSING PRE-AMP TUBE PROBLEMS
Because your amplifier is an all tube design, it is quite possible that
you will at some point experience minor pre-amp tube noise. Rest assured - this is no cause for alarm and you can take care
of the problem yourself in a matter of minutes by simply swapping tubes.
Let us begin by saying; It is a “very good” idea to keep at least a couple of spare pre-amp tubes on hand at all times to insure
uninterrupted performance. These minor pre-amp tube problems can take many forms but can generally be described in two
categories: Noise and Microphonics. Noise can be in the form of crackling, sputtering, white noise/hiss and/or hum. Microphonic
problems usually appear in the form of a ringing or high pitched squealing that gets worse as the gain or volume is increased
thus are more noticeable in the higher gain “HI” modes. Microphonic problems are easily identified because the problem is still
present even with the instruments’ volume off or unplugged altogether - unlike pick-up feedback which ceases as the instrument is turned down. Microphonic noise is caused by mechanical vibration and shock: think of banging a microphone around
and you’ll understand where the word came from.
The best way to approach a pre-amp tube problem is to see if it occurs only in one specific mode or channel. This should lead
you to the tube needing replacement. Then all that remains is to swap the suspect tube for a known good performer. If you
cannot narrow down the trouble to a specific mode or channel, the problem may be the small tube that drives the power tubes
which is operational in all modes and channels. Though rare, a problem with the driver tube would show up in all aspects of
performance - so if you can’t narrow the problem down to being mode or channel specific, you may want to try replacing the
driver tube. Driver problems generally show themselves in the form of crackling or hum in all modes of performance and/or
weak overall output from the amplifier. Occasionally an anemic driver tube will cause the amplifier to sound flat and lifeless, but
this is somewhat uncommon, as worn power tubes are a more likely suspect for this type of problem.
Sometimes making the diagnosis is more trouble than it’s worth and it’s faster and easier to merely replace the small pre-amp
tubes ONE AT A TIME with a replacement known to be good. But MAKE SURE you keep returning the tubes to their original
socket until you hit the one that cures the problem. You’ll notice that tubes located nearer to the INPUT jack always sound
noisier...but this is because they are at the start of the chain and their noise gets amplified over and over by the tubes that follow.
The tube that goes into this “input socket” (usually labeled V1) needs to be the least noisy of the bunch. The tube that goes at
the end of the preamp chain - just ahead of the power tubes - can be quite noisy without causing any problem at all. The tubes
in your amp have already been located in the most appropriate sockets and this is why you should NEVER pull them all out at
once and ALWAYS swap them one at a time. ALWAYS return a perfectly good tube to its original socket. Also it’s a good idea
to put the amp on STANDBY when swapping tubes to reduce the heat build up in the tubes themselves and to prevent explosive
noises (which can still occur even if you are pulling the tubes away from their sockets gently) from coming through the speaker.
Remember, take your time, be patient and chances are real good that you can fix your amp yourself by finding and replacing the
bad tube. It kills us to see someone who has shipped their amp back to us...and all it needed was a simple tube replacement!
If you must send back your amp, remove the chassis from the cabinet by unscrewing the four mounting bolts on the bottom
top. The chassis then slides back like a drawer and comes out from the back. Remove the big power tubes and mark them
according to their location from left to right 1, 2 etc. They need to be wrapped separately with plenty of wadded up newspaper
around them and put in a smaller box within the larger carton. Remove the Rectifier tubes and wrap them also. You can leave
the preamp tubes in or remove them and wrap them separately being sure to label their location. (See Tube Task Chart.)
To wrap the chassis, use plenty of tightly wadded up newspaper so there is at least six inches of “crush space” between the
chassis and the cardboard box. Bubble wrap also works well, but please DON’T use styrene peanuts - they will shift during
transit and get lodged inside your electronics as well as allowing your amp to end up at the bottom of the box unprotected and
possibly damaged.
Pre-amp tubes don’t normally wear out as a rule. Therefore, it is not a good idea to change them just for the sake of changing
them. If there isn’t a problem - don’t fix it. If there is no result from your substitutions, it may be possible that you have more than
one problematic tube. Though rare, this does happen and though it makes the troubleshooting process a little more intimidating,
it is still possible to cure the problem yourself.
NOTE: It is normal to hear a slight metallic ringing sound when tapping on the preamp tubes. As long as the tube does not break
into oscillation or start crackling or any other form of bizarre noise, it is considered normal and functional.
PAGE 23
SPEAKER IMPEDANCE MATCHING & HOOK-UP GUIDE:
IMPEDANCE
Wiring up speakers to provide the most effective load and making sure that all of them are in phase will help
in creating the best sound possible. This is not too difficult, as long as you understand a few things about loading and how to
connect your speakers to provide an optimal resistive load.
MESA/Boogie amplifiers can handle 4 and 8 ohms effectively. Never run below 4 ohms in a tube amplifier unless you are
absolutely certain that the system can handle it properly; this can cause damage to the Output transformer. A few amplifiers
can handle 2 ohms effectively without damaging them (for example the MESA’s Bass 400+). You can always have a higher
resistance (16 ohms, for example) without damaging results, but too low of a resistance will likely cause problems.
MIS-MATCHING:
When running a higher resistance (for example: 8 ohm output into 16 ohm cabinet), a slightly different
feel and response will be eminent. A slight mismatch can provide a darker smoother tone with a little less output and attack.
This response is a result of the amplifier running a bit cooler. Sometimes when using more than one cabinet a mismatch will
be the only option.
WHAT IS MY CABINET’S IMPEDANCE? If you have only a single speaker, you just match that single speakers impedance
to the amplifier, and you are done. In many cases, you will have a number of speakers, and then you must calculate the “load”
that the amplifier will need to support. There are generally three ways to wire multiple speakers together. They are as follows:
SERIES:
When you wire (hook-up) speakers in Series, the speakers resistance (as measured in ohms) is additive - i.e.
putting two 8 ohm speakers in Series results in a 16 ohm load.
NEGATIVE =
A
CK
SHA D
BL
BL
SHA D
Speaker A = 8 Ohms
OW
CK
OW
A
POSITIVE =
Speaker B = 8 Ohms
SERIES: Connect the Negative side of Speaker A
to the Positive side of Speaker B
PAGE 24
SPEAKER IMPEDANCE MATCHING & HOOK-UP GUIDE: (Continued)
PARALLEL:
When wiring in parallel, the resistance of the speakers decreases. Two 8 ohm speakers wired in (hookedup) Parallel results in a 4 ohm load. It’s easy to calculate the effect of a resistive load when all the speakers are all the same
resistance. It is really not suggested to wire different resistive load values in Parallel (8 and 4, 16 and 8 etc.) The formula for
figuring the total impedance in Parallel is the multiplication of the two loads divided by the sum of the two loads - i.e. putting
two 8 ohm speakers in Parallel results in a 4 ohm load. Connect the Positive side of Speaker A to the Positive side of Speaker
B - Connect the Negative side of Speaker A to the Negative side of Speaker B.
Speaker B
8 Ohms
BL
BL
Speaker A
8 Ohms
NEGATIVE
POSITIVE
Total Load = 4 Ohms
All 4 Spkrs. are 8 Ohms
SPEAKER A
COMBINATION OF SERIES & PARALLEL:
SPEAKER B
This is really just two sets of Series wired speakers connected in Parallel. This is how you maintain a consistent
load with multiple speakers. The importance of this is
more evident when you have more than one cabinet to
connect to your amplifier. This is when you need to figure
out the loads and how to wire them up without applying
too low of a resistance on the amplifier.
A
BL
A
BL
A
A
Simply connect the Positive side of Speaker A to the Positive
side of Speaker C.
SPEAKER D
BL
Connect the Negative side of Speaker A to the Positive side
of Speaker B. Next, connect the Negative side of Speaker
C to the Positive side of Speaker D.
A
A
A
A
BL
SPEAKER C
And lastly, connect the Negative side of Speaker B to the
Negative side of Speaker D.
POSITIVE
4 Eight ( 8 ) Ohm speakers wired in Series Parallel =
NEGATIVE
PAGE 25
WIRING SCHEMES...Amplifier to Speaker Cabinets
1
2
Partial back view of amplifier
8 OHM
4 OHM
8 Ohm Cabinet
3
8 OHM
4 OHM
4 Ohm Cabinet
4
Partial back view of a Mesa amp
8 OHM
Partial back view of amplifier
4 OHM
8 Ohm Cabinet
Partial back view of amplifier
8 OHM
4 OHM
16 Ohm Cabinet
SAFE MISMATCH
SAFE MISMATCH
PAGE 26
WIRING SCHEMES...Amplifier to Speaker Cabinets
5
Partial back view of amplifier
4 OHM
8 OHM
16 Ohm Cabinet
8 Ohm Cabinet
SAFE MISMATCH
6
7
Partial back view of amplifier
8 OHM
4 OHM
Partial back view of amplifier
8 OHM
8 Ohm
8 Ohm
SERIES BOX
4 Ohm
4 Ohm Cabinet
4 Ohm
4 OHM
PARALLEL BOX
CORRECT
MATCH
16 Ohm
4 Ohm Cabinet
16 Ohm Cabinet
16 Ohm
CORRECT
MATCH
16 Ohm Cabinet
NOTE: Both Series and Parallel boxes may be purchased through Mesa/Boogie by contacting Customer Service: 707-778-6565, Mon-Thurs 9am-5pm PST
PAGE 27
WIRING SCHEMES...Amplifier to Speaker Cabinets
8
Partial back view of amplifier
4 OHM
16 Ohm
8 OHM
SERIES BOX
8 Ohm
8 Ohm
8 Ohm Cabinet
8 Ohm Cabinet
8 Ohm Cabinet
SAFE MISMATCH
9
Partial back view of amplifier
8 OHM
4 OHM
8 Ohm
PARALLEL BOX
16 Ohm
16 Ohm Cabinet
16 Ohm Cabinet
16 Ohm
16 Ohm Cabinet
SAFE MISMATCH
NOTE: Both Series and Parallel boxes may be purchased through Mesa/Boogie by contacting Customer Service: 707-778-6565, Mon-Thurs 9am-5pm PST
PAGE 28
BIAS ADJUSTMENT
A Feature Article by Randall Smith
Here’s a question we often hear:
“Why doesn’t MESA put bias adjustments in their amplifiers?”
Well, there’s a short answer and a long answer to this question.
CATHODE ( )
GRID ( , )
SCREEN
GRID ( )
...
....
.
...
.
...
.
....
BEAMCONFINING
ELECTRODE ( )
PLATE
( , )
The short answer is that during my 12 years of repairing Fenders,
Structure of a 6L6 / 5881
one of the most frequent problems I saw was bias controls that
Beam Power Pentode.
were either set wrong or that had wandered out of adjustment due
to vibration. As any honest tech will tell you, there’s lot’s of easy money to be made by sprinkling “holy water” on amplifiers
... uh, what I meant to say is “Your amp needed biasing.” See what I mean? What customer is going to argue with that?
It only takes a moment and a volt meter: The Fender diagram shows how: “Adjust this trim pot for - 52 volts.” That’s it. Nothing more.
Now don’t be fooled into thinking that tubes “draw” more or less bias, they don’t. The way a bias supply is connected to a tube
is akin to a dead end road, it just trails off to nowhere without really completing a circuit. It’s a static voltage and regardless of
what tube is in the socket — or even if the tubes aren’t plugged in at all, it doesn’t change the bias voltage a bit.
So the end of the short answer is this: Since a bias supply needs to put out the right voltage and never vary, I wanted to build
amplifiers that were individually hard wired to the correct values and NEVER needed adjustment. And for 25 years, that’s how
MESA/Boogies have been built.
Time to change tubes? Just plug our tubes into any one of our amps and you’re DONE. No tech needed. NO bills and no BS
about biasing. And most important: The bias is RIGHT because it can’t change!
Now, you want the long answer? Here’s more information on how our hard-wired bias avoids trouble. Please read on.
But first, let’s make an important distinction. Our business is designing and building high performance amplifiers. And for this we
need tubes whose variance is within a narrow range. Our warehouse is full of rejects ...oh, they work — they just don’t perform
within our tolerance range. We have a very sophisticated computer - based tube testing system (nicknamed “Robotube”) that
matches and measures tubes over seven important parameters. It can even predict which tubes are likely to have a shortened
lifetime — even though they work perfectly during the test.
Because our business is building quality amps, we can afford to reject a lot of wayward tubes. The guys you hear complaining
because Boogies don’t have bias adjusters are primarily in the business of selling tubes - not amps. They don’t want to throw
away 30 percent of their inventory, so they promote the idea that tubes outside our parameters can be used to “customize”
amplifiers and they criticize us because our amps can’t be adjusted to accommodate their out-of-MESA tolerance tubes.
Now you might be thinking, “But I thought you just said that tubes don’t “draw” bias, therefore they don’t effect the bias supply
and thus it doesn’t need to be adjustable.”
When you set the bias (whether it’s by selecting the right resistors, as we do, or adjusting a trimmer — which is quicker) what
you are doing is establishing the correct amount of idle CURRENT that flows through the power tubes. But you can’t adjust
the current directly, you can only change it by adjusting the amount of bias VOLTAGE that goes onto the tubes’ control grids.
Voltage and current are NOT the same. Current is the AMOUNT of electricity, the “quantity” — and is measured in amperes.
Voltage is the degree of electric charge — like the “pressure” to use the old water analogy. Let me illustrate how different
PAGE 29
BIAS ADJUSTMENT (Continued)
voltage and current are: When you scrape your feet across a carpeted floor in dry, wintery conditions, your body can become
charged with 50,000 to 100,000 volts of static electricity. And when you reach for the door knob, a spark jumps and you feel
it! The voltage is super high but the current (measured in micro-amps) is tiny - otherwise you would die from electrocution.
Contrast this with your car battery, which puts out a mere 12 volts. You can lay your hands right across the terminals and not
feel a thing. Yet the amount of current available can run to several hundred amperes .. enough to turn over a cold engine and
get it started.
So current and voltage are two totally separate electrical parameters — though when you multiply them together, you get
POWER, which is measured in watts.
When you set the bias of an amplifier, you are adjusting the static VOLTAGE at the control grid of the tube in order to produce
a desired amount of idle CURRENT flowing to the tube’s plate. A small change in grid voltage, produces a large change in the
amount of current flowing — and that’s basically how a tube works. Say that again because it’s super important: A small
change in voltage at the grid causes a large change in current flowing to the plate. See, that’s the essence of amplification: A
small change causing a large change. And here it’s a small voltage change causing a large current change.
The bias conditions are what determines how much current flows through the big power tubes when you’re not playing. And
what drives your speakers is fluctuations in that current flow when you ARE playing. If the amount of current increases and
decreases 440 times per second, then you’ll hear an A note. If the fluctuations in current flow are large and still at 440 per
second, you’ll hear an A that is LOUD!
But for purposes of biasing, it’s the amount of “plate current” flowing with no signal applied that’s important. Unfortunately
current is hard to measure because the circuit must be interrupted — as in “cut the wire” — and the meter spliced “in series”
with the broken circuit. But measuring VOLTAGE is easy. It is not necessary to interrupt the circuit because a voltage reading
can be taken in PARALLEL with the circuit intact.
Thus, as a matter of convenience, most bias settings are given in volts at the grid ... even though current through the plate is
the important factor. In fact plate current is so inconvenient (and dangerous) to measure that Fender doesn’t even state what
the correct value should be. They only give the grid voltage that will produce that current. (That’s the minus 52.) But that only
happens if the tubes being used are “in spec.”
As long as the tubes ARE “in spec”, the right bias voltage will always give the correct plate “CURRENT” — but then there’s no
need for the bias voltage to be adjustable!
If the tubes are NOT in spec, then the only proper way to re-set the bias is to cut the circuit and measure the current while
adjusting the bias ... but no manufacturer I know even STATES the desired current value! Be that as it may, when the original
bias voltage is altered far enough, it will compensate for the tube’s abnormal performance and the correct amount of idle current flow may then be restored. Clearly this is something most repair techs should not attempt.
Some newer amps have LED indicators connected to the circuit which will turn on when the right threshold of current flow
has been reached. This is an improvement, and almost worthy if you’re willing to accept resistors and lights added into your
amplifier’s audio path — which we aren’t.
The other “advantage” of this system is that it allows some amp manufacturers to avoid matching their power tubes. The
thinking is that adjusting the bias to each tube separately eradicates the inherent differences between the tubes by insuring that
the same current flows through each one.
Again, this has some merit .. but it’s still not as good as using tubes that are matched in the first place because compensating
for the mis-match causes the push-pull circuit itself to become unbalanced. Two wrongs don’t really make a right.
PAGE 30
BIAS ADJUSTMENT (Continued)
Some of the other recommended biasing, “methods” — such as -”.. tubes running red hot, increase the bias .. sounds harsh
and runs too cool, turn it down ...” are guesswork at best. Luckily, one of the great things about tube amps is that they can
usually stand some abuse without causing any real harm ... at least not immediately. But don’t these alterations imply that you
are second-guessing the amp designer and that there’s a better set of operating conditions that the designer missed but the
tube sellers have discovered?
Now some players may like the sound of their amp altered by tubes with extreme characteristics and with the bias set to help
compensate. But often it is the mere novelty of change that they’re really responding to and when the amp goes back to the
proper original way, we’ve seen them be far happier still!
Because every part in every one of our designs has been meticulously evaluated, compared and stressed over — no matter how
seemingly insignificant it might be. And with every design we look for a “sweet spot” where all the parameters — including the
bias — come together to give the best sonic performance, consistently and reliably. Every part and voltage is important — yet
no one complains that these other parameters aren’t available for tinkering.
Consider our patented Simul-Class circuitry where there are two different bias voltages used for separate pairs of power tubes
... and changing one voltage also changes the other. Great care goes into getting this just right and we think we’d be asking for
trouble to have it adjustable for the world to play with ... unless you like paying to have your amp messed up. Sorry, I meant
to say, “Uh, ... your amp needed biasing.”
If that doesn’t appeal to you, then merely plug a matched set of MESA tubes into one of our amps and you’re ready for tone.
Guaranteed. You’d be amazed at the number of service calls we field every day that lead to a diagnosis of out-of-tolerance,
non-spec tube problems. To think these would be prevented by including a bias adjustment is something of an insult to you
and us. If you put the wrong size tires on your car, do you think changing the pressure will make them right?
Please, don’t think this is a blanket indictment of the other guys selling tubes — it isn’t. And their tubes aren’t all bad either.
It just doesn’t make sense to pay more of your hard earned cash for tubes that were probably made in the same Russian or
Chinese factory and which have the possibility of being outside the performance window we select for your amp. And it pains
us to hear the hype and mystique built up around biasing when twenty-five years of evidence affirms our decision to make bias
circuits that “never need adjustment”. How much money and trouble that has saved MESA/Boogie players you couldn’t estimate.
Our rigorously tested and hand selected tubes are available at your nearest MESA/Boogie Pro Center or from us directly. Nobody
offers better price, quality or warranty than we do ... so why swerve?
Cheers!
Randall Smith
Designer & President
MESA/Boogie Ltd.
PAGE 31
ON TRIODES, PENTODES & IRISHMEN:
A Look Into The Inner Workings of the Vacuum Tube:
A Feature Article by Randall Smith
With apologies to Friends and Relatives from the Emerald Isle - who will make their appearance soon enough - the humor which
follows is dedicated to the memories of Spec McAuliff and Fae (Rafael) McNally, two of the True Greats.
As their numerical references suggest, the terms Diode, Triode and Pentode indicate the number of elements within the vacuum
tube i.e. two, three or five. All tubes also require a filament or heater which is not included in the count. Its purpose is to excite
electrons from the cathode coating by raising the temperature such that they are able to boil out of the electron-rich coating
material and form a cloud of free electrons in the vacuum space surrounding the cathode.
Although the term filament and heater are often used interchangeably, there are specific differences: A filament is a directly
heated cathode where cathode coating is applied directly to the heating element. Examples are 5U4 twin diode rectifier and
300B triode amplifier tubes. A heater, on the other hand, is a heating element which is separate from the cathode and is usually
inserted within the tubular cathode sleeve. Examples are 12AX7 twin triode amplifier and 6V6 or EL84 beam power pentode
tubes. In all cases this fundamental aspect of each tube’s construction is clearly visible, especially when the heating element
is glowing red hot.
The cathode, then, would be considered the first numbered element because it is the source of the electrons. The word itself is
from the Greek literally meaning completely down, which implies a sense of central origin - like the center of the earth where
Tone begins. It might be said that an ecstatic audiophile experiences a positive catharsis, his soul being purified when his system
transports him to Audio Nirvana. The only trouble with taking this positive imagery too far is that the cathode is, unfortunately,
negative... at least electrically speaking. However this is easily remembered since virtually all musicians and audiophiles have
also experienced the more common negative catharsis when they emerge from the emotional rebirth kicking and screaming
in rage and frustration.
Once heated, the intrinsically negative electrons are energetic little fellows of almost no mass. Thus they may be accelerated
almost instantaneously and will travel through a vacuum a nearly the speed of light. Being of like, negative charge, they tend to
repel one another and thus within the electron cloud surrounding the cathode, there is much jostling and elbowing as each one
tries to maintain his distance from all the others... unless there is a strong and universal attraction from an outside influence.
Visualize, if you will, a group of sub-atomic Irishmen milling about and in a repellent, negative state of mind. All are scowling
and none wants to have anything to do with the other. Now introduce a strong attraction say, a public bar, and you can easily
picture an orderly, if rapid movement of the lot in a single direction. This is what happens when a positively charged element
called the anode or plate is introduced into the vacuum.
The plate is the large metal element most prominently visible through the glass of an electron tube. It is the outermost element
of a tube’s structure and it surrounds all the others. The cathode is at the center radiating electrons outwards. As higher and
higher positive voltage is applied to the plate, the attraction for the electrons surrounding the cathode is increased and with nothing standing in the way, full uninhibited flow to the plate occurs...sort of like removing the doors and offering free drinks to the
crowd of surly Irishmen milling around outside. As electrons flow to the plate, the space charge will continually be replenished
by further ‘boiling’ of the hot, electron-rich cathode as you can easily imagine other Irishmen impatiently taking up the places
of those who’ve gone inside - until the entire village is deserted.
Now, where do they come from and how do they emerge? Well, a grand and elegant lady once showed me how to revive
flat champagne: She dropped a raisin into the glass. There was a dramatic and immediate increase in effervescence with the
introduction of a cathoding surface. Thousands of tiny bubbles suddenly appeared - and continued to flow from the raisin. Of
course the bubbles were made up of gas dissolved in the beverage, but the analogy makes it easy to visualize the loosely bound
electrons dissolved in the rich cathode coating as they effervesce from its heated surface.
PAGE 32
ON TRIODES, PENTODES & IRISHMEN: (Continued)
But back to the electron flow. If the electrons are strongly attracted to a positively charged plate, then it follows that they are
strongly repelled by a negatively charged plate and they are. Thus, if an alternating current - such as comes from a transformer
- is applied to the plate, electrons will flow only during the times when the plate is positively charged. During periods of negative
plate charge, electron flow is stopped and the space charge of electrons remains compressed in the area around the cathode.
Thus a diode tube - one with a cathode and an anode - is mostly used to rectify alternating current into direct current by passing it without restriction, but in one direction only. This also explains why closing time is strictly enforced at Irish pubs: During
normal operation, the traffic flow is similarly unimpeded and uni-directional toward the bar and this process rectifies the workday negativity. It goes without saying that no one leaves as long as the atmosphere around the bar remains positively charged.
TRIODES
This section is a continuing technical treatise on the workings of Irish Pubs but to make it easier for the layman
to understand, it is explained in terms of vacuum tube technology. Enter the original bar - free beer and no doors. Well, it turns
out that some control over the flow can be a necessary and useful advantage. This led to the invention of those swinging louvered saloon doors which are open at the top and bottom. They are patterned after the control grid of the vacuum tube, which
is a loosely wound coil of thin wire located between the cathode and the plate.
In a Triode the plate is always positively charged with high voltage D.C. and even though the grid is blocking the path, those
negative electrons can still FEEL the strong attraction - just as the Irishmen can see in through the louvers of the bar doors.
They know what pleasures lie beyond, but to get there requires overcoming the negative influences controlling the access. This
negative influence is typically called a Bias. In electronic terms that means the grid is supplied with a voltage which is slightly
MORE NEGATIVE than the already negative electrons. The more negative the Bias, the more it tends to neutralize the attraction
of the plate and repel the electrons back toward the cathode.
The Irish can be similarly charged with Bias, but unless you are Irish yourself, this type of Biasing may be more difficult to
understand. The effect is similar though: The more negative the Bias, the more it impedes forward progress. Generally speaking though, the electronic Bias of the grid is easier to overcome, and for two main reasons: First, the Bias is set - like the bar
doors - to allow some passage. Second, the grid is mostly NOT THERE, like the louvered doors which are mostly open spaces.
Unlike the plate which is solid, the grid is like a coiled bed spring. It can create a repelling field but mostly it’s empty space in
between widely separated windings of wire. It’s very easy to control the electrons as they pass through the grid’s force field:
Changing the grid voltage only slightly will have an enormous effect on how much current flows through...and that’s what
AMPLIFICATION is: a small change in voltage at the grid causing a large change in current flowing to the plate.
The purpose of the louvered bar doors is similar to that of the grid, namely, to give momentary pause while still revealing the
promise within. Hesitation mostly gives way to temptation, but there are those few stalwart Irishmen who think twice and decide to come back later. Most just pause slightly then go on through. That is the purpose of the bar doors: to prevent everyone
from crowding in all at once - and as the door is made less of a barrier, wider spaces between the louvers, more of the bar’s
attractive influence is felt outside thus amplifying the  customer flow and increasing the crowd at the bar.
PENTODES
Occasionally though, bar doors - even the louvered type - were found to be too effective, and too many customers turned away. Something further was needed to increase the attraction of the bar and overcome the resistance created
by the door. Thus the cocktail waitress was invented.
Once again the idea was inspired by the vacuum tube. It had been discovered in some tubes, often large power types, that the
distance to the plate was too great to attract enough electrons past the negative influence of the control grid. So another grid
coil of fine wire was inserted between the first grid and the plate. This was called the screen grid and carrying a highly positive
charge, it functioned as a “bait” for the plate.
In a properly designed power tube such as an EL84 or a 6V6, the windings of the screen grid are precisely aligned to fall in the
shadow of the control grid. This way the electrons responding to the pull of the screen grid are lined up in sheets as they pass
between windings of the inner control grid... only to find that they have been fooled! Once past the control grid and drawn toward
PAGE 33
ON TRIODES, PENTODES & IRISHMEN: (Continued)
the screen grid, they discover...there’s almost nothing there. The path they’re on has them aligned to zing straight through the
spaces BETWEEN screen grid windings. So rather than a close and personal encounter, they just fly on past - and once they’re
out that far, there’s no stopping them. The influence of the plate takes over and - being solid metal and of the highest positive
attraction - it is at this final destination that the electrons congregate.
Thus the proper cocktail waitress - visible through the louvers - is scantily clad so as to be all the more effective at reinforcing
the attractive influence of her bar and by being located in between the door and the bar, she serves as bait to lure customers
past the door’s negative influence. Once through the door however, it is the rare Irishman who actually comes in personal contact with the cocktail waitress as, for all intents and purposes, she - like the screen grid - turns out to be a vanishing illusion.
Yet, having come this far, the solid influence of the bar itself now takes over and attracts the customers to congregate, having
happily reached their destination.
If you’re still following this and haven’t lost track of the count, you’ll know we’re still one element short of the five needed to
make a Pentode. This last part is a pair of beam-confining shields which being negatively charged, serve to direct the flow right
toward the plate. This is much the way a short entrance hall to the bar prevents wandering accidentally into the Men’s room
on the way.
Once at the bar though, the circuit is complete and the process of soul-nourishing works its ritual magic. Biases having been
overcome, illusory nightingales having vanished, the spirits truly soar and the once surly Irishmen now are filled with warmth,
wit and kindred friendship, enjoying the music and glowing nicely with their heaters on.
With appreciative thanks to the inhabitants of the Land of the Leprechaun, we have now concluded our little diversion into the
mechanics of proper bar lay-out.
Randall Smith
Designer / President
MESA/Boogie Ltd.
PAGE 34
MINI RECTIFIER 25 - Front
MINI RECTIFIER 25 - Back
590794
POT #
GAIN
590739
POT #
POT #
590737
POT #
590738
MID
590736
POT #
TREBLE
590148
POT #
PAGE 35
CAUTION:
Unplug power
before replacing
fuse
AC RECEPTACLE
pt# 613714
1A
SLO
BLO
DOMESTIC FUSE
HOLDER pt# 790347
FUSE DOMESTIC
pt# 790101 1A
120 V~
60 Hz
1A
EXPORT FUSE
HOLDER pt# 790346
FUSE EXPORT
pt# 795205 .5A
TUBE GUIDE
5 x 12AX7
2 x EL 84
D Y N A - W AT T
RECTIFIER
M I N I -TW E N T Y-FI V E
POT #
592596
PRESENCE
590738
POT #
10 WATTS
25 W
10 W
25 WATTS
HANDBUILT IN PETALUMA, CA
WARNING!
SWITCH
pt# 607231
POT #
590739
MASTER
590792
POT #
SWITCH
pt# 607231
CAUTION: CHASSIS SURFACE MAY BE HOT!
ALL TUBE AMPLIFIER
CAUTION!
POT #
590148
BASS
590150
POT #
ALL FRONT PANEL KNOBS pt# 408300
PARTS CHART: MINI RECTIFIER Twenty-Five Head, Back
JACK
pt# 619112
MODERN
VINTAGE
SWITCH
pt# 607231
FT SW
PUSHED
CLEAN
SWITCH
pt# 607231
INPUT
JACK
pt# 619112
PARTS CHART: MINI RECTIFIER Twenty-Five Head, Front
NI
PATENTED
6,621,907
7,602,927
TWENTY~FIVE
MI
CH 2
CH 1
FT
SW
POWER
4 OHM
8 OHM
SWITCH
pt# 600631
ON
ON
SWITCH
pt# 600631
STANDBY
ALL JACKS
pt# 619357
RETURN
SEND
CH2 LED RED
pt# 394201
CH1 LED GRN
pt# 394232
3 POS SWITCH
pt# 607233
SPKRS
FX LOOP
Thank you for trusting MESA/Boogie to be YOUR amplifier company.
We wish you many years of toneful enjoyment from this handbuilt, all tube instrument.
.
.
.
1317 Ross Street Petaluma CA 94954 USA
Customer Service
707 778 6565
Fax 707 756 1503
. .
. .
14/04/24
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