MIG-L Tube Preamp for Leslie Organ Speakers

MIG-L Tube Preamp for Leslie Organ Speakers
MIG-L Tube Preamp for Leslie Organ Speakers
By Carsten Meyer & Tonewheel Tubeworks, cm@tonewheel.de
Want that sweet tube sound for your transistorized organ speaker? Install a real tube
amplifier in your solid-state Leslie!
The MIG-L Leslie Tube Overdrive works with a real miniature tube. The MIG-LR even
has a remote control input for switching between tube limiter and bypass mode.
But wait... don't those tubes require a letal high anode voltage and a big transformer for
heating? True for most, but not for mine. I found some miniature tubes that work perfectly
with the Leslie's 28V power supply, and they need only a few milliamps for heating.
Let the Russians do the work!
Remember russian pilot Victor Belenko who defected to Japan in his MIG-25 Foxbat fighter
back in 1976? U.S. military officials were stunned when they examined what they thought
was the most advanced fighter jet in the world. The Russians, it turned out, were still using
old-fashioned vacuum tubes instead of state-of-the-art transistors and computer chips. For
all their vaunted military reputation, the Soviets seemed incredibly backward. Eventually
though, it dawned on the Americans that the Soviets had figured out the old tubes would be
less vulnerable to the electro-magnetic pulse of a nuclear blast than some newer components.
The MIG-25 fighter was equipped with thousands of miniature vacuum tubes, and most of
them are still available, since the russians literally made millions of them for military stock.
Since they are military grade quality, chances are good that they outlive your Leslie.
What makes the "Tube Leslie Sound"?
The 122 is "the" Leslie to have. Equipped with big 6550 tubes, a mysthic glowing stabilizer
and lots of iron, it adds the characteristic growl and distortion to your organ. Since 122 and
147 Leslies are pretty expensive nowadays, Hammond players wonder if they could achive
the same sound from their solid-state Leslies (700, 800 and 900 series, in particular).
Before thinking about certain modifications of a transistorized Leslie, let's talk about some
issues that contribute to the Leslie's sound. First, the speakers, rotors and the cabinet do
most of the work. Also, the Jensen V-21 treble driver has a heavy rolloff from about 8 kHz
up, so it smoothens distortion coming from the amp. I also think that the narrow louvers in the
wooden cabinet contribute to the sound. In fact, the stage Leslies sound more bright than their
wooden counterparts. So, if you own a genuine 770, you're on the right way.
The Tube Deal
Without driving it into distortion, differences between tube and transistor amplifiers are
subtile - the audience won't hear it, if you don't tell. The 122 amp has some distortion in the
3% range when putting out around 10 Watts and an almost flat frequency response from 30
Hz to 10 kHz (there is a slight rolloff for frequencies from 8 kHz up). However, there are
differences in their output damping: Transistor amps do have very low internal impedances
in the Milliohms range, so they control speakers very exactly. Tube amps may have
internal impedances in the 2 Ohms range, especially when overdriven (loss of negative
feedback), so speakers connected to them tend to live their "own lifes". This emphasises the
speaker's/driver's characteristics. BTW: We are NOT talking about speaker impedances here!
Emulating the relatively high internal impedance of a tube amplifier is pretty easy: Just put a
1.8 Ohm/11 Watt resistor (for 8 Ohm drivers) in one of the Jensen's speaker leads and a
1 Ohm/11 Watt resistor in one of the woofer's leads. That marginally cuts off the amp's
power, but it makes the speaker act like connected to a tube amp. As the audiophools would
say: "You will definitely hear a difference", not only from the lowered power output (loss is
in the 5 Watt range). Give it a try, it won't cost you a fortune.
When it comes to an overdrive condition, differences are way more big. Solid state amps
just cut anything above their margins, so a sine wave becomes a sharp clipped trapezoid
shape. That sounds harsh and very unpleasant, because it adds lots of higher odd harmonics
(those kill your treble driver fast). Overdriven tube power amps also clip signal shapes, but
with a decent rounded behavior (see picture below), so higher odd harmonics are present at a
way much lower level. To achive the distorted tube Leslie sound, you have to add a limiter
that imitates the push-pull power stage exactly.
An overdriven Leslie 122 amp (this one equipped
with new Electro-Harmonix 6550) produces a
smooth round clipping above the amp's power
margins (lower curve). The upper curve (done by a
distortion analyzer) shows the distortion product.
As you can see, most of the "overdrive condition"
distortion is "k3" (three times the frequency of the
1 kHz test signal).
The Solution: MIG-L
I've made up a little gadget that "tubifies" your transistorized Leslie. It works well in solidstate Leslies with an active crossover board as seen in the picture below. First, by putting a
real miniature tube (russian 1SH18b) in the signal path that adds those (triode-like) pleasant
soundig even harmonics (k2, k4), second, by adding an overdrive stage that works with
integrated field effect transistors (FETs) instead of silicon diodes (as seen in many guitar
stomp boxes) to add the odd harmonics (k3, k5) produced by saturated 6550s. FETs act
much more like tubes than transistors, since they use the electrical "valve" principle.
The MIG-L resembles the sound of an overdriven
Leslie 122 amp perfectly. It produces a
smooth clipping (lower curve) which is a little more
rounded than the 122 amp's cut to emulate
somewhat "used" tubes. As you can see, even
the distortion product (upper curve) comes very
close to the original. You can even modify the MIGL to get a "new tubes" sound with a clipping
exactly as pictured above, if you prefer.
The MIG-L has to be inserted in the Leslies's signal path right behind the input gain pot. It
gets a +28V supply (green wire/plug) and ground (black wire/plug) from the active crossover
circuit board. Remove lower backplane of your Leslie and note position of plugs. Remove
plugs from power supply and speakers. Locate the active crossover board, it's the uppermost
board above the two power amps. Remove four screws from active crossover board. Desolder
capacitor C66 (next to Volume Control) and insert two wires instead (see picture below).
These lead to MIG-L input and output pins, respectively. The grey wire next to the Volume
Control pot leads to MIG-L input, the violet wire comes from MIG-L output pin and send
the signal to the crossover. The green wire is +30V supply, solder to connector marked with
green dot. Don't forget the ground pin (brown wire, barely visible) lead to Leslie ground
(connector marked with black dot).
This is a MIG-LR affixed to a Leslie 760 amplifier panel. To the left you can see the
Gain and Volume potentiometers. I have drilled them and glued a long M4 screw as an
axle into the hole for easier access. The MIG-L board measures 80 x 65 mm; screw holes
fit well above the crossover board so it can be mounted on top with two 6-32 UNC
1" screws and two 15 mm plastic spacers. Be careful not to short-circuit the MIG-L
power plane with the screw/bolt.
Wiring is very easy (only four wires), but some soldering necessary. C66 (marked red)
has to be removed before inserting and soldering of input/output wires into circuit
board holes. Be careful not to short-circuit the MIG-L power plane with the screw/bolt.
(Sorry for the b/w reproduction; see www.keyboardpartner.de/hammond/pics/MIG-Lcrossover.png for a color presentation.)
The proper adjustment is very important for a real tube Leslie sound. Clipping should solely
occur in the MIG-L, and by no way in the transistor amps. As the most solid state Leslies
come with a master Volume Control input potentiometer and two Bass/Treble Gain pots
(for horn and rotor amps), it is possible to play an overdriven Leslie at very decent volume
levels. When installed, lower leslie volume by decreasing the Bass/Treble Gain volume,
then increase Master Volume Control until the MIG-L overdrive gives you a nice growl
when organ volume pedal is floored. Then increase both bass/treble Gain volumes until
loudness is as desired. If you own an oszilloscope, you may check when clipping occurs in the
transistor amps and mark that setting as a maximum on the Gain pots.
As you can see in the picture above, I drilled the potentiometers and installed a long M4
screw as an axle to fit a 4mm knob on the outside. This makes adjustments very easy -- no
fiddling with a screwdriver!
The MIG-L and LR are intended for use in a genuine Leslie speaker with transistor
amplification; they need about 1 Vrms input for full distortion. This is true for Leslies
controlled by a Combo Preamp or an organ high-level output.
If you need more gain for some reason (organ volume too low), you may replace R2
(originally 56kOhm) on the MIG-L(R) board by 100kOhm. For even more volume, you may
swap R1 and R2 values, so R1 becomes 56kOhm and R2 becomes 330kOhm. If output
volume of MIG-L(R) is too high (true for non-Leslie or Leslie clone use), you may swap R9
and R10 values so the output voltage becomes attenuated before entering your amp.
“The” Sound
Organ players often want to reproduce the “Jimmy Smith” sound. This sound is a result of
playing techniques, typical drawbar registrations and an decent overdrive that just kicks the
percussion to distortion. Do not forget the mikrophone and studio issues; a miked leslie will
sound very different dependant on the mikrophone position. Some people even forget that the
typical “growl” will need certain left-hand and bass pedal work.
Try a 88820000 setting on right, percussion 2nd short, soft, vibrato off and raise the volume
until the MIG-L begins to distort on the percussion decay. There should not be much
distortion on decayed notes to achive the JS sound. Try 88882000 setting on right, percussion
3nd long, soft, vibrato C3 or V2, leslie slow. Distortion should occur on the percussion.
Distortion and growl may be emphasised by holding a left-hand chord and a bass note on
Calamity Clones
If you have a Leslie clone, it will need some work to achive the smooth sound of a Jensen V21 driver with it's phenolic diaphragm and characteristic frequency response. Leslie clones
often use high-range drivers (tweeters) and a 1500 Hz crossover network, while the Jensen
works from about 700 Hz up (official crossover point at 800 Hz). Typical tweeter drivers used
by brands like Elka, Solton, Allsound, Echolette, Dynacord sound shrill and aggressive
compared to the V-21, so think about replacing the crossover network and the tweeter by a
mid-range PA horn driver like the Monacor KU-516 when you own a non-Leslie Leslie. For
two of my Leslie clones, I used old military surplus midrange PA drivers from a command
truck speaker. They are loud as hell and need hefty attenuation to match up with the woofer,
but they come very close to the V-21's frequency response.Non-Leslie Use
In the MIG-LRL version for Leslie clones the amplification yields full distortion at 100
mVrms input, so it can be used with line-input amplifiers often used on Leslie clones. You
will need a +24 to +35V @ 50mA supply for the MIG-LRL. This voltage may be derived
from the positive power amp supply rail. If power amp voltage exceeds 35V on idle, you will
need a power resistor of 100…270 Ohms/4 Watts in the supply wire to achive a +30V supply
for the MIG-LR (recommended voltage). The MIG-LR uses about 50mA, mostly for the tube
heater, so you need about 270 Ohms for a +45V supply (Ohm’s law).
Remote switching (MIG-LR only)
The MIG-LR version has a remote control input for switching between tube limiter and
bypass mode (pin in upper right corner, see picture). Normally, a jumper is installed to switch
on the overdrive effect. If you pull the jumper, you are in bypass mode (effect off). You have
to connect the remote input pin PL4 to ground now (via a footswitch, for example) when
you want the overdrive to be on. To use this feature, pull the jumper installed, and connect a
latching footswitch (like Boss FS-5L) to the remote pin (plug tip to pin, plug shield to
ground). The pedal toggles between distortion and bypass mode.
Changes to MIG-LRL version with 100 mVrms input/outpul volume setting:
R1 = 1k94
R2 = 220k
R9 = 47k
R10 = 3k9
Changes to MIG-L version without bypass remote input:
U3 omitted, replaced by jumper wire from pin 1 to 2. R11…R15 omitted.
To achive a smoother “worn tubes” overdrive effect, remove Zener ZD1 5.1V and replace ba
a 8.2V type. This modification gives a higher output level, so also change R9 to 27kOhms.
Some parts are selected to give the intended overdrive effect, especially the CMOS overdrive
stage. Using other manufacturer ICs will give poor results.
For support and complete modifying instructions, visit our webpage
Under rough use (vibrations due to transport or high volume levels) you should secure
the tube with a thick bead of silicone sealant to the MIG-L circuit board. This also
minimizes microphonic effects.
Board layout of MIG-L printed circuit board
Schematic Diagram of MIG-LR
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