ALPHA 8406
6 Meter
© 2010 RF Concepts, Inc
634 S Sunset St
Longmont, Colorado 80501
Phone 303 4739232
All rights reserved
Specifications subject to change without notice - Rev. 1.2
December, 2010
Quick Start Information
Station Engineering Considerations
Unpacking and Preparation
Theory of Operation
Operations and Maintenance
Trouble Shooting Hints
Specifications 8406 6 Meter Amplifier
Input drive Level
Power Output
SWR Tolerance
Duty Cycle
Inter Modulation Level
Mode of Operation
Input AC Voltage
AC Current
Input Impedance
Output Impedance
RF Connectors
T/R Relay
RF Bypass Level
50-54 MHz
50 Watts Nominal
1500 watts
100 %
34 dB minimum, Two Exciter
-72 dBc
100-240 Selectable
<13 5mps @240 VAC @1500 watts
50 Ohms
50 Ohms
“N” Female
Forced Air
17.3” W x 7”H x 21.0”D Including Fan Space
70 lbs., 31.8 kg
Vacuum QSK
Bargraph LED
Against all common faults
Less than 200 watts
This amplifier meets or exceeds all technical specifications listed above.
1 Introduction
1.1 General Description
Congratulations on your purchase of a professional quality Alpha 8406 amplifier! With
proper installation and care, you can expect to enjoy your Amateur Radio hobby with this
amplifier improving your signal for many years to come. Please study this manual
carefully before operating your amplifier for the first time.
The Alpha 8406 is a self-contained manual tune VHF linear power amplifier capable of
continuous operation at 1500 W peak power output on SSB, keyed CW, SSTV, RTTY,
digital modes or FM, with no time limit.
See our web site at or call 303-473-9232 for further information.
1.2 Amplifier Capabilities
It is extremely important to thoroughly review the Installation and
Operation sections of this manual before attempting to us the Alpha
8406. Failure to do so could result in serious damage that may not
be covered under warranty.
Continuous RF Output. The 8406 is capable of 1.5 kW continuous RF output on
all commonly used modes and on any authorized amateur frequency from 50-54
Compatibility with popular amateur transceivers and exciters. The 8406 requires
approximately 50-65 W peak RF drive for 1.5 kW output.
Capable of full CW break-in, QSK and all digital modes when used with any
appropriate transceiver.
Protective functions are built in. The control system incorporates protective
functions that minimize the probability of accidental damage to the amplifier or
its power tubes. In most cases, when one of the protective functions is “tripped,”
the amplifier will go to Standby.
1.3 Shipping Information
The Alpha 8406 amplifier ships in two heavy-duty double cardboard cartons. One carton
holds the power transformer and weighs 43 lb (20 kg) and the second carton contains the
amplifier and weighs 50 lb.(17 kg).
Alpha recommends that you retain the cartons after installation in the unlikely situation
that you need to ship the unit later. Contact RF Concepts at 303-473-9232 for shipping
advice and assistance.
1.4 Safety Information – Installation and Operation
Make sure the Alpha 8406 is located where there is good air circulation all around
and on top of the cabinet. The unit may become hot during operation.
The Alpha 8406 weighs approximately 70 pounds when the transformer is
installed. Use proper lifting techniques and two people when moving the
amplifier. Note that amplifier should NEVER be transported with the transformer
in place.
The Alpha 8406 is designed to meet international safety standards and FCC
regulations. However, one should always remember that the equipment works
with high voltages that can be LETHAL!
This operating manual holds information, cautions and warnings that must be
followed to ensure safe installation and operation. Read Chapter 1 before
attempting to unpack or operate the Alpha 8406 amplifier.
Warnings: What Not to Do
Never open the amplifier case without unplugging the unit from
the wall outlet.
Always resist the temptation to immediately hit the ON button after
the amplifier faults to power off.
Never touch an antenna during transmission.
Never attempt to turn on the amplifier without the cover
securely in place (all attachment screws reinserted).
Never turn the amplifier back on after a hard fault without
waiting at least 20 seconds.
Never allow liquids to enter the amplifier through the cover holes.
Never stick objects into holes in the case.
Warnings posted in this manual should be read and thoroughly
understood by users. Failure to perform procedures properly
may result in amplifier damage, fire hazard, or electric shock.
1.5 Owner Assistance
Technical Assistance from RF Concepts is available from several sources.
The RF Concepts web site is Click on Support and
follow the instructions. Many typical problems and their solutions are listed
on this site. On this site you can get the following assistance:
Technical Support
Repair Information
Software Downloads
Tech Tips
Legacy Equipment Information
Alpha Forum
You can email us for customer support by filling in the support request at or you can send your request by fax to 303-473-9660, or
call us at 303-473-9232
2 Quick Start Information
2.1 Overview
This section explains in brief the items you need to consider when setting up your
Alpha 8406 amplifier.
If you already have a well-designed shack and have used an amplifier before, please
review the items below to make sure you have considered all the critical items for
proper installation and operation.
If you are using an amplifier for the first time, please skip this section and go to sections
3, 4, and 5 for a more detailed explanation of how to set up your shack for maximum
safety and operating enjoyment.
If you have installation questions, do not hesitate to contact Customer Support. We
much prefer to address questions prior to power up.
2.2 Station Engineering Considerations - Checklist
Make sure you have properly addressed the following concerns (Section 2.3 below)
before installation of your Alpha 8406 amplifier. If you are unsure of any of these
items, please read the noted sections carefully.
2.3 Preparation
__ 240V AC Power in shack? (Section 3.2)
__ Amplifier placed with proper airflow? (Section 3.3)
__ Antenna ready for 1,500W? (Section 3.4)
__ Adequate RF cabling? (Section 3.5)
__ RF Exposure Safety (Section 3.6)
2.4 Unpacking
__ Unit Checked for Damage?
__ Transformer Installed? (Section 4.2.1, 4.2.2)
__ Power Cord Connector Attached? (Section 4.2.3)
__ AC Primary Voltage Set? (Section 4.3.6)
___Amplifier Grounded Properly? (Section 4.4.1)
__ Amplifier Cover Replaced and Secured? (Section 4.3.6)
2.5 Operation
__ All Exciter Interconnections Set? (Section 6.2)
__ Exciter Drive Correctly Set? (Section 6.1.1)
__ Amplifier Tuned to Antenna System? (Section 6.3.6)
3 Station Engineering Considerations
3.1 Overview
The Alpha 8406 is capable of dramatically improving the performance of your amateur
station. It is important that you observe good engineering practices to achieve all the
benefits of such a station in a safe and reliable manner. This section provides a few
hints for important operational considerations, but it is recommended that the user also
consult a good source of general information such as “The Radio Amateur’s
Handbook” by the ARRL, especially if this is the first high-power amplifier you have
3.2 AC Power Source
This amplifier runs best when powered by a 200V - 240V AC circuit. If you do not have
a 240V AC outlet in your shack, you will need to get a licensed electrical contractor to
install one. A minimum of a 20 amp capacity is required. A 20 amp breaker on your
240V circuit is sufficient. There are many styles of plugs, some of which are countryspecific. For this reason, the amplifier is not shipped with a power plug. Select a
location for the outlet as close as possible to where you expect to operate the 8406. If
you are not sure, or contemplate moving the amplifier, you may choose to get a second
outlet installed at the same time. Ask your contractor for two or three matching plugs
during installation as there are several styles of connector available. Ask the contractor
to measure the voltage and record it, so you can set the line voltage tap on the 8406
appropriately. If possible, have the contractor measure the line voltage with a 10 amp
current draw, and use this value for setting the transformer tap. The Alpha 8406 can run
when connected to a 120V AC outlet. However, you WILL NOT achieve full legal limit
output in this case. If the amplifier is connected to a 120V AC outlet, you should not
expect more than 1000 W output.
3.3 Air Flow
It is critical that airflow around the Alpha 8406 remain unimpeded at all times.
Keep the top of the amplifier clear of any restrictions. If you are mounting the
amplifier in a console, make sure that the exhaust air is properly and fully
removed from the console.
Figure 3.3 -Minimum clearance for proper airflow
Keep top tube exhaust clear
Keep rear intake clear
Poorly designed consoles can result in outlet air being drawn back into the amplifier air
intake and recirculated, thus getting hotter and hotter and resulting in degraded amplifier
performance or even failure. If you are designing your own console, consider putting in
additional fans and/or ducting to deal with waste heat. Try to minimize the possibility of
dust or other contamination getting drawn into or falling on the amplifier. It is also
advisable to periodically (at least annually) clean the dust out of your amplifier for
continued flawless operation. RF Concepts recommends the use of compressed air for
dust removal.
Too ensure adequate
cooling, make sure the top
and rear of your amplifier
have at least 3” of clearance
to allow unobstructed
Make sure the exhausted air
cannot be recirculated back
into the amplifier air intake.
3.4 Antennas
Many antennas that are suitable for general use are unsuited for operation with a full
1500 W of power. At this power level in a 50-ohm circuit, the RMS current is 5.5 amps
and the peak RF voltage is 387 volts. With a 2:1 SWR, these values double to 11 amps
and 775 volts. The actual voltage and current at various points in or on your antenna may
actually be many times these values. On a simple dipole with sharp wire ends, corona
(localized ionization) can easily occur. Corona can (and has!) led to fire in nearby
objects. Traps in beams and verticals can heat up significantly during high power
operation. Instances of melting or flashover of traps have occurred in many installations
where insufficient thought has been given to their ratings. If an antenna has been
deployed for a long period of time, it may be worth taking it down for inspection prior to
full power operation.
3.5 Coax and Connectors
The importance of a well-constructed feed-line system cannot be overstated. After all, the
purpose of the amplifier is to provide approximately 2 S units (12+ dB) of improvement
in your radiated signal. All too often, installation problems are encountered where cheap,
poor or under-rated coax and connectors are used. These often are responsible for at least
one S unit of degradation. (This means you could have bought a 375 W amplifier and
achieved the same radiated signal by buying good quality feed-line components!) Use
the lowest loss 50-ohm coaxial cable you can obtain. Use new, clean connectors installed
according to the manufacturer’s recommendation. Clean the connectors after soldering
them, and before mating them with the amplifier. Make sure any excess solder is
removed from the connector; likewise remove any fragments of braid etc. Never use old
coax, which may have had moisture penetrate under the jacket. Support the coax
frequently using non-compressive clips so that it does not hang or stretch under its own
weight. Avoid sharp or kinked bends (most manufacturers will specify a minimum bend
radius for their product). Make sure the connection from feed-line to antenna is
waterproof. Provide for disconnection of the feed-line when it is not in use; this protects
against damage caused by power surges and lightning strikes, which are not covered
under the amplifier warranty.
3.6 RF Safety
The FCC requires users to check their installations for compliance with published values
for allowable exposure to RF fields. This information is available in ARRL publications,
FCC printed rules, and on the web. RF Concepts strongly recommends that this be done
for any installation, both fixed and at an expedition or contest site.
If you have any questions regarding engineering your 8406 into your amateur radio
station, please visit our online technical support website at:
The below calculations are taken with the RF Exposure calculator located at Three representative calculations are presented
below. Two safe distances are represented. The first is a controlled area. A controlled
area, is where a person knows the radiation levels and exactly how long they can be in the
area before having to leave the area for a specific period of time.
The second is an uncontrolled area. An uncontrolled area is generally where pedestrians
may not know they are in an exposure area and as a result this area the radiation levels
must be below a point where a person may remain
¼ Wave Vertical Antenna
Interpretation of Results
1. The power value entered into these calculations should be the average power
seen at the antenna and not Peak Envelope Power (PEP). You should also
consider feedline loss in calculating your average power at the antenna.
2. If you wish to estimate the power density at a point below the main lobe of a
directional antenna, and if the antenna's vertical pattern is known, recalculate
using the antenna's gain in the relevant direction.
3. Please also consult FCC OET Bulletin 65 Supplement B, the Amateur Radio
supplement to FCC OET Bulletin 65. It contains a thorough discussion of the RF
Safety regulations as they apply to amateur stations and contains numerous charts,
tables, worksheets and other data to help determine station compliance.
na The above calculation is for a vertical ground plane antenna at 20 feet elevation feed
150 with a short piece of coax with the station located just below the antenna. Minimum
0 safe distance is represented in Red. The antenna is located at ten feet above ground.
na 6 Element Beam at 100 Feet
dBi The above calculation represents a 1400 watt RF signal feed into a 6 element yagi
Dis located 100 feet above the ground. As a result the down ward lobe 100 feet from the
tan base of the tower is at a 45 degree angle. The RF measured is out of the main lobe
ce and as a result the antenna has a negative 10 dB of gain over the normal 12 dBi gain
to if the person was standing in the main pattern lobe.
3 Element Beam at 35 feet
Average Power at the Antenna
1400 watts
Antenna Gain in dBi
5 dBi
Distance to the Area of Interest
100 feet
30.48 metres
Frequency of Operation
50 MHz
Are Ground Reflections Calculated?
Estimated RF Power Density
0.0613 mW/cm2
Controlled Uncontrolled
Environment Environment
Maximum Permissible
Exposure (MPE)
Distance to Compliance 24.7994 feet
From Centre of Antenna 7.5589 meters
Does the Area of Interest
Appear to be in
55.3913 feet
The above calculation represents a four element beam at 35 feet above the ground feed
with 1400 watts of power. At 100 feet most of the pattern gain is still over the person
standing at 100 feet.
4 Unpacking & Preparation
4.1 Unpacking
Inspect both boxes for physical damage. Save all packing material for possible future
use. Contact 303-473-9232 if shipping damage is found. Carefully unpack the amplifier
and transformer.
Carefully remove the amplifier and place it on a workbench or table where you can install
the power transformer.
4.2 Installing the Power Transformer
It is recommended that the power transformer be installed when the amp is at or near the
place it is to be used. The chassis of the 8406 is designed for the mechanical loads it
experiences when the amplifier is on a flat surface with the tilt-bail up or down. If the
amplifier is tilted too far, such that the transformer is cantilevered or “hanging out” to any
degree, the chassis of the amplifier can distort. If the amplifier is moved, even if only
from one site to another locally, remove the transformer to avoid the possibility of
Do not operate amplifier without the cover in place and all cover screws
installed. Do not operate the amplifier without a good ground connection
on the rear panel ground terminal.
Make sure all RF cables have a solid ground shield in place.
The transformer is very heavy and must be moved with due caution using
only the lifting handle.
PROCEED SLOWLY to avoid bumping and damaging adjacent wires,
connectors or components.
While the top cover is removed, make the tube is firmly seated in its socket, rubber
exhaust chimney is fully and correctly installed, and anode connector is tightly clamped
to the tube. The silicone rubber chimney installed on the 4CX1500B tube is a critical part
of the cooling system. Make sure the chimney is straight and fully installed so the
bottom of the chimney is firmly against the tube deck and completely covers the airflow
openings in the deck. Make sure that the silicone rubber chimney is flush with the top
cover when it is placed back on the amplifier. Tube cooling exhaust must exit only
through the tube anode fins; it must not be allowed to escape outside them. Failure to
ensure proper cooling airflow may result in tube damage or destruction, which is not
covered under warranty.
4.2.2 Connecting the Transformer Power Plugs
Connect the 9-pin white Molex connector to the matching plug mounted on the
back wall of the amplifier
Connect the 8-pin orange connector to the matching pins on the upper (Mains)
Connect the 6-pin yellow connector to the matching pins on the lower (HV) PCB
Check to ensure that all connector pins on these three connectors engage fully and
4.2.3 Power Cord Connections
To avoid the hazard of a potentially fatal electric shock and/or
severe damage to the ALPHA 8406 and other equipment, always use
an AC plug that is appropriate for the primary mains voltage, current
rating and configuration.
NEVER use 120V-type plugs and power receptacles for 190-250V
ALWAYS use grounding type AC connectors which conform to local codes and
ensure that the green wire in the Alpha 8406 power cable is wired only to the AC
mains safety ground (or to neutral, as may be necessary with a 240V circuit
configured 120V-N-120V without a separate ground, commonly found in the US).
The green conductor in the power cord is wired to the ALPHA 8406 chassis. It MUST be
connected only to the power source safety ground or neutral. The black and white power
cord wires connect to the two “hot” wires of the AC source; either wire may be connected
to either side of the line. For best results use a dedicated 200-240 V branch circuit of #10
AWG copper wire or equivalent, rated at 20 A, to feed the amplifier.
4.2.4 Important Information about Operation from 90-130V AC
Electrical power equipment will draw twice as much primary current from 120 V
mains as from 240 V mains. Therefore, operating the ALPHA 8406 on a typical 120
V/20 Amp household circuit without exceeding the 20 A circuit rating will limit
maximum peak power output to about 600-1000 W. Maximum possible RF output
power for any particular primary AC voltage and current capacity may be estimated as:
P max = (V x I ) / 2.3
For example, if the Alpha 8406 operates from a circuit that is capable of delivering 120 V
AC at a maximum current of 20A, with no other loads connected to the circuit, maximum
peak RF output possible without tripping the 20A breaker (or fuse) is approximately:
Po max = (120V x 20A) / 2.3 = 2400/2.3 = 1043 W
If the same circuit also supplies a transceiver drawing peak line current of 5A and a lamp
drawing 1A, only 20-5-1 = 14A is available for the amplifier and maximum possible
output is about:
Po max = (120V x 14A) /2.3 = 1680/2.3 = 730W
Below are some considerations at the high end and the low end of this voltage range that
are rarely encountered.
Low Voltage vs. Power Output At the low end of the voltage range, do not expect to be
able to get 1,500 watts output if your line voltage is below 120 volts. If your line voltage
is between 110 and 130 volts, then 1,500 watts PEP operation (CW or SSB) may be
possible if your AC line service has sufficient current capacity (30 amp circuit
recommended). However, 1,500 watts continuous should not be expected. If your line
voltage is between 90-120 volts, then power outputs above 1,000 watts should not be
expected from the amplifier. Tune (adjust) the amplifier for no more than 1,000 watts
output, and simultaneously for maximum efficiency.
Place where normal air flow is not restricted because the cord could overheat. If other
equipment is drawing current from the same circuit as the Alpha 8406, then the
considerations in section 4.2.3 should be taken into account.
High Voltage and Tube Life At the high end (sometimes encountered when using poorly
regulated generators) the plate voltage and tube heater voltage may be too high. If
voltages above 250 volts are applied for any length of time, the lifetime of the tubes may
be reduced. If this is your situation, the first line of defense is to contact your utility
company and ask if they can reduce your line voltage. If this is not possible, you may
want to consider placing your own step-down transformer in line between the AC outlet
and the amplifier. If this is necessary, a transformer with at least 4-kVA rating is
required, due to the nature of the current waveform in the primary. Another choice for
voltage control, a ferro-resonant voltage regulator, is an expensive solution, but is a good
way to stabilize primary voltage.
Note: If you intend to operate the amplifier on any of the 90 - 130V settings, the two
lower 2 amp fuses on the rear panel will have to be changed to 5 Amp. to allow for the
increased in-rush current.
RF Input
FIGURE 4-2-1 Amplifier Interior
4CX1500B Tube
Output Tank Circuit
Output RF Connector
Cooling Fan
Low Pass Filter
Input RF Connector
HV Safety Switch
AC Transformer Molex
Air Inlet
Input AC safety Switch
Transformer Relay and
AC tap PCB
RF Out
HV AC Transformer
AC Fuses
AC Cord
FIGURE 4-2-2 Amplifier Rear
USB Input
Input AC tap settings
can be made by
moving the flying lead
to the marked AC
spade posts marked in
the appropriate
voltages. The proper
taps must be made
before the AC power is
applied. When making
changes make sure the
amplifier is unplugged.
Flying lead attachment points.
Figure 4-3 AC Power Setting
Never use a tap other
than the proper voltage
or damage to the tubes
and amplifier will result.
With the top cover removed, the primary voltage taps are located on the top of
the Mains Board, which is between the transformer and the front panel. There is
a row of 5 “fast on” connectors (J1 through J5) and a “flying” jumper connector
which mates with them. See Figure 4-3 above. There are 5 “nominal” primary
voltages, which cover all the line voltages normally encountered around the
world. Selecting the appropriate tap for your situation will optimize amplifier
performance, safety and lifetime. The nominal mid-range voltage for each tap is
printed on the Mains Board circuit board. These voltages are 100,120, 200, 220
and 240 Volts. The acceptable line voltage for each tap is the center voltage plus
or minus 10 Volts. One of these taps is suitable for any of the “nominal” line
voltages encountered worldwide.
When using any high power amplifier, failure to connect ALL station
equipment to a good common ground may allow RF feedback to leak
into the transceiver and cause severe signal distortion.
4.4.2 Replacing the Amplifier Cover
Replace all attachment screws. Use only the 6-32 screws supplied with the amplifier
and do not tighten any of the screws until all are started. Do not attempt to operate the
amplifier with the cover removed or only placed back on the unit without the
attachment screws. This WILL cause damage to the Alpha 8406 and may also lead to
injury or death to the operator.
4.5 Amplifier/Station Interconnections
Once the power transformer is installed, properly configured, and the cover replaced,
place the amplifier in its operating position. The amplifier with the transformer installed
is heavy so you may need assistance to safely move it. Make sure it is placed on a
stable surface and that there is sufficient space to the rear, sides, and top to allow good
air flow and safe placement of cables.
4.5.1 Coaxial Cable Types & Connectors
Connect the transceiver RF output to the ALPHA 8406 RF INPUT connector with 50ohm coaxial cable- RG-58C/U or equivalent. Coaxial cable from the 8406 RF OUTPUT
connector to the antenna should be RG-8A/U, RG-213/U, or LMR-400. ALC cable is not
interconnected when used with a modern transceiver.
4.5.2 T/R Control Cable
The Alpha 8406 has a full break-in vacuum relay QSK system requiring only the normal
interconnection when used with a modern QSK transceiver. The Alpha 8406 requires a
contact closure (short circuit) on transmit from its RELAY jack center pin to chassis.
This function is supplied by the transceiver, usually from a dedicated relay that is
normally open in receive and closed in transmit. Shielded wire should be used for the T/R
control cable. The Alpha 8406 end must be fitted with a common phono (RCA-type) plug
and the other end with a connector suitable for the transceiver. The T/R relay contact
must close before application of RF drive. Modern transceivers have the proper time
delay between key up and the start of the transmitted signal to allow the Alpha 8406 to
follow the CW keying. If a T/R timing problem is suspected, connect the CW keyer to
the RELAY jack on the Alpha 8406, and connect a cable from KEY OUT on the
amplifier to the keying input of the transmitter.
4.5.3 ALC
The Alpha 8406 does not generate or use ALC voltages to control an exciter.
For proper operation the exciter transceiver should have its power output set so as not to
over drive the amplifier input. Today’s modern transceivers have very good power output
limiting. As a rule ALC is not needed if the transceiver is less than 15 years old.
Should your transceiver be older then the power output should be reduced so voice peaks
will not overdrive the transmitter under any modulation condition.
Theory of Operation
5.1 Theory of Operation - Overview
The Alpha 8406 uses a tetrode vacuum power grid tube (4CX1500B) as the amplifying
devices. The main power supply is an unregulated transformer/rectifier/capacitor power
supply for the high voltage (HV) and heater circuits. All other power supplies are
regulated. The control circuit uses a microprocessor “in the loop” to monitor and control
amplifier operation. There are 7 circuit cards in the amplifier. In addition to these, the
tube, tank circuit assembly, and transformer complete the main sections of the amplifier.
These major blocks are described below. The 8406 amplifier includes a 5 V power
supply that is mounted behind the front panel. Any time the amplifier is plugged into the
mains power, this 5 V supply is active and there is power to the micro-controller on the
main control board. This feature allows the amplifier to be turned on or off remotely as
well as remote monitoring and debugging though a USB cable connected to a computer.
5.2 Tubes
The amplifier is designed to use the 4CX1500B tetrode tube. The amplifier design uses
this tube well within ratings. The tube is operated in Class AB1, with a plate voltage of
3,400V (nominal, full output, key down), a grid 1 voltage of -50 to -60 volts, and a grid 2
voltage of +230 volts. The tube has an inductance resistor in series with the cathode. This
resistor stabilizes the tube bias and provides negative feedback, which improves linearity
(and hence IMD performance). Electronic bias switching (EBS) increases the negative
grid 1 voltage in pauses in speech or between Morse code elements. This reduces the
standing bias on the tube, resulting in less waste heat, longer tube life and higher overall
amplifier efficiency. The artifacts of EBS are not noticeable under normal
communications conditions.
5.3 Output Tank Circuit
The output tank circuit of the Alpha 8406 is designed to provide reliable high efficiency,
low distortion performance in a very compact volume. The basic topology is “pi-C”,
which provides harmonic attenuation adequate to meet the requirements of all countries
globally that permits power outputs of 1,500 watts. The amplifier is designed to operate
with only a single tube in order to reduce tube capacitance.
5.4 Tube Deck
The tube deck is a mechanical assembly built around the tube deck PCB. The tube deck
PCB has the tube socket mounted on it, as well as those critical circuit elements that need
to be in close proximity to the tubes. The tube socket contains integral screen grid (grid
2) RF bypass capacitors. Also on this PCB is the input bypass relay. This relay is under
microprocessor control and in one position switches the input RF to the tube and in the
other it switches the input RF to the tube matching circuit. The tube is operated as a
“swamped grid” tetrode design. The tube grid is tied at RF to a 50-ohm swamping
resistor, which absorbs most of the input drive power. The RF voltage across this resistor
is added to the grid 1 DC bias to provide the net low-impedance tube grid 1 bias. The RF
impedance represented by grid 1 and its capacitance is compensated for by a series
inductance to provide less than 2:1 SWR on each band at the amplifier’s input.
5.5 Mains Board
The power supply functions are split between the mains board and the high voltage
(HV) board. The mains board mostly deals with the primary side of the transformer. The
various taps for the transformer primary are routed through this board and so is the AC
line input. Relays on the mains board connect the AC line to the appropriate taps on the
primary. One of 5 tap options is selected by using a 5-way jumper field. See section 4.3
for more details on how to set the jumper. Also on the mains board is a step-start circuit.
This circuit consists of a relay and a resistor, which are time-sequenced to limit the
inrush current into the amplifier when it is first turned on. When initially turned on, the
tap relays operate from a voltage derived from resistors from the AC line. They hold
via contacts on the trip relay on the HV board. The regulated minus 12 volt and minus
124 volt supplies are also located on this board. Many of the important voltages for the
amplifier are brought to test points on this board.
5.5 High Voltage Board
The main high voltage for the amplifier is created on this board using a full-wave bridge
rectifier and a bank of capacitors. This power supply has two 10-ohm resistors, one in
the positive (B+) lead, and the other in the negative return, which goes to ground. The
combination of these two resistors limits the surge current in the case of a B+ arc. The
voltage across the resistor in the negative return is used to monitor tube plate current in
the control board. This voltage is also used to generate the “hard fault” condition. When
the power supply current exceeds about 2 to 2.5 Amps, a relay operates to open the coil
circuit of the mains tap relays on the mains board. When these relays release, the
amplifier goes to the power-off state. This hard fault circuit operates independently of
microprocessor control. The regulated screen supply is also located on this board. All
power supply filter capacitors on this board have bleeder resistors which will discharge
the capacitors in less than 60 seconds. If it is necessary to work on this board, it is
nevertheless recommended that the discharged condition be confirmed with a voltmeter,
due to the remote possibility of bleeder resistor failure.
5.7 Control Board
The control board is the heart of the amplifier. It is based around a PIC microcontroller.
This microcontroller has a built-in multi-channel analog-to-digital converter, which is
used to monitor all the critical voltages and currents in the amplifier, as well as the input
power and output forward and reflected power. It uses these converted values to control
the amplifier’s operation and to drive the display board on the front panel. A USB port
is provided for remote monitoring and is found on the back of the Alpha 8406. The
USB driver for the amplifier is provided on the CD shipped with the unit or can be
found on the Alpha website at
5.8 Display Board
The display board uses a MAX7219 LED driver chip. It receives data from the
controller via an SPI interface. It contains a regulator to drop the +12 V to +5 V for the
5.9 T/R Board
The T/R board contains the input and output relays, as well as the input power detection
and output directional wattmeter. The voltages from the detector are connected to the
control board. There is a trimmer capacitor on this board that is adjusted with the
amplifier operating into a good 50-ohm dummy load. The capacitor is rotated to
minimize the reflected power voltage. The board also has a static protection inductor on
the RF output for 50 -54 MHz operation. This is located between the PCB and the flange
of the output connector.
5.10 Center Partition Board
This contains the RF decoupling circuit on the B+ line as well as the “crowbar” safety
circuit. This safety device consists of a piece of spring metal, which shorts out the B+ line
when the top cover of the amplifier is removed.
Do not defeat this safety circuit. It is placed there for your
6 Operation & Maintenance
The ALPHA 8406 is extremely easy to operate, but failure to carry
out each procedure exactly as described in this manual is likely
to lead to amplifier damage, which is not covered under warranty.
Damage to other station equipment may also result.
6.1 Before Operating Your Alpha 8406
6.1.1 Setting Input Drive
You must set the transceiver output power properly. Virtually all damage to has resulted
directly from severe overdrive. The ALPHA 8406 requires about 50 W drive for full
rated output. Damage caused by applying several times rated drive power to the
ALPHA 8406 will not be covered under warranty. Fortunately, most modern
transceivers maintain quite consistent output from band-to-band and mode-to-mode when
set up properly.
CAUTION: Setting only the transceiver POWER or RF PWR control IS NOT
Several popular transceivers can generate RF spikes of 200-300 W. Make
sure your transceiver does not produce over wattage spikes. Some
transceivers are capable of 200 watts out. Male sure the transceiver power
output is adjusted correctly.
6.1.2 Dealing with Faults
The ALPHA 8406 “faults” into STBY or OFF when unsafe operating conditions
occur. This is shown when the amplifier changes from the OPR LED being ON to the
Fault and STBY LED’s If the 8406 encounters unsafe operating conditions, it enters a
protective “Fault” mode. When this happens, the tubes are biased off and the relays are
placed in the bypass mode, so that RF from the radio goes directly to the antenna. The
FAULT LED on the front panel will flash on and off for a period of about 4 seconds,
after which the amplifier will attempt to go back into OPERATE mode, unless the
OPER/STBY switch has been placed in the STBY position. This 4 second period can be
shortened by toggling the OPER/STBY switch.
While the FAULT LED is flashing, the Ip and HV LED’s are used to indicate which
of the four fault conditions caused the amplifier to enter fault mode.
Fault type 1
Neither Ip nor HV LED’s are lit. Output relay did not close. This fault will rarely be
encountered, but if it is, it must be investigated further immediately, as continued
operation with this fault could cause additional damage to the amplifier.
Fault type 2
Ip LED is blinking. This indicates that the plate current in the tubes exceeded 1.5 amps.
This could be caused by the amplifier being mistuned or by overdriving the amplifier. If
neither of the above is the case, then a problem in the bias control circuitry may be
Fault type 3
Both Ip and HV LED’s are blinking. This indicates that the gain of the amplifier has
fallen below 10 dB. This is an important safety feature of the amplifier, as many different
problems in the amplifier or with its operation can be detected as a drop in gain. To avoid
this fault occurring while the amplifier is being tuned up, the gain fault detection is
disabled when drive power to the amplifier is below approximately 20 watts. This allows
correct initial amplifier tuning to be achieved before going to full power.
Fault type 4
HV LED is blinking. This indicates a reflected power fault. The 8406 is set up to trip
when the reflected power exceeds approximately 250 watts. At 1,500 watts output, this
would represent VSWR fault. The fault type codes above are also reported in the
amplifier telemetry data via the serial or USB ports on the rear of the amplifier.
If the tube current exceeds about 2.5 amps, the amplifier is shut off completely- that
is, the AC is shut off. The ON/OFF switch will need to be used to put the amplifier
back on line. To avoid problems, wait at least 20 seconds after this occurs before
attempting to put the amplifier back on line. If the amplifier trips again immediately,
investigate and cure the problem before attempting to turn the amplifier on again.
Repeatedly hitting the ON switch when the amplifier trips out is likely to result in
severe damage to components in the amplifier.
If a hard fault trips the amplifier all the way OFF, wait at least 20 seconds before
turning the amplifier power on again.
If you are certain that you have taken care of the problem that caused the fault, you may
turn the amplifier back to operate and proceed with use.
6.1.3 High SWR Considerations
On any frequency where your antenna VSWR exceeds 1.5:1, it’s important to
carefully tune the ALPHA 8406 for a proper match. The ALPHA 8406 does not contain
an antenna tuner. The SWR can be tuned via the antenna or an external tuner connected
to the output of the Alpha 8406. Nevertheless, if the system SWR is below 2:1, the
additional RF power loss of an antenna tuner can be avoided by tuning the 8406 into the
slight mismatch. There is no advantage to using a tuner to “tweak” the last bit of SWR in fact you will lose power this way.
6.1.4 Operating at Less Than 1.5 kW Requires Retuning
If you tune the amplifier for maximum power output and then decide to operate the
8406 at a power output much different from 1.5 kW, it must be re-tuned for efficient
and RF-clean operation. Under such “lightly loaded” conditions, the green grid LED
will probably be fully lit. Note that if you tune the amplifier at 1,500 watts and then
simply reduce power to 1,000 watts, the tubes are actually going to run hotter than at
1,500 watts. It is better to tune the amplifier with close to 50 watts of power, but
reduce the loading control to get 1,000 watts.
6.1.5 Lightning Protection
Induced energy from nearby electrical storms or other power transients may damage
components in the amplifier. Such damage is not covered under warranty.
It is important to use a good lightning arrestor, however the only lightning proof
solution available is to disconnect antenna feed lines and AC power when the
equipment is not in use.
6.1.6 Operating in Bypass Mode
Whenever the 8406 is in line, either off, in standby (STBY), or in warm-up with the
WAIT LED lighted, the amplifier is bypassed and the exciter is connected directly to
the antenna. The throughput limit in all cases is 200 watts when operating into a 1:1
SWR. This power level should be reduced accordingly for a higher SWR. Any power
level more than this may damage the RF switching relays in the 8406.
6.2 Transceiver Connections
The Alpha 8406 “relay” jack has approximately 12 V on it. When pulled all
the way to ground, a current of 10 mA flows. Consult your transceiver
manual for additional information on how to connect to an external amplifier.
6.3 Initial Setup and Tuning
6.3.1 Control Functions
The following chart shows the controls that allow you to adjust and monitor the
Used to select amateur band desired (in MHz).
Sets output tank circuit to resonance within each band. Higher
frequencies tend to tune toward the “0” end of the dial scale, while lower
frequencies tend to tune further toward the “100” end.
Sets amplifier plate loading and determines the power level at which best
efficiency and linearity are achieved. In general, loading is heavier at
greater scale settings. Higher frequencies tend to load more toward the
“100” end of the dial scale and lower frequencies toward the “0” end.
6.3.2 Tune-Up
The objective of tune-up is to adjust the amplifier (and the drive applied to it) to obtain
optimum efficiency and linearity at the desired output power. Any linear amplifier must
be adjusted for optimum efficiency and linearity at each specific power level. If
operation at higher power is then attempted without appropriate readjustment, the result
will be flat topping also known as “splatter,” and (usually) excessive amplifier grid
current. If operated at a much lower power level than it has been adjusted for, the
amplifier’s efficiency decreases considerably. Recommended practice is to tune first into
a dummy load or artificial antenna, then connect the antenna and make any slight final
adjustments that may be needed.
6.3.3 Grid Current Information
The ALPHA 8406 operates in Class AB1 when delivering maximum output power
consistent with excellent linearity. A small amount of grid current flows and the green
GRID MIN LED illuminates as drive approaches the optimum level. The green GRID
LED will flicker on SSB voice peaks and illuminate under CW/SSTV/RTTY carrier
As overdrive approaches, grid current increases rapidly and the red GRID MAX LED
illuminates. At maximum output and efficiency, the red LED lights dimly; full
illumination of the red LED indicates overdrive and must be avoided. If the red LED
lights up before the desired value of plate current and/or against grid current fluctuations.
On SSB, optimum output consistent with good linearity, occurs when the green forward
LED’s illuminates on most voice peaks and the red LED flickers dimly on only the
highest peaks. Excessive grid current results from overdrive and/or inadequate loading.
The solution is to restrict drive, and/or increase amplifier loading. The 8406's 4CX1500B
tube is well protected and these adjustments tend to be less critical than in many other
amplifiers. Grid bias is stabilized against grid current fluctuations.
6.3.4 ALC
The 8406 grid current limiting circuits provide substantial tube protection against
possible damage, therefore no ALC control is necessary. It is only necessary to set the
drive power from the radio as detailed in this manual.
6.3.5 Turning On The Amplifier
Please Note: Every time the ALPHA 8406 is powered up, there is a built-in 180
second warm up wait.
1. Place the OPR/STBY switch to STBY (standby).
2. Rotate the multimeter selector switch to HV.
3. Depress the POWER/ON switch. The fan and blower should immediately begin
to operate. If there is no air flow from the amplifier and no sound of blower
operation, immediately turn the amplifier off and investigate.
4. Within two seconds, the HV display should be all the way to the right, certainly
above 2,500 Volts. If it is lower than this, investigate further- perhaps the
primary taps are not correctly set.
5. At this time the red LED representing 1800 W output power is illuminated,
indicating 180 seconds countdown remaining (1800/10 = 180 seconds). As the
timer counts down, the remaining time will be indicated by the currently
illuminated LED on the power output bar graph moving to the left.
6. Move the multimeter switch to the Ip position. There should be no current
indicated, and this should be true during the entire period the amplifier is
warming up. Leave the switch in the Ip position during amplifier warm up. The
“Wait” LED will be blinking about twice per second, indicating that warm up is
still in progress. The FAULT, OPER and STBY LED’s should not be
If exhaust air is not coming from the top vents, TURN OFF the amplifier immediately
and verify that the exhaust chimneys are properly positioned over the tubes. When the
warm up delay is complete, the WAIT LED will extinguish and the OPR or STBY lights
will stop blinking. Put the OPR/STBY switch to OPR and the Alpha 8406 is now
*Each ALPHA 8406 shipped from our factory will include an individual table showing
the tune and load settings we used to achieve full output power on that amplifier into an
AP 2100, a 50-ohm dummy load. These settings will vary from those in the manual
depending on your station setup.
If at any time in the following procedure the amplifier fails to respond as
described, remove drive immediately and turn the OPR/STBY switch to
STBY! Verify all connections and cables, turn the amplifier switch to OPR
and proceed with the tuning procedure.
Alpha Amplifiers recommended tune up procedure.
Tune for Maximum Efficiency
Your Alpha 8406 is most easily tuned with the multimeter switch in the efficiency
position. Efficiency should be in the 55 – 59% range – the efficiency scale ranges from
50% to 70%. You should watch the efficiency reading during long transmissions and
peak it for best performance.
“Dip and Load” Method
Tuning up for Operation at 1,500 W RF Output.
Preset BAND, TUNE, and LOAD controls to the nominal positions given in your amp’s
specific tune up table or Table 6.3:
Note: Final TUNE and LOAD settings will vary with the operating frequency, antenna
characteristics and power level.
1. Please limit the transceiver drive to about 60W for tuning up and operating the
2. Set multimeter to Ip (Plate Current). Set TUNE and LOAD controls to
numbers indicated in the tune up sheet originally included with the amplifier.
3. Key radio with 10W drive and adjust TUNE control for a peak in RF out
which should be at the same point as a dip in Ip.
4. Increase drive to get 500W output, going back and forth between the TUNE
and LOAD to peak the RF output. If more output is desired, increase drive
from radio slightly, increase LOAD for a peak in RF out, then peak RF out
with TUNE control.
5. Increase drive to get 1000W output, going back and forth between the TUNE
and LOAD to peak the RF output. If more output is desired, increase drive
from radio slightly, increase LOAD for a peak in RF out, then peak RF out
with TUNE control.
6. When the amplifier is tuned correctly on 6 Meters, the Ip should range
between 0.9A and 1.1 A (read on the 0-to-1.5A scale) for 1500W output and
input drive should not need to be more than about 70W.
7. Plate current (Ip) is the most useful parameter to monitor on the multimeter
bargraph during normal operation of the amplifier.
6.4 Normal Use
6.4.1 Tube
The 4CX1500B tube used in the Alpha 8406 is supplied as a selected and tested tube.
The tube is very rugged and normally operates with a large margin of safety. The tube
should provide outstanding service for many years if not damaged by abuse – such as
overdrive or blockage of cooling airflow. Allow at least three to four inches (10 cm) of
unobstructed clearance around the air intake and exhaust areas. Stacking equipment on
top of the amplifier is not recommended.
Never allow key-down plate current to exceed 1.5 A for more than one or two seconds.
If you do, and a plate current trip occurs, it will automatically reset in about 4 seconds if
the amplifier is returned to receive (key-up).
Never allow the red GRID LED to stay brightly illuminated for more than a second.
Frequent on-off AC power cycling may shorten the tubes’ life. It is less stressful to
leave equipment in standby for several hours than to cycle power repeatedly on-offon-off over the same period.
6.4.2 Interlocks
The ALPHA 8406 is equipped with a cover interlock switch intended to remove primary
power from the amplifier, and a crowbar to short-circuit the high voltage to chassis
whenever the cover is lifted. These interlocks are designed to protect against dangerous
electric shock resulting from accidental contact with the lethal voltages inside the
Cover interlocks are intended only as back-up protection against accidents. Never
depend on them! Always disconnect the power cord from the AC mains before
removing the cover! Interlock switches should not be disabled for any reason.
6.4.3 Fuses
Never replace any fuse with one of a different type or greater current rating. Blowing of
one or both primary line fuses indicates that the maximum safe average power capability
of the amplifier has been substantially exceeded or that an equipment failure has
occurred. USE ONLY 20 AMP, 250 VOLT RATED FUSES for 190-220 VAC service.
25 amp fuses may be used with caution for line voltages of 90 – 130 V.
The slow-blow fuses Fe, located below the primary line fuses, may prevent damage to the
stop-start resistors and HV rectifiers in the event of abnormal turn-on conditions or HV
faults. If the AC interlock is defeated and primary power is applied while the HV
crowbar is close, the step-start fuses will normally blow.
6.4.4 Plate Over Current Relay
This relay will quickly turn off the amplifier in the event of grossly excessive plate
current or fault in the high voltage circuitry. The relay will not prevent tube or other
damage due to either short or long term overdrive or improper tuning. It is the operator’s
responsibility to ensure safe tuning, drive, and general operating conditions. Should the
over current relay trip, remove AC power from the amplifier, then determine and correct
the cause of the trip before turning the 8406 on again. This “hard fault” trip circuit does
not rely on the microcontroller for it’s operation, and will protect the amplifier even if the
processor has been damaged or is malfunctioning.
6.4.5 Idling Plate Current and Electronic Bias Control (EBS)
Idling plate current of the ALPHA 8406 is approximately 350 to 400 mA during full
power transmission. A detector senses RF drive, and reduces plate current to 30-50
mA during pauses in speech and key-up intervals, thus substantially reducing average
power supply loading, heat generation, and wasted energy.
6.4.6 RF and Mistuning Protection
The ALPHA 8406’s exclusive circuit senses the beginning of any RF arc in, for example,
a TUNE or LOAD variable capacitor and automatically switches the amplifier to standby
within a few milliseconds. This system has virtually eliminated RF arc damage in current
ALPHA amplifiers. The system similarly detects severe miss-tuning of the 8406, and if
drive exceeds about 25 W switches the amplifier to standby. The 25 watt input trip
threshold permits safe tune-up at low power levels without aggravating and unnecessary
6.5 Standard Maintenance Tasks
The amplifier interior, particularly the high voltage area, should be cleaned with a
vacuum cleaner and a soft bristle brush frequently enough to prevent visible
accumulation of dust. In extremely dusty conditions it may be advisable to secure a thin
air filter of the type used for window air conditioners across the air intake on the rear
There are no user-accessible lubrication points in the amplifier. Do not apply oil or
grease to any of the components. The exterior of the ALPHA 8406 may be cleaned with a
mild household liquid detergent. Do not use chemical solvents, as these may severely
damage the front panel or cabinet finish. Never use an abrasive cleaner.
The amplifier should be cleaned at least once a year and all dust should be
removed. This includes blowing out the cooling fins in the 4CX150S0B
7 Troubleshooting Hints
7.1 Normal Troubleshooting
Amplifier must be plugged in to the AC mains for 5 seconds before
turning on the front panel AC switch.
8406 will not turn on; nothing happens when ON switch is pushed.
1) External AC wiring, fuse or circuit
breaker may be open.
2) Amplifier cover is not in place or
properly secured; cover safety interlock is
3) Fuse F1-F4 open or missing. Check
fuses with an ohmmeter.
Check & correct wiring, replace fuse, or
reset circuit breaker.
4) Step-start resistor open.
Check resistor. If damaged, replace.
Make sure cover is replaced and all screws
securely inserted.
Check and replace any blown fuses with
fuses of the same size.
Amplifier turns on but no HV is indicated by the multimeter LED bargraph.
1) Multimeter selector switch in wrong
position, e.g., Ip.
Set Multimeter switch to the correct
2) Possible HV circuit fault.
3) HV sampling resistor in power supply
4) Transformer plugged into power supply
Check power supply wiring and connectors
to be sure input and output connectors are
set properly and the voltage taps have been
set properly.
Amplifier turns on but no multimeter indication; other LED bargraphs are
1) Low voltage power supply problem.
2) Defect or damage on control board.
Amplifier turns on but time delay will not complete; WAIT LED does not turn off.
1) Defect or damage in timing circuitry on
control board.
Contact Alpha Customer Service.
Amplifier turns on, time delay completes but amplifier will not transmit.
1) Open T/R control line from transceiver to
RELAY jack.
Contact Alpha Customer Service.
Amplifier transmits but red GRID LED illuminates often.
1) Amplifier overdriven or under-loaded.
2) Load VSWR (reflected power) exceeds
200 W.
Check output cabling and antenna. Correct
impedance mismatches.
3) Exciter output poorly controlled.
Consult exciter’s User’s Manual for
assistance on controlling output.
Reduce transceiver output and /or increase
amplifier loading.
Receive signals disappear or are severely attenuated when switching from
1) RELAY (T/R) control cable from
transceiver is shorted.
2) Transceiver locked in transmit
Check T/R control cable to make sure it is
switching properly. Replace if needed.
Make sure transceiver is properly switching
between transmit and receive. See
transceiver’s User’s Manual for assistance.
Plate current indicated when amplifier is in STBY or receive.
1) 4CX1500B tube heater-to-cathode
leakage or short.
2) Tube bias supply or T/R bias switch
Contact Alpha Customer Support.
Contact Alpha Customer Support.
Distorted SSB signal; Grid bias is unexpectedly decreasing.
1) Excessive RF drive from transceiver
and/or insufficient amplifier loading.
2) Coaxial connector, coax feed line,
antenna feed point balun, tuner, or antenna
trap arcing on voice peaks.
Decrease drive from transceiver. Re-check
amplifier tuning.
3) RF feedback from antenna into
transceiver via the transceiver power cord,
microphone or key cable, or other
unshielded station patch cables.
Make sure all power cords, microphone and
key cables, or other cables are properly
shielded and grounded.
4) Poor station RF ground.
Be sure the amplifier and transceiver have a
proper RF ground. Correct if necessary.
Required drive to maintain 1,500 W is steadily increasing.
1) If the amount of drive required to
1,500 watts is steadily increasing, this is an
indication that the tubes are probably aging.
Contact Alpha Customer Support to see if
tubes need replacing.
8 Glossary
The following terms are used in this manual. Detailed explanations may be found in
various publications including the ARRL Handbook.
AB1-Modulation class AB1. Provides good linearity in push-pull configuration.
AC-Alternating current
ALC-Automatic Level Control
ampere-Current measurement
ARRL-American Radio Relay League
AWG-American Wire Gauge
CW-Continuous wave
EBS-Electronic bias switching
Exciter-The radio that provides RF drive for the 8406 to operate
FCC-Federal Communications Commission
FM-Frequency modulation
FSK-Frequency-shift keying
HF-High frequency (3 to 30 MHz)
HV-High voltage
Ip-Idling plate current
kVA-Kilovolts/ampere. kVA * 0.8 = kilowatts
LED-Light-emitting diode
LV-Low voltage
PSK-Phase shift keying. A digital modulation scheme
QSK – Quick shift keying. In this CW transmitting mode, the transmitter is
only on for the duration of each dot or dash and switches to receive between each
RCA - Radio Corporation of America. Also a type of interconnecting plug.
RTTY – Radio teletype
SSB – single side band
SSTV – Slow-scan television
STBY – Standby
SWR – Standing Wave Ratio. A measure of antenna and feed line efficiency.
T/R – Transmit/Receive.
UHF – Ultra High Frequency (300 – 3,000 Mhz)
US – United States.
VAC – Volts of alternating current.
VDC – Volts of direct current.
VSWR – Voltage Standing Wave Ratio
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF