This amplifier is designed around
the low cost Russian GS35B triode, and features:
1.5KW output using
grounded grid circuitry with about 100 Watts drive
A commercial (expensive)
tube socket is not required
Easy to fabricate
low cost air system chimney
Fast vacuum relay
bias to accommodate various operating conditions
circuitry helps prevent damage from mistakes or circuit
"step start" circuit limits cold inrush current;
extends tube life
Cooling air blower
remotely located for noise reduction
voltage requirement of 12.6 volts at 3.2 amps
using die cast enclosures
Click on each picture
for larger image
Top Panel Removed
QSK Relay Box
Click download link
to see complete five page schematic diagram in Adobe format.
complete 5 page schematic
Build a high power Low
Pass Filter for 50 MHz Amplifier.
to low pass filter plans page
to GS35B liquid cooling page
Russian data sheets show the GS35B anode cooling requirement
to be 2500 liters/minute. Other GS35B specification sources
include information on the cooling requirements of the GS35B
cathode and grid. By definition, one cubic foot equals 28.327
liters. Converting to cubic feet per minute (CFM), the anode
cooling requirements are about 88.3 CFM.
to the airflow diagram to the right. In this amplifier, the
under chassis area is pressurized. A number of screened airflow
holes are placed in the grid compartment and main chassis mounting
area. An inspection of the photograph for the grid compartment
will show some of the eight one inch diameter screened inlet
air paths. Two 2.5 inch diameter holes in the top chassis plate
provides an air path towards the bottom of the tube radiator.
Providing a large airflow surface as possible will reduce the
back pressure requirements of the external air blower fan. Passing
all of the airflow directly through the under-chassis shielded
grid compartment will easily satisfy the cathode and grid cooling
needs. It is very important to not ignore the smaller cathode
and grid airflow cooling requirements.
table below shows GS35B element cooling requirements in various
units of measure:
|GS35B Cooling requirements
| Cubic feet
per min (CFM)
| Liters per minute
free Agilent Technology (Hewlett-Packard) Conversion
that will do these conversion calculations (and much more).
3C5I kindly sent me an audio file of this amplifier as he copied
it on 4 December, 1999 at 1649Z from his African QTH in Equatorial
Guinea. Click the Six Meter QSL cards to to hear this unmodified
audio recording made by Alan.
Description of Amplifier Circuit Function
amplifier provides 1500 watts power output for the Six-meter
band. It uses the low cost GS35B Russian power triode in a grounded
grid or cathode driven configuration and requires about 100
Watts of drive power. The Russian tube is currently available
from USA and international sources1 at attractive
prices, especially when compared to the cost of traditional
American tubes of this power rating.
classic cathode driven or "grounded grid" circuit
is used. The secondary of the filament transformer is isolated
from the vacuum tube with a homemade bifilar choke wound on
a 1/2" diameter six-inch long ferrite rod. The center tap
of the filament secondary provides the connection to the tubes
bias circuitry. RF driving power from the transceiver is applied
to the tube's cathode using an input matching circuit.
Partial RF DECK Schematic
vacuum tube output circuitry is the familiar PI network design.
Home made high voltage RF chokes wound on 3/4" ceramic
forms are used to keep RF out of the high voltage supply. The
output Pi-network coil is wound from 1/4" diameter copper
tubing. A 5 to 30-pF vacuum tuning capacitor is shown on the
schematic. At plate voltages approaching 4kV, this capacitor
may be eliminated. The tube circuit can be resonated using the
vacuum tube output capacitance in conjunction with the output
Pi-network coil. A safe, easy way to adjust the output coil
is described in the section "Adjusting the Pi-Network".
This is also a "set and forget" adjustment. Considerable
expense is saved by elimination of the vacuum variable.
plate blocking capacitor value needs to exhibit a reactance
of less than about 5% of the plate load impedance. For this
amplifier with a plate voltage of 4 kV, this works out to a
minimum capacitance value of 200 pF. The current rating of the
plate blocking capacitor when used in this amplifier is 10 Amps
at 50.1 MHz. If you find a capacitor that has no RF current
rating marked on it or has no current rating certified by the
manufacturer, don't use it. It is probably intended for power
supply and not RF service. The basic specification for the blocking
cap required by this amplifier is 10 kV minimum voltage breakdown,
200-pF minimum capacitance, and 10 Amps RF current at 50 MHz.
A suitable commercial component would be one Centralab HTY57Y102
only remaining output circuit components are an inexpensive
200 pF 1kV air variable used as the output loading control,
and a tuned, 1/2 wave length shorted coaxial cable "stub".
This stub is located on the output of the RF deck enclosure
and serves two purposes. The first and most important is to
provide a dc path to ground should the plate blocking capacitor
fail. If you choose to omit this stub from your amplifier, you
MUST add an RF choke from the amplifier antenna output to ground.
The size is not important; it must present a high impedance
at 50 MHz. A one mH choke is adequate. The dc path this choke
provides will blow the fuse in the high voltage power supply
if the plate blocking capacitor were to fail. Leaving the shorted
stub or RF choke out is asking for BIG trouble. Essentially,
the plate voltage will appear on your antenna line when the
capacitor shorts, a potentially lethal situation. This voltage
on the output coax has caused welded vacuum relays, tube destruction,
and high VSWR antenna/feed line problems.
second purpose of the shorted stub is to help suppress second
harmonic radiation from the amplifier. This stub is made from
RG/8 type coax and is tuned for 100.2 Mhz with a SWR analyzer.
Physically mine is made from 52 inches of International # 9086
coax, with one end soldered to a standard PL259 connector, and
the far end has the center lead and shield braid shorted together.
(Hence, shorted stub). When hooked up to a SWR analyzer (like
the MFJ-259), and the frequency set to 100.2 MHz (the second
harmonic of six meters) the instrument panel resistance meter
shows zero Ohms. This means the shorted stub acts like a short
at this frequency, and helps stop the second harmonic from radiating.
The desired 50.1 MHz energy is not affected.
you use a different type of coax, just trim the coax length
until you get a zero Ohm reading on your SWR analyzer. This
type of filter is somewhat broadband, easy to make, and does
help in reducing the second harmonic. Again, the primary function
of this stub is to blow the fuse should the plate blocking cap
fail short, and not for harmonic reduction. This tuned stub
does not eliminate the need for an external low pass filter.
A suitable design is available
on this web site
transistor will turn on when supplied with plate high voltage
reduced by a large resistance divider. This signal is used by
the tube protection circuitry.
- RECEIVE CIRCUITRY
transmit relay is built into a separate die cast aluminum enclosure
and is also mounted on the RF deck rear panel. This vacuum relay
is a Jennings RB3. The 15 msec operate and release times for
the RB3 relay are not particularly fast, but the driving digital
circuitry is designed to compensate for this. Of course, a faster
relay may be used. This relay has been flawless in this application.
The CW keying or PTT line first goes directly to the amplifier.
This activates the vacuum relay, and a digitally delayed output
is then sent to the transceiver. This insures that no "hot
switching" of the vacuum relay occurs. When this amplifier
is turned off, the CW key signal simply passes straight through
to the transceiver as usual. A small 5-volt fast acting reed
relay in the T/R control circuit is activated to again regenerate
the PTT or CW keying signal. This approach will key any transceiver
you wish to use, regardless of the type of keying circuit it
employs. The digital delay circuit is crystal controlled, and
uses two series connected shift registers to generate a 15 millisecond
delayed keying wave form These commonly available digital circuits
are low priced. The schematic page labeled "T/R Control Timing"
shows detailed digital timing diagrams. No trouble has been
experienced with this circuit in over three years of operation.
front panel mounted panel meters measure plate current (0 to
1 Amp) and grid current (0 to 500 mA). The basic meter movement
in both meters is 50 micro Amps. The four 1% resistors are meter
shunt resistors. Other meter movements may be used, just scale
the shunt resistors accordingly. If you don't have 1% values
in stock, simply measure with your ohmmeter and make up the
necessary resistor series/parallel string to get within 1% of
the target values. This will insure your panel meter readings
6 amp 1 kV diodes protect the mechanical meter movements in
the event of circuit failure.
cathode driven circuit of this amplifier requires that a positive
voltage be applied to the cathode of the vacuum tube with respect
to the grounded grid. This establishes the operating point of
the tube, and determines the class of operation. Older circuit
designs have used high power discrete Zener diodes. Some designers
have used low wattage Zener diodes driving a high power external
series pass discrete transistor. Both approaches were tried
in earlier versions of this amplifier. A third circuit using
a variable voltage source was suggested by G3SEK 2.
This circuit uses a precision programmable current source driving
an external PNP transistor. This emulates a variable high power
Zener diode. The external transistor is mounted on a heat sink.
This circuit is superior and provides smooth variable tube bias
that is stable over the full power output range of this amplifier.
Ian White, G3SEK mentioned in an email message that when plate
voltages of 4 kV++ are used, the 36 Volt maximum rating of the
TL431 is exceeded. He has an application note available
on his website that describes this. The circuit here was
suggested by Ian and provides variable bias in the range of
27 to 45 Volts, without exceeding the regulator voltage rating.
The TL431 is available in two package styles, costs less than
one dollar, and is widely available. The schematic diagram shows
the pin connections for the eight-pin mini-dip package. The
variable resistor voltage adjust control needs to be available
from outside the amplifier, so mount it accordingly. The one
amp fast blow fuse in series with the filament transformer center
tap protects the tube from excessive current in the event of
circuit failure. Likewise, the fuse needs to be replaced without
disassembly of the amplifier, so mount it in a good spot on
the chassis wall.
low voltage regulated power supplies provide regulated +26 volts
and + 5 volts for the amplifier control circuitry.
lamp driver and circuit protection logic is assembled on a small
circuit card and resides under the main chassis. Feed through
capacitors of 1000 pF are mounted in the under chassis walls,
and decouple the circuitry signals. Signals from the high voltage
power supply (warm-up and high voltage OK), RF deck (grid fault),
and front panel (operate/standby) are combined to provide a
signal to allow application of RF drive to the amplifier. RF
drive is prohibited if high voltage is not present, if a grid
fault has been detected, or if the front panel function switch
switches the amp to standby. Four colored status lamps are used.
This amplifier uses 28-volt incandescent lamps and jeweled glass
lens covers. These look good, are bright, have a very wide viewing
angle, and are highly visible in a busy ham shack. If you prefer
to use LED indicators, simply substitute the LED of your choice
for the lamp, connect the cathode of the LED to the integrated
circuit driver, and connect a (typical) 1.6 kOhm resistor in
series with each LED.
amplifier front panel has a three-position rotary mode switch.
The first position is OFF. The second is STANDBY, while the
third position is OPERATE. When the amplifier is first turned
on, both the WARM-UP and STANDBY lamps are illuminated. After
a two-minute filament warm-up period, high voltage is applied
to the amplifier RF deck. If the mode selector is on STANDBY
and the high voltage detector logic senses proper high voltage,
the WARM-UP lamp goes out, but the STANDBY lamp remains illuminated.
If the WARM-UP lamp is off, the mode selector is on OPERATE,
but the STANDBY lamp stays on, suspect that the high voltage
is not connected to the RF deck. Turn everything off and then
check the high voltage connections. This protection circuit
will not allow RF drive if proper high voltage is not present
on the RF deck. Normally, turning the mode selector to OPERATE
removes the STANDBY lamp and illuminates the OPERATE lamp. Should
a grid over-current condition be detected, the FAULT and STANDBY
lamps only will illuminate, and the protection circuitry will
automatically bypass the amplifier preventing damage. Momentarily
depressing the front panel RESET pushbutton will restore the
amplifier to the normal OPERATE condition.
two highest priced single components in RF amplifier construction
have usually been the vacuum tube and the high voltage power
transformer. The availability of Russian tubes has brought the
tube part of this cost down to manageable levels, but the power
transformer price can remain a stumbling block 3.
I had an existing 2 KV transformer from an earlier project.
Rather than use 2 kV in this new GS35B amplifier, a voltage
doubler circuit was employed so that the existing transformer
could be used. If you are starting from scratch, consider a
design voltage of about 3 kV This will keep the plate voltage
within published tube specifications. 4KV has presented no problems
with this amplifier; especially since the tube plate dissipation
rating is not being exceeded. Some amateurs in Europe (PA3CSG,
9H1PA, DL4MEA, G0RUZ) and the USA (K0PW, K7CW) are now using
this approximate voltage level on the GS35B with good success 4.
on surge delay relay is used to reduce the initial power on
current surge required to charge the filter capacitors. Special
high voltage connectors are used to connect the high voltage
to the RF deck. This amplifier has the power supply remotely
located, so high voltage wire is used to make this connection 5. Exercise extreme caution when working with the
voltages present in this power supply.
INITIAL TESTING, ADJUSTMENT, AND TUNE UP
tubes tested in this amplifier are surplus items from the Russian
military. The tubes may be unused, but have probably been subjected
to long-term storage. Filament conditioning is recommended 4,
and a high
voltage breakdown tester described on this web site
will be valuable in identifying tubes with problems before they
are placed into the amplifier.
circuits in this amplifier are built as small sub-systems. They
can be tested before placing them into the amplifier. Pre-testing
your circuitry will greatly help when the time comes to actually
insert your vacuum tube for final checkout. In particular, the
RF deck plate tank circuit, grid trip circuit, and QSK T/R relay
circuitry are easily adjusted and tested ahead of time. The
front panel meter Grid Current and Plate Current readings can
be verified easily as well. Having a tuned plate tank pi-network
ahead of time prevents subjecting your tube to grossly mistuned
conditions. The same is true of the grid trip circuit. Adjusting
the plate pi-network ahead of time is a practical necessity
if you choose to eliminate the vacuum variable tuning capacitor.
REMOVE ALL TUBE VOLTAGES. Leave the tube in circuit. The plate
load impedance of your amplifier is expressed approximately
as: [(plate voltage in volts) divided by (1.8 times the plate
current in amps)]. Assuming 4000 plate volts and 750 ma of plate
current, this works out to about 2963 ohms. Make up a resistance
value close to this number with low inductance resistors, and
temporarily place this resistor string from the tube anode connection
to ground. This simulates the plate load impedance of the amplifier.
The purpose of the pi-network in the amplifier is to change
this relatively high plate load impedance value to 50 ohms for
your transmission line. Now, hook up a SWR analyzer to the amplifier's
RF OUTPUT connector. Adjust the SWR analyzer for a frequency
of 50.1 MHz Adjust the copper coil windings (slightly expand
or squeeze together) in conjunction with adjusting the output
loading capacitor. Adjust for a 1 to 1 SWR reading on the SWR
analyzer. When the SWR reading is flat, your adjustment is finished.
The top RF deck shielding cover on this amplifier had a small
effect on this setting, so a tiny adjustment was necessary when
the amplifier was running at full output. It's amazing how close
this procedure gets your amp to the final settings. If you don't
have a SWR analyzer, consider getting one. Remember to remove
the temporary resistor you installed during this procedure.
the grid trip circuit:
REMOVE ALL TUBE VOLTAGES. The GS35B and GS31B tubes are designed
to run much higher grid currents than American tubes of the
8877 variety. A typical Russian GS35B will run about 25 to 30
percent of the plate current value for a grid current. So, if
the plate current is 800 ma, a grid current of about 240 ma
is common. A grid trip setting of about 300 ma or so is an approximate
number for this amplifier. To set up the grid trip variable
resistor, you will need the low voltage plus five VDC supply
activated. You will also need a current limited low voltage
external power supply, and a multimeter. Connect the minus lead
of the external power supply to the amplifier chassis ground.
Set the voltage setting of the external power supply to zero
volts. Connect the plus lead of the external power supply to
the "B minus" connection of the amplifier. Using the
multimeter, adjust the external supply for 300 milliamps. The
front panel Grid Current meter should also now read 300 ma Adjust
the grid trip variable resistor until the small LED indicator
illuminates, indicating the grid trip relay is now latched.
Remove the external supply. Pressing the front panel RESET button
should extinguish the indicator LED and the relay should now
be deactivated. This completes the grid trip adjust. This is
a "set and forget" adjustment. The variable resistor
and LED indicator can be buried inside the amplifier since no
external adjustment is required.
Plate Current meter operation can be verified in a similar manner.
Using the minus lead of the variable external supply connected
to the amplifier "B Minus" connection, and the plus
lead connected to the plus terminal of the Plate Current meter,
your multimeter current reading should be the same as indicated
by the front panel meter.
T/R relay circuit can be tested ahead of time. ABSOLUTELY REMOVE
ALL TUBE VOLTAGES. The low voltage +5 and +26 volt regulated
supplies need to be active for this test. Temporarily remove
the "Standby Signal" input connection. Apply a hand
key or CW keyer to the "Key In" connection. Hook up
a code practice oscillator to the "Key Output" line
(or otherwise verify activity at the output line). Keying the
input causes the output to key, and the keying relay should
activate. If you have an oscilloscope, you can verify that the
delayed keying activity is 15 milliseconds delayed. The logic
timing diagram shown on the schematic page 4 shows the circuit
activity for typical CW keying speeds. The delayed keying times
are fixed, and no adjustments are necessary for proper relay
activity. Momentary grounding of the "Standby Signal"
input should disable the relay activity, but normal output at
the "Key Output" will still occur.
variable bias circuit is adjusted with all normal tube voltages
applied. With ZERO power input (turn your transceiver off),
ground the "Key input" signal line, and adjust the
variable bias resistor for a resting plate current reading of
input SWR adjust is best set with the amplifier running normally,
and about ten watts or so of RF power applied to the amplifier.
A suitable dummy load or antenna must be connected to the amplifier
output. Apply RF drive, and adjust the small variable capacitor
for a 1 to 1 SWR between the transceiver output and the amplifier
input. You should be able to obtain a very low SWR reading.
Six different tubes were tested in this amplifier, and each
was easily matched with a slight adjustment of the capacitor.
This is a "set and forget" alignment, but make sure
that you place the capacitor so you can adjust it while the
amplifier is running. This amplifier has the adjustment shaft
available on the rear RF deck chassis.
this point, more input power can be applied to the amplifier
and the output loading capacitor adjusted slightly for best
volts plate voltage
ma plate current
ma idle current
ma grid current
numbers will change with different tubes. Some tubes tested
in this amplifier show outputs greater or less by a couple hundred
watts or so.
thanks for help and support go to B-N "Bob" Alper
of Svetlana, Jim Tonne WB6BLD, Paul Kiesel K7CW, and Paul Goble
Alex, UR4LL has
provided good service in obtaining tubes.
Board" by Ian White, G3SEK.
source for custom transformers has been Ed
Dennis at Heritage Transformer Co.
from informative Russian
tube web site of Paul Goble, ND2X/5.
components are available from RF