50MHz GS-35 PA
by Bill Jones, K8CU
Click for K8CU web site

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Front View - The front panel is made of aluminum and like most aluminum parts in this project has been bead blasted to give a smooth satin finish. The enclosure is a standard 19-inch wide rack panel size.


Front Panel Removed - Wiring connections from the amplifier chassis to the front panel are made with Molex connectors. Feed through bypass capacitors are installed on the main chassis wall. Visible in the lower left is the grid bias adjust control and the fuse for plate over-current. The main chassis enclosure is made of 1/8" aluminum and aluminum angle and has been bead blasted both inside and out. It appears to be painted a silver color, but is actually a bare aluminum matte surface.


Top Panel Removed - The heated air exhaust screen is visible. The 4-inch air inlet hose has been removed for this photograph. Notice the QSK relay box on the chassis rear panel. A large access hole in the rear enclosure panel allows easy access for the blower air hose, cables, and the high voltage power supply leads.


Chassis Top - This amplifier chassis was originally made for four 4CX250B tubes. One hole has been enlarged for the GS35B tube. The remaining three are used as screened air inlet holes from the pressurized chassis. The circuit board on the rear enclosure wall is a voltage divider for the "HV Okay" signal that's sent to the control circuitry. The home made Teflon sheet air exhaust chimney is visible around the vacuum tube.


Chassis Rear - The QSK relay box and tuned shorted stub are visible. Below the relay box is a knob that controls the tuning capacitor to allow adjustment of the input SWR. The screened air inlet hole is shrouded by the plastic adapter that holds the four-inch dryer hose for cooling air. Visible above the cooling inlet is the high voltage connector that connects to the remote power supply.


QSK Relay Box - The vacuum relay is mounted on home made rubber sound insulators. This helps to keep the relay quiet. Control signals for the relay are connected using feed-through ceramic bypass capacitors and series RF chokes. This particular relay is too slow for fast keying speeds, but the driving logic compensates for this. Operation has been flawless with no "hot switching" possible.


Chassis Bottom - Visible in the upper right is the QSK relay T/R Control logic housed in a small die cast box. Immediately to the left is the plus 5 and plus 26 volt regulated power supplies. The large circuit board is the cathode keying and lamp logic circuitry. The lower left shows the relay for filament step-start action and the heat sink mounted grid bias transistor. The filament power transformer is just to the left of the grid compartment die-cast enclosure. The right center shows the input T circuit SWR matching coils and adjustable capacitor.


Grid Compartment - The simple grid circuitry is revealed when the cover is removed. A home made bifilar filament choke is connected to two feed-through capacitors that connect to the filament power transformer. Connections to the GS35B grid and filament connector rings are done using home made circular copper clamps. Notice many screened air inlet and exhaust openings.


Click on link below to see 5-page schematic diagram and timing logic in PDF form.
Download Complete Schematic    

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Narrative Description of Circuit Function:



This amplifier is designed around the GS35B triode, and features:

  • 1.5KW output using grounded grid circuitry
  • Drive power of 100 watts or less
  • A commercial (expensive) tube socket is not required
  • Easy to fabricate low cost air system chimney
  • Full QSK break-in system using a vacuum relay
  • Variable cathode bias to accomodate various operating conditions
  • Tube protection circuitry helps prevent damage from operator mistakes or circuit failures
  • Tube filament "step start" circuit limits cold inrush current; extends tube life
  • Cooling air blower remotely located for noise reduction
  • Modest filament voltage requirement of 12.6 volts at 3.2 amps
  • Modular construction using diecast enclosures gives good shielding and aids assembly/troubleshooting
  • THE AMPLIFIER CIRCUIT

    This 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 sources (see note 1 at end of narrative) attractive prices, especially when compared to the cost of traditional American tubes of this power rating.

    RF DECK SCHEMATIC

    The 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.

    The 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 "Tune Up". This is also a "set and forget" adjustment. Considerable expense is saved by elimination of the vacuum variable.

    The 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 capacitor (see note 5).

    The 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.

    The second purpose of the shorted stub is to help suppress 2nd 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.

    A small 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.

    TRANSMIT - RECEIVE CIRCUITRY

    The 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 waveform. 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.

    Two 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 are correct.

    The 6 amp 1 kV diodes protect the delicate mechanical meter movements in the event of circuit failure.

    The 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. 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.

    The low voltage regulated power supplies provide regulated +26 volts and + 5 volts for the amplifier control circuitry.

    LAMP LOGIC

    This 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.

    The 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.

    POWER SUPPLY

    The 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 (see note 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 (see note 4).

    A power 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

    The 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 (see note 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.

    Some 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.

    Adjusting the pi-network:  ABSOLUTELY 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.

    Adjusting the grid trip circuit: ABSOLUTELY 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.

    The 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.

    The QSK and 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 QSK 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.

    The 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 100 mA.

    The 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.

    At this point, more input power can be applied to the amplifier and the output loading capacitor adjusted slightly for best power output.

    Typical operating conditions:

    • 4100 volts plate voltage
    • 700 mA plate current
    • 100 mA idle current
    • 230 mA grid current
    • 85 watts drive
    • 1500 watts output

    These numbers will change slightly with different tubes. Some tubes tested in this amplifier show outputs greater or less by a couple hundred watts or so.

    Thanks

    Special thanks for help and support go to B-N "Bob" Alper of Svetlana, Jim Tonne WB6BLD, Paul Kiesel K7CW, and Paul Goble ND2X/5.

    Notes
      1.  

    Alex, UR4LL and Gintarus, LY3NML or website have both provided tubes at good prices. One USA source is Tom’s Tubes.  Check among the different sources because prices vary considerably. I have purchased some tubes for as low as $67 total, with airmail delivery included.

      2.  

    See description of “Triode Board” by Ian White, G3SEK.

      3.  

    A good source for custom transformers has been Ed Dennis at Heritage Transformer Co.

      4.  

    Data taken from informative Russian tube web site of Paul Goble, ND2X/5.

      5.  

    Some amplifier components are available from RF Parts Co.

    Email K8CU at k8cu@arrl.net or go to his website at http://www.realhamradio.com/.


    Review the requirements for Air Cooling of the GS-35B.

    Build a Low Pass Filter for 50MHz PAs.


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    Last Updated: 18 May 2002
    Feedback: Paul S. Goble, III, ND2X
    Copyright © 2000 - 2002 by ND2X & K8CU, all rights reserved


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