GS-35b HF Power Amplifier
by Gary Podgorny, EW1MM
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Fig. 1 - PA Schematic

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The amplifier described here is an update of the High Power linear amplifier described in the July, 1995 issue of the "Radiolubitel. KW i UKW" magazine in Belarus. In English, this is "Radio Amateur. HF and VHF". The author, EW1MM Gary Podgorny (callsign in the 1970s was UA6LFC) constructed his 1st high power amplifier in 1976 using GS-7B metal-ceramic triode (see the black and white picture at left - click for zoom). The GS-7B metal-ceramic triode provided the same power output as the GS-35B because the parameters of both tubes are the same. It looks like "GS-7B" might be the name in the 1970s of the "GS-35B" today.

In the color picture at right (click for zoom) one can see the latest model of EW1MM project. The modified schematic diagram of the amplifier can be seen in Fig.1 above. The Russian high-mu triode, a GS-35B, is connected in a grounded-grid configuration which provides about the most simple layout possible. Although the input impedance of the grounded-grid GS-35B is close to 50 ohms and would provide a good match for a driver with fixed-impedance output, a pi-network input circuit would be recommended for better linearity and to lower the driving power requirements.

Typical Operating Characteristics:
  • Class of operation B2

  • Input impedance close to 50 Ohm

  • Plate Voltage 3000 V
  • Cathode current (sum of Grid and Plate currents) 1 A

  • SSB idle current 120 mA

  • CW idle current 60 mA

  • Grid current (single tone) max. 30% of the Cathode current

  • RF Plate Load Resistance 2000 Ohm

  • Drive Power 80100 W

  • Output Power 1500 W

  • Efficiency 5565%

  • Required Cooling 150 cubical meters per hour (90 CFM) for 24 hour work

  • Radio parts used in the RF deck of the Amplifier (Fig. 1, above)
    RFC1 25 bifilar turns of 2 mm dia enameled Cu wire on ferrite rod (length 150 mm, dia 8-10 mm).
    RFC2 0.5 mm dia enameled Cu wire on 30 mm OD ceramic form, 70 mm long close spaced; then 17 more turns spaced 1 mm at anode end. The total inductance is 195 microHenries.
    RFC3 2.5 mH, 0.3 A RF choke.
    L1 10/15 m 3.5 4 turns, 10 mm by 1 mm copper strip with 1 mm space between turns, 40 mm ID. Tap for 10 meters at 2.5 turns from C1 end of coil.
    L2 20 m 5.5 - 6 turns of 6 mm dia Cu tube with 6 mm spacing between turns, 50 mm ID.
    L3 40/80/160 m 20 turns of 2.5 mm dia Cu wire on 75 mm OD ceramic form, 2.5 mm turn spacing.
    To cover the WARC bands one must use a 9-position switch for S2.
    PA1 0-1.5 A dc meter Cathode Current.
    PA2 0-0.5 A dc meter Grid Current.
    D1D7 Russian D815A Zener diodes 5.6 V, 8 W (Use a heat sink).
    D8 Russian D817A Zener diode 56 V, 5 W (No heat sink is used).
    D9, D10, D11 1N4007 or 1N5408.
    C1 TUNE 500 pF maximum, 3-4 mm spacing (or combination of 170 pF variable capacitor with fixed RF type high voltage 100 pF for 80 m, 330 pF for 160 m capacitors and RF relay).
    C2 2200 pF, 10 kV RF type doorknob capacitor.
    C3 LOAD 2000 pF maximum, 1.5 kV air variable.
    C4 1000 pF, 5-kV doorknob capacitor.
    C5 2200 pF, 5-kV disc-ceramic capacitor.
    C6 0.01, 1 kV disc-ceramic capacitor.
    C7 0.01, 500 V MICA.
    C8, C9 0.01, 1kV disc-ceramic capacitors.
    K1 RF type relay, 24 V dc coil, should be rated for 100 W.
    K2 RF type relay, 24 v dc coil, should be rated for 1500 W.
    K3 relay, 24 V dc coil, 2 A contacts.
    K4 relay, 24 V dc coil, 1 A contacts.
    S1 switch, should be rated for 2 A.
    S2 5-position (9-position if WARC bands used), high-power ceramic RF switch.

    Radio parts used in external High-Voltage Power Supply (Fig.2 below)
    S1 Power relay 1015 A contacts.
    K4 relay, 220-V ac coil, 1015 A contacts.
    K5 relay, 220 V ac coil, 5 A contacts.
    T1 34 kVA plate transformer, 240 V ac primary, secondary 2400 V ac at 1 A.
    T2 filament transformer, 240 V ac primary, secondary 12.6 V ac at 4 A.
    D1D20 5 diodes in each leg.
    The high voltage rectifier diodes are built up with a series combination of 3 A, 1000 V PIV silicon diodes. Each diode is shunted by 500 k resistor and 0.01 mF, 1000 V disc-ceramic capacitor to equalize the reverse voltage across the diodes and to eliminate "white noise" generated by the diodes which can appear as sideband noise on transmitted signal.

    The exact value of the shunt resistor is different and depends on the PIV of the diodes. R=PIV rating of the diode should be multiplied by 500 Ohm. Example:  The PIV of the diodes are 1000. 1000 x 500=500 000 Ohm (500 k), so each diode should be shunted by 500 k resistor.

    The secondary side of the power supply is conventional. The power supply provides approximately 3200 volts under no signal conditions and 3000 volts at a cathode current of 1 ampere. C23 and C24 are filter capacitors. The total value is 50 mF, 6000 V. I would not recommend using the electrolytic capacitors here. A high-voltage meter is included in the power supply. A low voltage power supply is shown in Fig.3 below.

    The linear amplifier operates in a grounded-grid circuit. The GS-35B amplifier requires a driver that can deliver at least 80 watts p-e-p. Actually, it is best to use a driver that is capable of 100 watts p-e-p, to assure that sufficient driving power is available on 21 and 28 MHz, the frequencies at which the efficiency of coupling circuits is often poor in comparison to that of the lower bands.

    The bias of the amplifier is provided with D1D7 D815A, 5.6 V, 8 W zener diodes (Use a heat sink). It is a good idea to use one of the well known adjustable Bias Circuit with high power NPN transistor or something like that instead of D1D7. The bias voltage for GS-35B is between 2835 V.In standby mode, the amplifier is biased to cutoff (near-zero cathode current) by the D8 D817A, 56 V, 5 W zener diode.

    The grid current indicators are a portion of the grid-trip overload circuit. When grid current exceeds a preset value, grid current is drawn through the 10 Ohm resistor (R1, Fig.1) and the voltage drop across it turns on transistors VT1 and VT2 which operate the red and green LEDs. Trimpots R3 and R6 provide the means to select grid current levels at which the LEDs are activated. In this amplifier, the Green LED turns on when the grid current is 30% of max cathode current, and the Red one at 3233% of max cathode current. Any flashing of the Red LED indicates excessive drive power or improper plate circuit loading.

    A grid-trip module is included in the amplifier to protect the tube against excessive grid current. Again, high levels of grid current can be caused by excessive drive, improper plate circuit loading or lack of a proper load on the amplifier. When grid current exceeds 35% of max cathode current, relay K4 is tripped (Fig.1), so the amplifier will go to the BYPASS position. Pushing the "Reset" (Fig.1) button places the amplifier back in service. The grid-trip protection circuit is simple but it works very well.

    It is dangerous practice to place the plate current meter in +3000 lead. The meter here is placed in the Cathode return circuit, so it reads the Cathode Current, which is the sum of the Grid and Plate Currents.

    Grid current is monitored by measuring the voltage drop across a 10 Ohm, R1, 2 watt (see Fig.1 above) resistor through which grid current passes.

    The Grid current of the GS-35B with full drive power and good SWR in the antenna system can reach maximum 30% of the Cathode current. The max data sheet of GS-35B cathode current is 1.4 A. So the conditions are absolutely normal when working cathode current is 1 A, the grid current is not more than 300 mA, plate voltage is 3000 V under the load, SWR in the input and in the output of the Power Amplifier is around 1.2:1 or better.

    In this amplifier the tube is placed in the horizontal position, but it can also be placed vertically with no problem. The cooling system provides cooling air in the direction from anode of the tube to the heater.

    The output tank circuit is a pi-network with air variable capacitors. If adjustments are correct in the output tank, maximum grid current, minimum plate current, and maximum power output take place at the same time. The 2.5 mH RF choke is connected between the output end of the pi-tank and ground to prevent +3000 V from appearing at the antenna terminals should C2 develop a short.

    The air flow system must always be used when any power is applied to the GS-35B even filament. The RF drive power should never reach the GS-35B unless the tube has plate voltage applied.

    Good DXing!
    Gary Podgorny, EW1MM.

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    Fig. 2 - 3KV Power Supply

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    Fig. 3 - +26 VDC Supply

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