GS35B 2mtrs Poweramplifier


Goal was to have for contest operation a stable and virtual no tune amplifier capable of delivering 1KW of output power. This goal has during 2003 been accomplished, based on the design of DL4MEA. This design is using a Russian GS35B tube. At some points this design has been altered and those will be described below in more detail.

  • Measurement of the kathode and grid current
  • Anode lecher lines changed to single tubes and seperate (doorknob) anode capacitors
  • Added is a novel controlcircuit for fault monitoring and TX-RX sequence
    • Cathode current
    • Grid current
    • Anode supply voltage
    • Heater supply votage
    • Blower air temperature
    • Blower air flow

Mechanical design

As pointed out before the mechanical design is more or less equal to the original dl4mea design

Anode lecher transmission lines

gs35b_10_300_400Both lecher transmissions lines (at both sides of the pedistal) are constructed as shown. 25mm diameter copper tubing was used.

Via two 100pF/3kV parallel connected doorknob capacitors the transmission lines are connected to the anode plate.

The transmission line lechers are firmly grounded by copper clamps. Provision should be made that those clamps can be adjusted along the transmission line for course anode resonance adjustment


Air flow measurement

gs35b_2_400_300On top of the cavity a "butchered" air inlet assembly normally used for cooker hoods is fitted. All flappers apart from one were removed. Together with an optocoupler circuit this is used to signal airflow, or better: loss of airflow

When the blower is in operation and the airflow is not blocked, the air will flow first through the grid/kathode compartment via the lecher transmission lines through the anodecooler and will "flip" the flapper in a more or less upright position, clearing the optocoupler. Upon loss of airflow the flapper will go to his horizontal position and the optocoupler is set. This type of cooker hood is available in a number of diameters and will fit nicely on the gs35 anode


Electrical design

Based on the original dl4mea design the electrical design was enhanced by an on board RX/TX sequencer and fault monitoring

Cathode and Grid circuit

For accurate monitoring the cathode and grid currents and signalling towards the main central processing unit (CPU) some OPAMPS are used. Grid current measurement: Opamp U2A measures the floating differential voltage accros R4 and provides a single ended voltage to U2B, acting as a comparator. Upon reaching the threshold the output of U2B will go high and signal an overcurrent situation to the CPU unit.

In the same way the grid current is monitored by U2C and an overcurrent situation signalled to the CPU unit.

Remaining circuitry is as per DL4MEA's design and is straightforward.


CPU unit witn TX/RX sequencer and fault monitoring

schematic gs35b cpu

Functionality of the CPU unit

CPU unit is capable of:

  • Providing enable signals for (in the right sequence with accurate delay times)
    • Highvoltage powersupply
    • PTT RF driver
    • Coax relais
    • Switch on Quescient current of GS35
    • Blower mode slow (rx) or full (tx)
    • CPU watchdog safety
  • Act upon inputs from
    • Main PTT
    • Amplifier enable/disable
    • heater voltage available/unavailable
    • Anode High Voltage supply available/unavailable
    • Airflow present
    • Temperate of exchaust too high
    • Grid current exceeds limit set
    • Cathode current exceeds limit set

Under normal operations the CPU will enable all the outputs in the correct sequence with a fixed delay time (set in firmware) derived from the main x-tal oscillator clock.

When an error situation occurs, e.g. grid current overflow or loss of airflow the amplifier will go into standby mode, taken into account again the correct sequenc of de-activating all the outputs. To indicate what the cause was of the error condition the error led will output in CW the error code (UA, UFIL, AIRFLOW, AIRTEMP,IA, IG).

All inputs can be manual enabled/disabled by placing the appropiate jumpers.

Of course a warmup timer is installed, with a special sequence to override this (most of the time too long...) warmup delay.

CPU used is a single chip microprocessor with on chip RAM and EEPROM. Software development was done with "Keill" and the application is written in C. To get an idea an idea of the application the source code is here and is mainly a statemachine keeping track of the previous and present state and based upon inputs received decides what the next state should be.

Operating parameters


  • Anode voltage: 3200V at 0mA, 3000V at 1A
  • Trip setting for grid current: 500mA
  • Trip setting for cathode current: 1.5A
  • Cathode voltage: 100V during receive, tbd for 100mA idle current


  • Power output: 1 KW
  • Anode current: 500 mA
  • Anode voltage: 3000 V
  • Input power 40 W
  • Grid current: 200 mA

Copyright © 2011 Carel Mobach, PC5M

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