Tetrode Linear Amp

Tetrode linear amplifier boards

by F1FRV

* Rev 0: December 2005 *

* Rev 1: May 2007 *

* Rev 2: May 2010: Added low ripple G2 supply *

* Rev 3: november 2010: Global update, except heater regulated supply *

* Rev 3a: december 2010: Added IP & HV meters on HV/G2 sequencer board *

* sept 2011: improved logic board & alternative blower tempo *

* oct 2012: improved G2 board regulation with 1 resistor *

The new logic board can now be used, either for tetrode, or for triode amplifiers.

This is only for those who dont want to built a wiring mess, without schematics, and without enough safety for them and equipment, but accept to pay what is minimum necessary (only a few among hams).

This versatile design is only for G1, G2, heater with soft start, PTT sequencer, safeties logic chain, and sequencer for application of G2 voltage AFTER and WITH high voltage (HV MUST be present). RF circuitry, input and output matching networks are not described here, as a lot of descriptions are available on the web (or elsewhere). These boards can be used with all tetrodes, from the smallest (not very interresting) to the more powerful (more interresting).

If you think that safety is excessive for you in this design, remove all safeties you want, at your own risks ...

All printed circuit boards are single side only, with a few straps, and large pads and traces, for easy realisation by the average amateur who knows that ferric chloride is not an ingredient for exotic coktails.

Printed circuit boards used in this design:

1 - Logic board, with mains circuitry, start-up timers, blower fuses and shut-off timer, safety chain, PTT logic timer, tube soft start heater. Revision 2 with low cost and easy to find relays.

2 - Grid 1 shunt regulated supply (separate voltages for grid blocking and grid adjustable bias), antenna and amplifier by-pass relays DC supply, and high voltage detection.

3 - Grid 2 shunt regulated supply, with overcurrent trip and heater voltage detection. Tested voltage stability is better than 0.1 V at 350 V between 0 and +/- 100 mA.

4 - Sequencer for application of HV and G2 voltages, in the right and safe way for the tube, when all safety devices are OK. Rev 2 with lower cost relays. Rev 3a, provision for IP & HV meters.

5 - Optional, but useful front panel LEDS and switches PCB and front plate printable label.

A special transformer and choke inductors for G1, G2, antenna relays and other necessary voltages has been designed. This avoid a bunch of transformers. It can be ordered directly to the French listed manufacturer. Some EXCEL and PDF files are included in the downloadable files, to help you in your own design.

Some simulations of very good, good, bad and very bad grid supplies are available in this page. Dont laugh, some people, in our 21th century, still never heard about negative G2 current.

NOTA: As 24/27V DC supply is with negative at ground, be careful if using these boards with R-140 amplifiers.

For information only, see a typical wideband 1.8 - 30 Mhz input circuit.

To download data and pictures: Entree Tetrode.zip

Logic Board

SIMPLE delayed blower stop, timer assembly (thermal switch & heater).

Delayed blower stop, ALTERNATIVE electronic timer Download

PTT logic sequencer & start up timers

Adapt timing resistors and capacitors to your needs. An EXCEL calculation sheet is in the downloads, to help you. Calculated timings are not extremely accurate, as FETs gates voltages for conduction are depending of a lot of parametres, and capacitors leakage current is not taken into account. Do not use resistors higher than ~ 2 Megohms, to limit influence of FET gate and capacitor impedances.

Heater soft start: Calculate " Rx " resistor according to Ohm's law (if you heard about it...), for nominal current of heater under short circuit. As example, for heater 27 V @ ~ 4 Amps (GU-78b), resistor shall be ~ 6.8 Ohms. This limits inrush heater current to ~ 4 Amps, (instead of more than 50 Amps), and gives ~ half nominal heater voltage and current to the tube after a few seconds. Power of " Rx", for less than 10 seconds of service, can be reducced to ~ 1/2 of what is calculated (in this case, 25 W is OK)

For heater current higher than 10 amps, RL1 shall drive an external power relay.

For tubes like TH293 which needs extended time (>1 minute) at half heater voltage, timing capacitor C2 can be modified for longer time delay. In this case, " Rx" shall accept full heater half power without burning ...