for G1 of GU78b
Voltage regulator for G1 of triode PA
This voltage regulator has been developed for a 2m PA with a GU78b
tube. It is a parallel regulator similar as the one for G2. It consists of
the regulator and protection circuits. The regulator thank to low
transistor gain (about 100) has no tendence to oscilations. The protection
circuit limits the cathod current in case of discharge in the tube.
Without this the discharge current can even (depending on the capacity in
the anode power supply) burn the printed circuit with the regulator and
thus completely destroy it..
The resistor 5k/10W is to set so there's current between 10-20mA through
the regulator. I hope everybody who can build a PA can also calculate this
Just for your information. The voltage will be 0.6V higher (the PN
junction voltage) then on the Zener diodes. :-). 1.6W diodes are OK.
The voltage for a GU78b tube is about 96V and every volt means the cathode
current change about 40-80 miliampers. Therefore the regulator is
permanently under the current for better voltage stability.
The tube the maximal G1 voltage should be observed while blocking it for
RX so the tube does not get damaged.
The blocking capacitors should be at least 300V types and their dielectric
should work on 144 MHz, which is not always the case!
Note OK1VPZ (2016): check as well as these articles: