Hello I am just about to start building a 8877 amplifier for 6m.I have
looked at lots of designs for 6m all of which are using the standard Pi
tank.This part is no problem.What I find confusing is the correct bias
voltage and optimum eht volts.Eimac give the max as 4kv I am using all
vac variable so I was thinking of runing about 3800 off load which will
probably drop to 3500 on load.I can run any EHT volts up to about 5kv at
1.5 amps.
Now comes the bias bit.In Europe it is very important to have a clean
signal.The density of amateur population is at times incredible
particularly in IO93 and IO91 in the UK.6m is a very popular here and I
wish to happily co-exist with my fellow amateurs,so I am after optimum
IMD performance.Here are my concerns.
1.Do variations in the bias voltage severly degrade the IMD performance
of a 8877?My experience with tetrodes has taught me the importance of
keeping the screen regulation absolutely rock solid (look at G3SEK's WEB
site for an explanation).How does this translate to Triodes?I have never
run one and therefore don't know.Is a 50 watt zener adequate or should I
use a shunt stabaliser like the OE9PMJ one?This is also variable so you
can set the bais for the optimum standing current.
2.What is the optimum standing curent for AB1 operation on a 8877?Most
designers say use a 12 volt 50 watt zener and say that the idling
current will vary depending on EHT volts available!Surely that can't be
right can it?
3.What is the maximum grid current that should be expected in normal
operation with 3500 volts on the anode.I intend using a grid trip and I
would like to know where to set it?
73 de Conrad G0RUZ
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