I've seen a variety of values for GS-35 input impedance over the range
25 to 100 Ohms. It varies between tubes because the grid bias varies
with these russian tubes. I tend to believe the input impedance runs
towards the low end of that range. It runs lower than 8877 and the like
due to the high grid current. Cathode drive impedance is proportional
to Eg / (Ip + Ig).
Definitely halve the impedance for two tubes in parallel. It might well
end up 12-25 Ohms. Running high plate voltage will increase bias and
increase input impedance.
jeff, wa1hco
Tomm Aldridge wrote:
I am now at the stage of designing my tuned input for the dual GS35b
amp that is in process here. There seems to be a general concensus
that the tube presents a 50 Ohm impedance when cathode driven in GG
configuration. Can I assume a 25 Ohm input impedance as a good
starting point for my input filter design?
Regards
KD7QAE
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