I had a query about my finding regarding the higher than expected input
impedance of the pair of GI7B tubes in an amp I have been reworking. I
did some reading and found a post from Frank, W3LPL several years ago
regarding the same thing. He discovered that if the input tuning
networks are not located very close to the base of the tube the added
capacitance of the connecting coax is enough to radically alter the
input SWR, resulting in loss of efficiency unless the networks are re-tuned.
That is the case in this GI7B amplifier-the tuned input is fed through
approximately 10" of coax to the tubes, and it appears that this is the
source of all the problems with this particular unit. The elevated
input impedance of 150 ohms rather than the expected 100 ohms for two
tubes appears to be explained by a flaw in amplifier design.
73, Bill VE3NH
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