Bill magoo at isp.ca
Thu Dec 19 07:30:34 EST 2019

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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