From my work on my 10-5 project amp, I noticed one idiosycrasy about the 6CG7 tube.
These tubes DO NOT like lower-than-spec heater voltage!
Bringing the amp up on the variac, I had WILD variation in transconductance (as shown by differences in bias current with the same B+ and cathode-bias resistors on both sides) between the two sides, under low voltage.
HOWEVER- once the voltage was set at normal (with, naturally, normal 6.3v heater voltage), both sides were almost EXACT match, as the tube showed in the tube tester.
I checked several 6CG7/6FQ7 tubes (RCA, GE, etc)... and they all exhibited the same behavior!
So... if you ever have a balance problem between the two halves of a 6CG7/6FQ7 tube, be sure and check the heater voltage at the socket where it's installed!
These tubes DO NOT like lower-than-spec heater voltage!
Bringing the amp up on the variac, I had WILD variation in transconductance (as shown by differences in bias current with the same B+ and cathode-bias resistors on both sides) between the two sides, under low voltage.
HOWEVER- once the voltage was set at normal (with, naturally, normal 6.3v heater voltage), both sides were almost EXACT match, as the tube showed in the tube tester.
I checked several 6CG7/6FQ7 tubes (RCA, GE, etc)... and they all exhibited the same behavior!
So... if you ever have a balance problem between the two halves of a 6CG7/6FQ7 tube, be sure and check the heater voltage at the socket where it's installed!
