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Do you mean the 6SA7 or 6SC7? The 6SC7 is the dual triode that acts
as the first audio and phase splitter. Pulling that leaves the 6V6's
active. CH-2, the bass boost resonator, connects from the grid of V-14.
one of the output tubes, and one plate of the 6SC7, the side that's used
for the audio pre-amp. Just lift one side of the choke. I don't think
its the choke. See what the Bass switch is actually doing. According to
the diagram its shorting the choke when in the IN position, Since it is
resonating at the grid I think it should be the other way. In any case
disconnecting it should prove one way or the other. If it turns out its
boosting the bass in the OUT position it would explain why the hum gets
greater, simply because the gain goes way up, but not the source of the
hum. Also, does the volume control have any effect on the hum? If so
what effect? Also note the phase splitter grid comes from the junction
of the two 6V6 grid resistors at R-39. At this point it is getting the
unbalanced audio from the output of the two halves of the 6SC7 and
generates a balancing signal. This is the out of phase signal that dries
the other 6V6. It is also what puts the effect of the tone control and
bass boost on both sides. I have forgotten the name of this type of
phase splitter but its very common. Also, look at C-44 for ripple. There
should be very little. If the boost circuit works as I think any ripple
or even a heater to cathode leak in the 6SC7 will be magnified by the
resonant boost in the grid of the first audio and in the phase splitter.
A couple of minutes with the scope should tell you. BTW, did you
change the 6SC7, if its got a leaky heater it could be the cause of the
hum. More poking is needed.

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998