Let’s resume the case?:

Tom pulled the 6SC7 and the “hum” remained.

He shorted the two 6V6 grids to GND and the hum disappeared.

No hum when the “bass” switch is in the IN position (both plate loads of the 6SC7 equal and CH2 + C43 shorted).

Hum present when the “bass” sw in OUT position.

The “hum” is at 60Hz…

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Which suspects remains ??

Does the C44 is “grounded” at the same place than the filaments of the 6V6s and this connection is resistive ??

Clue: when the driver stage is “balanced” plates loads wise, no hum.

When it is not: hum !

Note here that the 4H. inductor reactance at 60Hz is only 1500 ohms… and that the C43 is practically an open circuit at 60 Hz.

Could it be that the 60Hz injection point is the C44 grounding lug ?

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My two SX-28As are too far to be quickly reached.

But If I manage to dig the “never touched” one out of the storage during the next weekend, I can check how it is wired.

Keep thinking….

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And about the meaning of the “Bass” switch: when it is IN, the bass is ENABLED (literally, “bass” is in).

When out: bass is out (DISABLED).

CH2 – C43 forms a resonant circuit at ~ 1.1kHz.

The obvious intent is to “tailor” an audio bandpass response for voice frequencies (300Hz – 3kHz)

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Gents, dunno about all that, but why the no hum in the In position? And when base is boosted? ??

don??? va3drl