Model T AC humm ,, bias.

[MarkG]

I am trying to rehab a 1st Generation Model T. Actually works quite well as is, but has a very slight 60hz humm way down in the background that comes on when the standby is switched to operate. It does not vary at all with any volume or tone control. The amp is pretty quiet otherwise,so I had assumed the source of the humm was old power supply filter capcitors. I have replaced them, that is: the 2  20ufd  C17+C18,as well as the duel can 80ufd C19+C20. (although I couldnt get an exact match,went to 100ufd. )   Replacing these didnt even put a dent in the humm. Still there. Looking at the schematic I now see a C15+16 that could also be a problem???   Any comments from the ModT gurus on if these or any others that should go next?? I believe I should be able to eliminate this humm,as my older 200S is perfectly quiet. Also regarding bias..is -55v correct for any brand 6550 in this amp??

[Anonymous]

C15 and C16 are the bias supply filters.  The bias supply is a 1/2 wave rectifier, and that makes it the only real source of any 60hz hum (full wave rectifiers, like on the B+ supply could cause 120hz hum).

I would replace them... after all, they are electrolytic and probably just as old as the others.

One point you want to keep in mind.  60hz hum that is introduced by the bias supply should cancel out in the power amp, since an equal phase signal is being applied to both the push-pull sides at the same time.  Any hum that doesn't cancel out, is usually due to some imbalance in the power amp - like mismatched tubes normally, and in extremely rare cases a mismatched output transformer.

As for bias... yep, I always run mine at -55 volts.