Another problem people commonly miss is voltage drop in the
filament choke.
A normal properly sized filament choke has a few tenths of a
volt AC drop across the windings from filament current.
This is meaningless in a center tapped filament transformer
feeding a directly heated tube, but when one side of a
heater is connected to the cathode you either need a
hum-compensating pot at the DC return for the cathode, you
need a third winding only for dc that ties to the filament
pin common th the cathode (a trifilar choke), or an entirely
separate dc return choke that does NOT carry any filament
current.
It's surprising how little ripple voltage in bias at the
cathode can cause hum on the signal.
By the way, I see quite a few Internet pages on constructing
amps with Russian tubes that ignore this problem. It's also
a problem with the 3CX5000, 3CPX5000 series of
heater/cathode tubes that have the cathode tied to one side
of the heater. You can't run the cathode DC back through the
same choke winding as the heter uses without inducing some
unwanted hum. The only exception is when the transformer end
has a hum-balancing pot.
I know a fellow in Mississippi who worked on his 3CPX5000
for weeks and never figured that out. His solution was to
run a filtered DC supply!
73 Tom
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