> > The problem is the hum it adds, as it changes bias.
>
> I though you liked AC on the filaments and saw problems with
> DC? RF amps with AC filaments don't seem to have hum problems from
> AC filaments. What did I miss, here?
I think you missed the fact the bias was applied to one end of the
filament, instead of the middle of the filament. I was responding to
the Hi Fi amp with the directly heated filaments, and the tubes
being moved from socket to socket periodically.
Isn't that where the tube-life issue came in?
> > I'd worry about the bias being uneven from end to end of the
> > filament. That can't be good, if the filament voltage is a large part of
> > the total bias voltage.
>
> I tried to make the point that with 60 Hz AC, the filament voltage
> varies from 1.4 time RMS one way to 1.4 times RMS the other way
> and it varies so slowly compared to RF or modulation that effectively
> appears as a constant ... like DC but with even more variation in filament
> to grid voltage. If the bias effect existed, then DC should improve it,
> not make it worse
>
> Some Hi Fi amps run DC on the heaters for the low amplitude portions
> of the chain. But I thought that prevented capacitive coupling between
> filament and cathode from introducing hum.
There is no separate cathode in the Hi Fi amp tubes that were
discussed. Or did I miss something?
I would think running the filaments on AC far above the frequency
limit of hearing would be better. The Hi Fi snake oil salesmen could
make a mint selling supplies like that along with oxygen free
copper. Or haven't you seen some of the $5000 2A3 amplifiers?
73, Tom W8JI
w8ji@contesting.com
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