Steve Thompson wrote:
<snip>
> Yes, I had thought the same - 50/60Hz sidebands low down don't hurt
> anyone else, 30kHz away would. Any tiny imbalance in the heater feed
> would do that.
>
> Just to be clear, given variations in terminology, my primary question
> is about use with indirectly heated cathodes - for example 8877, 4CX250 etc.
>
> Steve
> _______________________________________________
Steve,
We have experienced problems with indirectly heated cathodes which are
INTERNALLY connected to the filament like the GS-35B and the GI-7B such
that the output was modulated with hum from the filament supply. This
problem can be easily eliminated by using a separate cathode choke and
being very careful to connect it to the ring which has the filament and
cathode common (larger of the two in these cases). Never use the common
center tapped filament transformer to feed the cathode of one of these
tubes. Although this has been done in amps with a pair of tubes, the
filaments (and cathodes of course) were criss crossed so the modulation
canceled itself in the output. With proper connections and the use of
the separate cathode choke all traces of the modulation can be
eliminated. There should be no problem using any LOW frequency filament
power you like, even DC. In fact, I prefer regulated DC on these
filaments which is easily obtained using a small switching supply
(select one that isn't creating lots of RFI) or your favorite analog
regulator. It is still good practice to place a .01 uF capacitor across
the filament/cathode pins at the tube to insure there is no RF unbalance
on the filament.
73, Tony W4ZT
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