Hi Jeff,
> > The only problem is with directly heated filament-cathodes and low
> > bias voltage tubes, the voltage drop across the filament can be a
> > significant portion of the bias voltage. That would mean the filament is
> > not supplying equal current over the entire area of the filament.
>
> Even with AC on the filament, the voltage between grid and filament
> varies along the length of the filament at any instant in time. The
> filament voltage varies very slowly compared to both the RF
> carrier current pulses and most of the modulation envelope.
The problem is the hum it adds, as it changes bias.
> > That shouldn't hurt life, but it certainly could affect IMD and other
> > parameters.
>
> I can't see how DC could effect IMD. DC creates less variation
> in filament to grid voltage than AC because it doesn't have the
> peaks over RMS that exist with AC.
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.
As for distortion, I was thinking in terms of the Hi-Fi amplifier the
one person had. I'd try to do something to match the bias more
evenly across the filament, to not ruin the good sound that goes
along with all that cotton covered oxygen-free wire and those old
clear sounding triodes.
I think the hi-fi buffs would be better off running the filaments on 200
kHz AC, and I think I have a supply that will run the filaments and
get rid of that distortion that is worth...oh say...$15,000.
73, Tom W8JI
w8ji@contesting.com
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