----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <amps@contesting.com>; "William Fuqua" <Wlfuqu00@pop.uky.edu>; "jeff
millar" <jeff@wa1hco.mv.com>
Sent: Wednesday, November 01, 2000 9:03 PM
Subject: Re: [AMPS] DC Filaments???
> 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.
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?
> > > 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.
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.
jeff
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