[AMRadio] 813 filament voltage

Jim candela jcandela at prodigy.net
Sun Nov 5 15:54:38 EST 2006

Steve WD8DAS said:

I'm resurrecting a very nicely-built homebrew grounded-grid 813 amp.  I
noticed that the filament voltage at the pins is 8.5 vac.  Nominal on the
813 is 10
vac as I recall.  Is the 8.5 OK or will emission suffer?   I guess I could
also be into an RMS versus average metering issue... hmmm.

Reply from Jim WD5JKO,

Steve, the accuracy of your voltmeter is always a question in cases like
this. The load is resistive, so if the filament transformer is putting out a
decent sine wave the True-RMS versus peak responding and RMS calibrated
meter should give the same answer.

More than likely you have some resistance between the transformer and the
tube pins. There is likely a filament choke in there since this is a cathode
driven amplifier (grounded grid). Look at all the connections, see if any
get hot, or maybe measure across each connection looking for a voltage drop.
What does the transformer secondary have across it? If the transformer is
behaving, and the 1.5 vac drop is from the choke, then you might consider a
12 volt transformer with a resistor in series with the primary such that you
have 10 volts AC at the 813 filament pins (not the socket). The series
resistor will cushion the turn on current surge a bit, and that is a good

I was just going through my transformer junk pile, and I have a nice Stancor
10v CT @ 10 amps; gathering dust.

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