[Amps] Measuring filament voltage

[Fuqua, William L.]

That reminds me, maybe 30 years ago I bought a 3-500z GG amplifier from a guy that said he could not get it to
work well. I took it home and found that considerable voltage drop across the filament choke.
I added another smaller filament choke secondary in series with the primary of the 3-500z transformer as a
boost transformer and it worked just fine.

________________________________
From: Amps <amps-bounces at contesting.com> on behalf of Steve <g8gsq72 at gmail.com>
Sent: Sunday, June 5, 2022 2:29 PM
To: amps at contesting.com <amps at contesting.com>
Subject: Re: [Amps] Measuring filament voltage

CAUTION: External Sender


Good additional and practical points from John. Higher value resistors
might suit in some situations - don't forget to allow for the error
introduced by the 'pot down' of the resistors against the DVM input
resistance. With 2 x 10k and 10Mohms it's 0.2%.

Steve G8GSQ

> Steve's approach is an acceptable method to read the filament on a common grid HF amplifier. The 10 K resistors would optimally be carbon film or comp and not wirewound, so that they are non inductive and have lowest capacitance from end to end. There will be RF voltage across it!  I would probably put a 10 nF cap to ground on each cold lead to the meter as well, keeping all of that close to the resistors in an enclosure in the amplifier. The capacitors should be mica with short leads or quality ceramic. The resistors should be bigger than 1/2 watt just for the RF voltage. Not for dissipation. Just remember to test it with and without RF drive applied to see if there is RF affecting the reading or not. An analog rms meter will likely ignore it.
>
>
> 73
>
> John
>
> K5PRO

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