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Re: [Amps] Measuring filament voltage

To: Steve <>, Amplifiers <>
Subject: Re: [Amps] Measuring filament voltage
From: "Fuqua, William L." <>
Date: Sun, 5 Jun 2022 19:51:53 +0000
List-post: <>
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 <> on behalf of Steve <>
Sent: Sunday, June 5, 2022 2:29 PM
To: <>
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

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