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Re: [Amps] Measuring filament voltage

Subject: Re: [Amps] Measuring filament voltage
From: Steve <>
Date: Sun, 5 Jun 2022 19:29:49 +0100
List-post: <>
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.




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