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[Amps] Measuring filament voltage

Subject: [Amps] Measuring filament voltage
From: John Lyles <>
Date: Sun, 5 Jun 2022 11:19:21 -0600
List-post: <>
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.




On 6/5/22 10:00 AM, wrote:
    1. Re: Why does power output increase? (Steve)


Message: 1
Date: Sun, 5 Jun 2022 16:41:35 +0100
From: Steve <>
Subject: Re: [Amps] Why does power output increase?

Tack a 10k resistor to each filament pin, 10nF decoupling between the
loose ends then twisted pair out to your DVM. Should filter out the rf
and you don't have to worry about shorting the wires together.

Steve G8GSQ

I did measure the voltage at the tube pins. I haven't measured it while 
transmitting, because the RF drives my meter crazy. I assume the voltage drops 
a bit. Unfortunately I no longer have my trusty analog voltmeter. I wish I did.

I'll measure the line voltage. But the only thing I can think of that would 
cause that to increase after long transmissions would be if the breaker on the 
line heated up and lost resistance. I have never heard of that!

Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
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