[Amps] Measuring filament voltage

[John Lyles]

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

On 6/5/22 10:00 AM, amps-request at contesting.com wrote:
>     1. Re: Why does power output increase? (Steve)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 5 Jun 2022 16:41:35 +0100
> From: Steve <g8gsq72 at gmail.com>
> To: amps at contesting.com
> 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!
>>
>>
>> 73,
>> Victor, 4X6GP
>> Rehovot, Israel
>> CWops #5
>> Formerly K2VCO
>> https://www.qsl.net/k2vco/