[Amps] Measuring screen current in a tetrode amplifier.

[Victor Rosenthal]

Google "Wheatstone bridge." It requires three resistors and a pot. You 
can adjust the pot to balance the circuit so the meter reads zero at any 
point.

73,
Victor, 4X6GP
Rehovot, Israel
formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/

On 13/11/2024 13:47, flynth at gmail.com wrote:
> I've built an amp roughly based on the Pa0fri design (with some changes
> that include better stabilisator for the screen voltage - thanks to this
> group, 3000V - under load, HV PSU from an old R-140 military trx) based on
> Gu-43b. I'm pretty pleased the amp has had its first QSO yesterday and both
> the reports as well as SDR pictures and Imd measurements are good.
> 
> However, I'm having an issue measuring the screen current. Specifically as
> part of the design there are many resistors (in addition to MOVs and
> capacitors) soldered between the screen and the ground in the tube socket
> itself. They all add up to about  25k ohms. So there is 14 miliAmps of
> current flowing through them.
> 
> As such my screen current meter is always offsett 14mA to the right. It is
> a bidirectional +-30mA meter. It is not possible to correct half of the
> swing.
> 
> So I wonder, how is it that commercial, and other amps manage to measure
> their screen currents without this offset? I believe the resistors are
> necessary to provide extra protection in case the tube arcs over.
> 
> I'm trying to keep the design as simple as possible, but if someone can
> suggest some circuit I could use to get rid of that offset that would be
> great.
> 
> Regards,
> F
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps