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@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
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