[Amps] Measuring screen current in a tetrode amplifier.

[donroden at hiwaay.net]

The positive offset allows you to monitor for negative screen current.

( not that a little negative current is a bad thing )

The resistances to ground also gives the screen more stability.

Don W4DNR

On 2024-11-13 5:47 am, 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
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