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[AMPS] Metering a GG amp

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Subject: [AMPS] Metering a GG amp
From: (Ian White, G3SEK)
Date: Mon, 1 Feb 1999 20:14:22 +0000
Arthur Moe wrote:
>I have some very vice 1ma meters that I would like to use in my
>amp project. Question is, which method to use, wind shunts or use
>them as volt meters and measure the voltage drop across a 1 ohm
>resistor? Meters to be used to measure both Grid and Plate current.
Shunts are better when everything is working normally, because they
produce the minimum voltage drop,
in the event of current surge (due to an arc or whatever) it's more
difficult to protect shunted meters effectively using diodes.

For the best protection, use the meter to measure the voltage drop
across a resistor. If the resistor is adjusted to give 0.5V full-scale,
and it is shunted by silicon diodes that are big enough to handle the
surge, the meter can never see much more than about twice its normal
full-scale current.

0.5V is a kind of "magic number", quite close to the threshold voltage
of silicon diodes, but not close enough to affect the accuracy of the
meter readings. 

If you also want to operate a grid-current trip using the metering
resistor, 0.5V will not be enough because it takes 0.6-0.7V to turn a
transistor on. That will need a higher-value resistor and two protection
diodes in series. 

A minor problem when metering triodes is that the voltage drop across
the grid-current meter appears as unwanted extra grid bias, so the
resistor should not be any higher value than is needed to do the job.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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