Re: The "Sussex" Homebrew Valve Tester.
HI Eddie - Les (Top Cap) has kindly produced an excellent document summarising the Sussex design. This can be found at post #502 which should appear on page 26.
He says this about the GM test.
Start quote:
"Gm is a measurement of gain and is in fact the change in Anode current (mA) from a change in Grid voltage (V). In the Sussex we apply 0.1V to the grid of the valve under test and measure the change in Anode current across a 10 Ohm resistor R12. Let us assume a valve under test has a gain of 25, if we were using 1V as a grid voltage we would expect a change in Anode current of 25mA and therefore there would be 250mV developed across R12. But as we are using 0.1V signal then the voltage developed
will be a tenth of this i.e. 25.0mV. This figure is what will be displayed on the front panel Gm meter.
Due to failures encountered with this meter under fault conditions, for example if the Anode supply FET goes short circuit, a protection diode placed across R12 is being trialled.
The diode has a forward turn on voltage of 600mV approximately so should not affect the 100mV max expected in normal use.
Whether a simple diode on its own will be sufficient has yet to be proven!"
End quote.
If you cannot find the Les's document I can email it to you if you send me a personal message.
Regards - Alan G3WXI
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