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Old 11th Oct 2018, 5:56 pm   #1499
Top Cap
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Join Date: Mar 2006
Location: Watford, Hertfordshire, UK.
Posts: 1,270
Default Re: The "Sussex" Homebrew Valve Tester.

I must confess to not keeping up with the changes since I built an early model and it has given faultless operation. The sine wave generator was either a work or no work circuit which seemed to rely on VR1 (200 Ohms) being set to the right point, I added the value that I found to work on mine which was 85 Ohms. Adjusting the 100mV drive, if I may call it that, required a RMS Voltmeter though I did wonder at the time, if it was possible to use the front panel AC mV Meter. Using a switch, one should be able to set the meter to 100mV by VR2 (4k7) then monitor the sense resistor R12 (10 Ohms) as normal, any metering errors will apply to both switch settings and thus give a reasonably accurate gm reading. You would of course have to set the Bias Volts Pot to 0V when checking 'drive' or the AC mV meter may complain. Maybe the switch could be made to do this in the CAL position?

I kept putting off trying this arrangement until the Sussex required service but that has not happened and maybe someone is in a better position to try it. If it works, the CAL pot and switch could be mounted on the front panel close to the sense resistor?
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