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Old 20th Feb 2010, 4:51 pm   #174
Join Date: Feb 2010
Location: Hong Kong, China.
Posts: 5
Default Re: The "Sussex" Homebrew Valve Tester.

Hi, Mike,
Thanks very much for your quick reply. The reason for my questions is that I have built a prototype of the anode voltage regulator (only the part from D3 to TR1, TR2, TR3, and output via R5, R6, and R7) on a breadboard with all the components (except I replaced the voltage selector switch with a 1mega ohm pot and the mosfet is a IRF730) as stated on the schematic, but it doesn't work properly as expected. May be I should report some values I have measured:
Power transformer secondary: 250 vac
After a half wave rectifier: 324 vdc
Output from the anode regulator: 250 vdc, can be adjusted to all the way to 0 v. Voltage remained stable even if left on for over an hour.

I then tested the regulator with a 12AU7, and later with a 6922. The cathode (pin 3 and 8 were connected to ground). The moment the tube was inserted the voltage dropped to 90 vdc at the anode, grid negative voltage was set at -6.5vdc. Anode voltage could still be adjusted but only downward by the 1 megaohm pot. Adjusting the grid voltage could changed the anode voltage but the max value was still 90 vdc. It seemed the regulator just woundn't hold the voltage at the preset point. I also noticed that the mosfet didn't get hot at all. Should it get hot when working properly?

Any idea on what went wrong?

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