UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

ahjohn002 · 21st Feb 2010, 5:08 pm

Mike and Les,
Thanks for your suggestions. The regulator is working properly now after replacing the IRF730 (it's brand new) with an IRF820. After setting the anode voltage at 250 vdc, a 12AU7 is inserted and the voltage only dropped to 247 vdc, with both sections of the dual triode connected for test. The reading remained stable even with variations on -ve grid voltage.
One observation though. There was a 1 vac present, noise and hum presumably, on the anode after a tube is inserted with grid input coupling capacitor grounded. I just wonder would it affect the gm measurement because a 1 khz sine wave should be injected at the grid which would be amplified at the anode. The amplified signal together with the noise signal would affect the gm measurement. Wouldn't it?
Regards,
John

21st Feb 2010, 5:08 pm	#182
ahjohn002 Retired Dormant Member Join Date: Feb 2010 Location: Hong Kong, China. Posts: 5	Re: The "Sussex" Homebrew Valve Tester. Mike and Les, Thanks for your suggestions. The regulator is working properly now after replacing the IRF730 (it's brand new) with an IRF820. After setting the anode voltage at 250 vdc, a 12AU7 is inserted and the voltage only dropped to 247 vdc, with both sections of the dual triode connected for test. The reading remained stable even with variations on -ve grid voltage. One observation though. There was a 1 vac present, noise and hum presumably, on the anode after a tube is inserted with grid input coupling capacitor grounded. I just wonder would it affect the gm measurement because a 1 khz sine wave should be injected at the grid which would be amplified at the anode. The amplified signal together with the noise signal would affect the gm measurement. Wouldn't it? Regards, John