Fluctuating B+ and low filament voltage problem

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12/24/1999 5:30 AM

Bill Watson

Fluctuating B+ and low filament voltage problem Greetings,     I'm building a Dumble clone and have the power supply, bias, filament, and foot/panel switch hooked up. My voltage at the first cap is jumping from 466 to 478 to 482 to 492. It will stay at any of the voltages for minutes at a time. Also, my filament voltage is reading about 5.7. The transformer is from an old Earth bass amp that ran 3 12Ax7s and 4 6L6s wich is exactly what I have for the new amp. The bias voltage also seems to vary proportionatly along with the B+. I've unhooked everything and hooked up each winding seperately with about the same results. Tryed the 1 ohm resistor trick in series with the power cord and the drop reads about .18 volts which is supposed to be good. I also have a seperate 12V transformer for the relay, so that shouldn't come into play.     I have another trans out of an old Hammond organ that ran a kazillion dual triodes and 2 6L6s that I might try. Anyone have any suggestions? Should I trash the trans? Do you think the organ transformer will run 4 6L6's biased to AB.     By the way I'm using Steve Ahola's schematic for the the Dumble power supply.     I would appreciate any response.

 

12/24/1999 5:05 PM
Daver	Is the AC from the wall also fluctuating at the same time?   Daver

 

12/24/1999 10:07 PM

Bill Watson

Thanks for your response Daver.     I plugged my meter into the AC outlet and, yes, it fluctuated between 116 and 121 volts. If the primary and secondary trans voltages are proportional, that would make the B+ vary about the amount I noted. Am I correct in thinking they are proportional? If so, I'll have to find the source of the problem in my house wiring.     Also, to everyone, sorry for the loose use of the "D" word. This is a hobby for me and I don't get to talk to other real amp people often.

 

12/25/1999 3:10 AM
Daver	Bill,   If you can get ahold of two meters, you can watch the AC coming in and your B+ at the same time. If they both change in the same direction at the same time that's probably the "problem". Other things on the same line that constantly cycle like heaters and refridgerators can cause the line voltage to fluctuate.   Daver

 

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