Negative Plate Voltage on one 6v6?

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yello
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Negative Plate Voltage on one 6v6?

Post by yello »

I'm having a strange amp issue (Filmosound 385), asking the 18w mindtrust for insight:

It's a dual 6V6 push pull amp I am working on, and I am getting an expected ~333vdc on one 6v6 plate, and -5vdc on the other plate. I've personally never experienced this. All other voltages in the amp seem to be within spec.

Here is the schematic:

http://www.acofs.org.au/part_5_files/Be ... iagram.PDF

Amp is fully recapped (it was needed) and serviced. I updated the input to typical guitar style, and added 470 ohm screen resistors on the power tubes. 6v6 tubes swapped, problem stayed with the socket. Tried another pair of 6v6 to be sure, same. Swapped the 12ax7's, no change.

I then changed out the power tube coupling caps, grid resistors, and added grid stoppers - was going to do that anyway as the multi-cap/resistor part in that section of the circuit was measuring wrong and leaking DC, and the grid stoppers hep with oscillation when volume and tone are cranked. Did not affect the negative voltage issue.

The amp passes signals, and sounds mostly good, but with a grungy/buzzy decay on the notes, especially low notes.

Any thoughts on what I can test next? Or what the issue might be, if someone else has experienced this negative plate voltage issue?

I will put together a voltage chart this weekend, and can supply pictures also - for now I'm hoping for some insight that can help guide me if anyone else has had a similar experience with negative power tube plate voltage.
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Daviedawg
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Re: Negative Plate Voltage on one 6v6?

Post by Daviedawg »

Off the top of my head, look closely at the affected valve base for shorts by whiskers or failure of the base. Clean the pins and sockets. Presuming that it is the original base.
Move the lead around to see if it was a break in it.

Dd
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yello
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Re: Negative Plate Voltage on one 6v6?

Post by yello »

Further exploration revealed that the OT had one side of the primary open.
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yello
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Re: Negative Plate Voltage on one 6v6?

Post by yello »

I'm glad I was able to sort the problem out with enough thinking and testing, given what I continue to pick up from the 18w crowd, and others. Sending the OT out today for a repair!
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