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D70 Output Tubes Drifting WHILE Biasing

Posted by Jonesy (A) on March 22, 2014 at 16:17:10

My bias on the 6550C tubes used to be easy to set. Never drifting much upon check ups during the year, always near the 65 mark.

Last time I checked, I was not able to even get a steady reading. It was bouncing anywhere from 64.3 to 65.7 without even trimming the pot. All 4 pots.

I know that's not much, but it used to just stay solid unless I adjusted the trimmer.

Here's what I've done so far...

Sockets were getting pretty worn, (twice over the years, cold solder joints raised their ugly head.) So I replaced all the original octal sockets with Beltons. Nice and snug but no change to biasing problem.

Replaced all octal tubes with spare set. No change.

Checked ALL tubes on tester. Test strong.

Sprayed biasing pots. I don't know if anything actually gets in there, but I've done this before, and they definitely turn smoother. No change though.

I browsed through the Asylum, saw that the 12ax7 tube has some impact on the power supply. Changed that out and saw a big improvement, but still a swing in all the tube read outs while just keeping the DVM on the test points.

Oh, I also touched up all the socket joints again just to be sure.

The amp sounds great. That's comforting.

But can I run it like this?

Is there something else I can check that may be causing this problem?

Jonesy




"I know just enough to get into trouble. But not enough to get out of it."

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