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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."
Follow Ups:
Be sure to try a new battery in your meter ;-)
Thanks CB. I tried new batteries and that didn't help.
I also replaced the 12ax7 tube socket.
No change either.
Hopefully some other ideas out there.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
The bias drift you have between the tubes is so minimal that it will have little if no effect.If coupling caps to the output tubes have the slightest amount of leakage,that can cause drift in your bias..I am more inclined to believe that you may be picking up a little RF generating from the signal tubes.
Take out the preamp signal tubes or even the driver tubes and then set the bias after being on for 30 minutes or so..Then recheck it with the tubes still out..If it holds,one of the small tubes could be oscillating and you may need to shield it better.
Honest amplification is better than excessive 2nd order distortion anytime.
I'll give the driver tubes a go.
I don't leave the preamp on, or connected, during biasing, per the manual. I just leave the speaker wire hooked up per info I picked up on the Asylum.
There's a phase-splitter tube as well. I have extras of all tubes so will do some switching around.
Hope it's not the coupling caps. I'm not sure which ones they would be. I'm the kinda guy that just replaces stuff and sometimes I get lucky, and sometimes it gets expensive. (LOL.) Still cheaper than sending it in for service. Just me, my DVM and soldering pencil. Thank goodness I have a decent Hakko. Best thing I ever did.
Thanks for your advice.
Will see how it goes tonight.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Not a problem.When I said the preamp tubes,I was referring to the ones in the amp's gain stages. The coupling caps it would be are the ones that go to the output tubes.
Honest amplification is better than excessive 2nd order distortion anytime.
Hi Mike,
I would like to remove some tubes as you suggested.
So I would leave in the output tubes, the power tube, and the 12ax7 that is part of the power supply.
Take out 2 driver and 1 signal tube per channel.
Take out the phase splitter tube, which I believe is part of driver and signal tube circuit.
Then I would turn the unit on, and check the bias on the output tubes.
Sound right?
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
I swapped out all the driver and phase splitter tubes, rather than leaving them empty. 1 trim pot appears to have tightened up but no change to the other 3. Baffling. I guess I should just take them out altogether. All the rolling and letting the amp warm up each time takes awhile. Going to watch some hockey and back at her tomorrow.
Thanks again.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
I like the hypotheses about the coupling caps and the bias pots. If they turn more easily you definitely got some in there. AND you saw some improvement. Try cleaning again. If that doesn't work try replacing one or more of them.
We had a preamp in once that I cleaned the hell out of the balance pot, and it would play properly for a while, then get noisy. To make a long story short, after about 5 cycles of cleaning/working/noise I replaced the damn pot. That fixed it. Learned my lesson--not everything will clean.
Lee
Thanks Lee.
Last night I was flipping through the Mouser catalogue looking at pots and capacitors, though it looks like I may have to get the "boutique" type capacitors elsewhere.
I will keep testing, cleaning and checking for solder joints that may have gone bad. Wish I had better test equipment than a multi-purpose DVM. Be nice to have one that checks capacitors though. I guess I can at least check them for shorts/resistance.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
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