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In Reply to: RE: D70 Output Tubes Drifting WHILE Biasing posted by Michael Samra on March 23, 2014 at 22:15:12
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."
Follow Ups:
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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