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Chuck, I have replaced all of the tubes, power and preamp with known good tubes with no change. The rhythm channel is fine but when kicked into lead the board becomes microphonic and the trouble starts. When kicked in the contour mode which reshapes the sound and boosts the volume the amp starts to howl and can go into runaway feeding back even with all of the volume controls are at zero. I have to test the amp hooked up to an attenuator just to keep the volume down while going through it.
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The bitterness of low quality is remembered
long after the sweetness of low price has been forgotten. -
Your yankin' my chain, right? The amp is going into uncontrollable feedback with all volume controls on 0? If that's true, all the pot and component connections in the preamp are good and all your LDR's and relay's are functioning then the problem is somewhere beyond V3a.
Just because a tube is new absolutely does not mean it's not microphonic. The Sovtek 12ax7LPS is one of the best sounding new made preamp tubes. And they're all too microphonic for use as V1 in most amps. All of them. Right out of the box.
It's not likely any of those transistors are the cause of this problem. I might check all the components and connections around V3b.
As an after thought... Has the OT been replaced or disconnected at any time?"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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I don't recall, did you ever check for any loose rear covers on any of the controls in the bad channel, as I suggested in post 7?Education is what you're left with after you have forgotten what you have learned.Comment
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The amp operates fine in the clean mode and that portion of the circuit V1 comes before the lead/ contour circuit at V2 which is why I didn't think it was after that section but I'll take any suggestions at this point.
I've done some further work on the amp and adding a 33k grid stopper seemed to tame it a bit but when switching channels from clean to lead and then to contour "boost" whether using the footswitch or the manual switch on the amp face there is a pop, not so much like an arching switch and a huge boost in volume and the lead channel volume starts to behave like a linear pot. 0 to 10 in nothing flat. The board is still microphonic everywhere but around the power tranny. Just a slight flex of the board causes noise.
Thanks again for any help or direction.
KevinLast edited by kramps; 09-23-2011, 09:14 PM.The bitterness of low quality is remembered
long after the sweetness of low price has been forgotten.Comment
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Enzo, I checked the pots out but will give them a better review today.The bitterness of low quality is remembered
long after the sweetness of low price has been forgotten.Comment
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