The amp is a 1973 or 75 Peavey model Vintage.
Fully SS preamp and 4 6L6 output stage.
I just fully recapped it but did not put the amp on a O scope to see what the output wave form looked like.
I tested the output tubes and there match is great, I also set the bias from a far too low .008 up to .025 amp.
Now here's the issue, at 60 watts of RMS output power the wave form is sweet,at 96 watts RMS the one sided cross over distortion starts to sneak in pretty good, and at its max power it looks like that last picture.
Anyone see this strange stuff before?
New problem, I get all the winners!
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Stevem
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New problem, I get all the winners!
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When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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martin manning
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Re: New problem, I get all the winners!
OT failure?
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Tony Bones
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Re: New problem, I get all the winners!
What do the signals driving the grids look like?
Check the coupling caps as well as any grid stoppers and grid leak resistors.
Check the coupling caps as well as any grid stoppers and grid leak resistors.
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Stevem
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Re: New problem, I get all the winners!
Well I am the 3rd tech to work on this amp in 4 months and the guy wants the amp back bad ! Real bad! he also never plays the amp loud enough where it's likely to produce the issue and on top of that he does not want to pay any more!
Long story short, he's getting the amp back as is so the cause of the issue may never be determined, I would surly like to know though!
Long story short, he's getting the amp back as is so the cause of the issue may never be determined, I would surly like to know though!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: New problem, I get all the winners!
Yeah, I'd vote to scope the output of the SS preamp. If it's there, the outputs are just doing what they're told.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Mark Twain
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Stevem
- Posts: 5144
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Re: New problem, I get all the winners!
I just can't rap my around how the output power could be exactly the same from both sides / halves of the OT like it was showing on the scope, yet crossover distortion was taking place on only one half of the output wave form!
My best guess is that it's a PI out coupling cap, yet I don't know by what mechanism that could be taking place from.
How about you R.G?
My best guess is that it's a PI out coupling cap, yet I don't know by what mechanism that could be taking place from.
How about you R.G?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: New problem, I get all the winners!
This one doesn't yet make an obvious cause appear in my head.
Did the preamp output not have the one-sided crossover?
If not, I would then measure the actual DC bias voltage on each tube grid, just to see if it was asymmetric.
If all was well statically, I would then suspect that something was letting grid conduction push one tube's grid bias under high drive levels. Just a guess. That could be a tiny leakage in one of the two PI coupling caps, I guess.
I dimly remember something about induced or apparent crossover distortion coming from leakage inductance, something like that. I think it was in the article on the "unity coupled" MacIntosh OT.
Did the preamp output not have the one-sided crossover?
If not, I would then measure the actual DC bias voltage on each tube grid, just to see if it was asymmetric.
If all was well statically, I would then suspect that something was letting grid conduction push one tube's grid bias under high drive levels. Just a guess. That could be a tiny leakage in one of the two PI coupling caps, I guess.
I dimly remember something about induced or apparent crossover distortion coming from leakage inductance, something like that. I think it was in the article on the "unity coupled" MacIntosh OT.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Mark Twain
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
1 others liked this
Re: New problem, I get all the winners!
Thanks for your thoughts!
Well if the amp craps out and I get it back I guess I will be forced to figure it out!
Well if the amp craps out and I get it back I guess I will be forced to figure it out!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!