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Topic: Session 400 distortion problem |
Cartwright Thompson
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Posted 27 Nov 2012 7:44 am |
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I've got a '76 Session 400 that is giving me fits. It has a subtle but obvious buzzing/distortion of any notes played. The whole tone doesn't break-up, there's just a somewhat higher buzzing when notes are played. It also is very noticeable as a note decays. It is not the speaker or cabinet, I've tried it with several different speakers and cabs. It's not the reverb circuit either. It reminds me of the description of "parasitic oscillation" that I've read in books on tube amps, although I've never actually heard an amp with that problem.
I just got it back from Peavey. They cleaned the pots, checked caps and did the usual routine maintenance. They sent it back saying that it tested as per spec.
I'm about to give up on it, anybody have any ideas? |
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Robbie Daniels
From:
Casper, Wyoming, USA
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Posted 27 Nov 2012 9:39 am
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Back in the late 70s I had the same problem with the Session 400 I had. It buzzed on 440 A. I changed speakers, had a local amp specialist look at it, sent it to Peavey, Peavey sent it back saying it met their tests. I finally gave up and sold it. About two months ago I bought another one for the jam sessions I sponsor and it produces flawlessly. Go figure?
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Carter D12, MSA S12, 12 String Custom Made Non-Pedal, Evans FET 500LV, Evans SE200, Peavey Nashville 400, Fender Steel King |
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Jack Stoner
From:
Kansas City, MO
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Posted 27 Nov 2012 10:38 am
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Does your steel, directly to the amp no volume pedal and no effects, do it? Do you have a different amp that you can try with the steel or a different guitar you can try?
Have to isolate for sure where the problem is.
On the older Session 400's, I recommend the Lemay mod (or maybe the Peavely mod) as it replaces all the electrolytic capacitors on the preamp board. I did that mod for a steeler and it made a world of difference to his old Session 400. "White" noise was gone. |
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Ken Fox
From:
Nashville GA USA
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Posted 27 Nov 2012 11:18 am
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I had that pronlem on a few after putting in new output transistors. Peavey advised to put a .01uf cap in series with a 1 watt, 22ohm resistor from tne speaker out put to ground. Same thing they did later on Nashville 400 and other amps. On the scope I could see a ringing causing the sine wave to in-fill at the top of the wave. This solution fixed it. Still, you may have another problem entirely. If it check the same on a different speaker then I would send it back to Peavey for abother go around.
Trouble-shooting noise in the output section of these amps is a difficult matter, to say the least! Almost the same power amp as a Webb and other Nashville series amps.
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Cartwright Thompson
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Posted 28 Nov 2012 4:27 am
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Thanks Ken. I'll try that fix. I don't want to send it back to Peavey and risk another $100+. They didn't even think anything was wrong with it, I doubt a second attempt would be any better.
I've got three of these amps (actually the other two are from '75) and I know what they should sound like. And yes, I've tried it with several different speakers and it is definately the amp.
Maybe I should send it to Fox Vintage Amps and get one of the last mod kits? 
Don't worry Ken, I wouldn't do that to you, I know how much you love chasing down problems like this! |
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