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Old 14th October 2009, 06:39 PM   #1
adamus is offline adamus  United Kingdom
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Default problem - scratchy sound, rythmical

I bought a cheap el34 valve amp to play with (it was sold as not working).

I have revived it by replacing coupling caps, a new set of valves (used 6pS3) and replacing the cathode resistors of the input stage.

All biases up fine, plays music..... but. There is a constant but quiet rythmical noise, scratchy noise.

to a beat of around 2 a second it has minor bursts of noise.

Anyone have any ideas of where to start?

Edit - And its from both speakers. Seems like B+ or maybe the screen supplies.

Last edited by adamus; 14th October 2009 at 06:58 PM.
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Old 14th October 2009, 08:32 PM   #2
adamus is offline adamus  United Kingdom
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I pulled all the input tubes, and the noise went. Put some old crap chinese 6n1 in, and the noise is gone.

Must be a couple bad 6n1p's, unless anyway can think of a reason why 6n1p may do this over the 6n1?
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Old 14th October 2009, 09:12 PM   #3
adamus is offline adamus  United Kingdom
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scratch that. the noise is back. I dont think its B+ or output tubes, as without the input tubes in there is no noise.
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Old 14th October 2009, 10:07 PM   #4
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schematic?
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Old 15th October 2009, 03:19 AM   #5
Enzo is offline Enzo  United States
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Got a cell phone sitting near it?

Also, set up an AM radio next to it, tune to where there is no broadcast and listen if the radio picks up the same noise. Angle the radio different directions, as they do pick up directionally.
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Old 15th October 2009, 06:17 AM   #6
adamus is offline adamus  United Kingdom
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I dont have a schematic (havent drawn one yet) but after a quick look it looks like a phase splitter to SRPP to output stage.

I dont think its interference, its not that typical buzzing you can get.
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Old 15th October 2009, 08:03 AM   #7
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well thats an odd architecture... to me anyway. I was more interested in the power supply configuration since it seems to be related to that frequency
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Old 15th October 2009, 03:13 PM   #8
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Quote:
Originally Posted by adamus View Post
I dont think its interference, its not that typical buzzing you can get.
Regardless of what you do or don't think, interference comes in many forms. Enzo gave you some good advice and is a proven diagnostic tool. And now is the time for the oscillocsope to be brought out and used as a signal tracer. If I could have only one instrument on the test bench, it would be the scope. With it you can replace the function of many instruments. It's really indispensable as a troubleshooting tool.
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Old 15th October 2009, 05:19 PM   #9
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I have a scope, and will be using it tonight.

Will report back. cheers for the input
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Old 15th October 2009, 05:57 PM   #10
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ok i scoped various points and nothing, looked ok, a bt too much ripple on B+ for my liking but that can be sorted.

then i plugged the speakers back in.... and nothing, no noise.

Then i turn the amp over to its (it was upside down for testing).... and the rustle is back.

This ihas to be mechanical or heat.
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