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Old 12th November 2012, 12:39 AM   #11
getonemaker is offline getonemaker  United States
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Join Date: Aug 2011
Default even more tests

OK, so there are two issues. Hum and scratching sound.

If I only have the rectifier and the power tubes, there is hum, but no scratching sound. I have tried with multiple tubes. Inserting just ONE 12au7 tube at either location gives no scratching sound, but if both are in, there is scratching noise. Again, this is independent of tubes, as I tried several of them. Did this shed any light?

Note: no 12ax7 tube was inserted.

Thanks again!
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Old 12th November 2012, 01:02 AM   #12
dgta is offline dgta  United States
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The scratching sound is likely bad components or bad pots in the tone circuit. Check everything or bypass the tone section altogether.
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Old 12th November 2012, 01:29 AM   #13
Wavebourn is offline Wavebourn  United States
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DUKANE tube PA help
Do timbre controls change the timbre of the scratching sound?
Nothing in the universe is perfect. The ideal things are the ones that are most optimal. Optimization criteria, what matters. When I hear "No Compromise Design", I want to take a sledgehammer and test how impact-proof it is.
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Old 15th November 2012, 09:27 PM   #14
getonemaker is offline getonemaker  United States
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Default identified the bad part, how to replace it?

As I was gently poking around I found that one of C14 was falling apart. I will replace it and hope that fixes the problem. Here is a question, however:

C14 is .003 mfd, 1600 V.

I guess it is non-polarized. I can't find a capacitor of that exact value.


1. a 3.3 nF 1600 V be OK
2. a .003 mfd 630 V be OK

I will need to buy those.

I have however three .01 mfd 600 V capacitors at home. If I put them in series, would that be OK? I guess I would do this and if it fixes the problem I will eventually buy option 1 or 2 if one of those is OK.

Please let me know what you suggest I do.

Again, I am really thankful for all of yours time.
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