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Old 8th November 2012, 09:21 PM   #1
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Default DUKANE tube PA help

Hello Everyone!

I got a nice Dukane tube PA amp. I brought it up slowly with a variac and it worked for about 40 hrs. Now there is a loud hum and some scratachy noise (as if you had a loose speaker connection, which I do NOT have). Since I tried spares of all the tubes, I know it is not that. I did replace the C6 which is two paralell 20 mfd caps and did not see a change.

Please look at the attached schematics and give some advise what other part might need to be replaced.

Thanks!
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Old 8th November 2012, 09:28 PM   #2
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Do you see any smoke going from somewhere?

Check RC1 diode, C5 caps, and R1 resistor. You should have negative voltage on the cap. Remove output tubes before powering on to measure!!!
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Old 9th November 2012, 12:34 AM   #3
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I did not have time to open the unit, but I removed the power tubes and turned the unit on. I could hear the scratching sound (although more quietly) and with that the pilot lamp also was changing its intensity.

C5 caps are brand new, rated 600V.

What would be the mouser catalog number for RC1?

Thanks!
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Old 9th November 2012, 12:39 AM   #4
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If pilot lamp is changing it's intensity and you hear scratching sound that means you need to check power plug, power cord, power switch, everything that goes from power plug to the primary of the power transformer, it seems to me you have some loose contact there.
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Old 9th November 2012, 12:49 AM   #5
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I did not have time to open the unit, but I removed the power tubes and turned the unit on. I could hear the scratching sound (although more quietly) and with that the pilot lamp also was changing its intensity.

C5 caps are brand new, rated 600V.

What would be the mouser catalog number for RC1?

Thanks!
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Old 9th November 2012, 02:24 AM   #6
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Quote:
Originally Posted by getonemaker View Post
I did not have time to open the unit, but I removed the power tubes and turned the unit on. I could hear the scratching sound (although more quietly) and with that the pilot lamp also was changing its intensity.

C5 caps are brand new, rated 600V.

What would be the mouser catalog number for RC1?

Thanks!
Forget about it, I thought that the problem may be in bias. But looks like you have the problem with powering your power transformer. The pilot lamp is powered from the same transformer with the rest of the amp.
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Old 9th November 2012, 02:47 AM   #7
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If you have a dim bulb tester you can tell if your power transformer is going bad.
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Old 11th November 2012, 11:46 PM   #8
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Thanks for the replies. I have checked all the connections running to the power transformer and resoldered them, with no effect.

I also removed all the tubes and turned the unit on. No scratching sound, no hum and the pilot lamp stayed the same.

Looking at the amp, I suspect that C4 which connects to R23 and R26 might be the bad apple. Would that make sense?

It is 25 mfd 25 V. I have a 20 mfd 50 V and two 2.2 mfd 63 V caps. Would it be OK putting these in parallel to replace that?

Thanks again!
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Old 11th November 2012, 11:55 PM   #9
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Interesting... So, without tubes it does not cause that popping corn sound, and pilot lamp does not dim?

Try inserting tubes back, one by one. First put power tubes and check. And so on, until it starts popping.
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Old 12th November 2012, 12:10 AM   #10
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Yes you can use 20 uF for C4, no need to be exact. Measure the voltage at that point and use a cap with equal or higher voltage rating. R23/26 provides a reference voltage to bias the filaments above ground. I doubt that's the problem, but simple enough to replace.
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