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Post subject: Reverb buzz/hum problem '69 Dual Showman
Posted: Thu Dec 21, 2017 6:26 pm
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Joined: Mon Sep 14, 2015 3:55 am
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Hey guys!

I'm working on a '69 Dual Showman, which has been modded to AB763. And the amp sounds great!

However I have some serious buzz/hum going on with the reverb, and the only thing that kills the buzz is disconnecting the input on the reverb tank (either at the amp or the tank itself), or turn down the reverb volume. So that leads me to think its in the reverb driver section somewhere. The reverb sounds loud and clear, with no buzz, if i leave the input disconnected and shake the tank. It's a brand new 4AB3C1C tank by the way, as the old one was completely shot.

The buzz, or hum, itself sounds to me like a grounding issue, but I have gone over the grounding in that part of the circuit many times now with no luck as to find the problem.

I have checked the tubes, the reverb driver and they are all good. I have cleaned all the connectors, checked the grounds, like I mentioned earlier, which all measure good, and just about everything else I can think of. The amp has all new electrolytic caps.

Any idea what can be causing this buzz?


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Post subject: Re: Reverb buzz/hum problem '69 Dual Showman
Posted: Fri Dec 22, 2017 2:07 am
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Joined: Fri Feb 05, 2010 4:31 am
Posts: 14045
Location: Province de Québec, Canada
Did you put new tubes or just check on tubes tester wich can't test that ?

The hum buzz may come from anywhere from the reverb channel , not from the reverb circuit itself.
Are input jacks shorting contacts to ground did their job on both channel ?
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How is the wires dress ? Wires *should be close to chassis with 90 degree crossing between them, no parallel running......

* Not heater

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