Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on April 04, 2012, 08:03:23 pm
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HI guys, I have a Vibro Champ that is buzzing like crazy when you turn it on. I have already swapped OT leads. Changed rectifiers. Fired it up on a current limiter and there are no shorts. I'm tending to think one or more of the main filter caps are questionable.
My question is, what does an amp sound like when a filter cap isn't doing it's job? In this case it would be open right?
There is 361vdc coming off the rectifier. There is one Astron cap left in the amp and it's the 10uf cap, and I planned on replacing it.
I guess I could sub in a 20uf filter cap and see if the noise dies down.
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Undo reverse of OT leads. You add regenerative feedback if you reverse them from as factory built.
Buzz is likely a failing filter cap. Tack in a new cap to test. Use a lamp limiter to test. :-)
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I did use a lamp limiter, and OT leads are back to normal. I have 391vdc on the plate of the 6v6 and also on the second node. I tacked in a 22uf filter cap onto the first node and it seems the buzz is reduced "slightly" but still there, I'll have to tack in two more. The buzz could be coming in from the screen or grid I guess.
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check also: one side of the 6.3V filament secondary is grounded. check that connection. are you hearing 120Hz buzz or 60Hz hum?
--DL
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Filaments OK, definitely 60 hz hum. I substituted three 22uf caps in place of the cap can, and it is quiet now; bad cap can.
Thanks for the help, I'd never heard that before and the first time is a little disconcerting.