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In Reply to: RE: Ampeg SVT4 pro hum posted by Pro Sound Guy on August 07, 2009 at 11:00:27
When was the amp last re-capped and the diode rectifier bridges replaced? Could be some noise getting through old parts (maybe)?Ampeg SVT often have a pots in the bias circuit to adjust the bias or for hum control. These pots often get noisy. They are usually on chassis below volume/tone controls & inputs. Prolly good place to start. After checking the electrolytic caps' age.
Other places to quickly look are the jacks (need cleaning & check their gournd points). And the ground points to the chassis, for bad solder points. Worn out ground lugs, etc.
Are the lines from the input jacks to the circuit cracked, brittle, burnt? Solder look ok? Pull the tubes and clean the pins and sockets.
while you are cleaning and looking at the guts, I'd check the resistance of the resistors in the power section and be sure to replace with 1% tolerance, like Mills.
Edits: 08/08/09Follow Ups:
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