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tube preamp ac hum

Posted: Fri Mar 30, 2012 10:39 am
by n2gbt
I bought an inexpensive tube preamp made by ART-- lots of people complain about an ac hum-mine has this also- the thing is- it uses a 9v ac walwart as the power supply so the hum would be coming from the walwart right?

any thoughts on how to eliminate this hum- I hate sending stuff back


Posted: Fri Mar 30, 2012 3:38 pm
by k9yli
that would depend a lot on the type of tube..
most need at least 90 volts on the plate.
that implies a step up transformer. then rectifier and filter.
hum from bad filtering.

or ground loop. as 9 volt ac likley doesnt have a third wire ground back to the house power.
you could try a ground lead back to the house power ground form the amp chassis.
really more data is needed as to tube type. power supply, chassis material,
printed circut or hard wired .????

Posted: Sat Mar 31, 2012 2:38 am
by N9LCD
It may be one of those "low" B+ tubes used in car radios or the R-392.

Can you post a schematic of the preamp? It may need a hum balance adjustment pot.