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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

Joe N2GBT

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 .????

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.

N9LCD