tube preamp ac hum

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n2gbt
Posts: 62
Joined: Sun Dec 14, 2008 5:50 pm

tube preamp ac hum

Post by n2gbt » Fri Mar 30, 2012 10:39 am

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

k9yli
Posts: 228
Joined: Fri Aug 12, 2005 11:35 pm
Location: northwest wisconsin

Post by k9yli » Fri Mar 30, 2012 3:38 pm

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 .????

N9LCD
Posts: 329
Joined: Sat Jul 01, 2006 8:32 pm

Post by N9LCD » Sat Mar 31, 2012 2:38 am

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

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