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Hum

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3/22/1999 11:12 AM

Hum

I've been experiencing alot of hum from my 70's blackface SuperTwin Reverb (385W Peak 180W RMS). It's the 2 channel model with 5 band EQ and a distortion section that's too nasty to use!! I just replaced tubes with all new matching Sovteks and can tell very little difference. The technician who recently did the work mentioned missing 2 tube shields. Could this produce hum? Any suggestions as to what I might try? The hum is at it's worst when I take my hands off the guitar..... I've tried two guitars so I don't feel it's in the guitar!! I appreciate any suggestions! Thanks in advance!!

3/22/1999 12:21 PM
Steve Jones	Search the archive for the word "hum" and you'll see lots of posts on this subject. Seems to be everyone's demon. If it didn't used to hum and hums now, a likely cause is a bad filter cap. If the filter caps have never been replaced, you may want to try that. Steve

3/22/1999 1:22 PM

Hi Joe,

Unshielded tubes can be susceptable to hum if the tubes are in the first pre-amp stages. So, it's always best to put shields on all the 9-pin tubes on the amp chassis.

As to the huge hum ocurring when not touching the strings...your *body* is being used to shield the back-side of the pickups. This is done by connecting the strings to amp-chassis ground and when you touch the srtings, you become part of the ground system and shield the pickups. When you don't touch the strings, the pickups are "unprotected", so to speak.

This effect is most notable with single-coil pickups of the sort found on most Fender guitars. Just a fact-of-life.

Regards,

Dave James
James Music Products
Los Gatos, CA.

3/22/1999 3:01 PM
Giacomo	>hum is at it's worst when I take my hands off >the guitar I have same problem too. Is your amp's hum affected by the volume control? If not maybe an output stage trouble (like mine). If turning down the volume defeats hum it's probably something in preamp stage. Just asking: did you tried with a different guitar cable?

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