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Hum in Sound City PA

4/9/2004 10:38 PM
slowblues	Hum in Sound City PA Got a Hummmmm problem with a Sound City PA 6 input head. Replaced the B+ electrolitics sill Hums. Removed the 3 preamp 12Ax7's (6 inputs) still Hums. Disconeced the preamp, wired in a jack straight into power amp (two gain stages, one Pi, 2 EL 34's) Sounds great no hum. Thought it might be a ground loop so moved the preamp grounds around sill hums and this is with preamp valves removed. Any idea what to look at next. This is a true PTP wired circuit very tight not much room between parts but has not been muck about with. The humm is only aparent with the MV above 5. Please help slow.

4/9/2004 10:38 PM

slowblues

Hum in Sound City PA

Got a Hummmmm problem with a Sound City PA 6 input head.
Replaced the B+ electrolitics sill Hums.
Removed the 3 preamp 12Ax7's (6 inputs) still Hums.
Disconeced the preamp, wired in a jack straight into power amp (two gain stages, one Pi, 2 EL 34's)
Sounds great no hum.
Thought it might be a ground loop so moved the preamp grounds around sill hums and this is with preamp valves removed.
Any idea what to look at next.
This is a true PTP wired circuit very tight not much room between parts but has not been muck about with.

The humm is only aparent with the MV above 5.
Please help slow.

Replies:

Ian Anderson these are notorious amps for hum pr... -- 4/10/2004 12:28 AM
slowblues Thanks Ian I may mod it later but w... -- 4/10/2004 6:54 AM