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Re: Peavey Classic 30 Hum Problem

6/3/2004 10:43 PM
Re: Peavey Classic 30 Hum Problem
New tubes are not always good, but apparently you have resubbed them.  
The 120Hz is the clue. 60Hz comes from grounding and shielding problems, but 120Hz comes from power supply ripple.  
Your probing has determined the noise is not from the power tubes, and by turning the controls to zero, you eliminate the reverb.  
The heaters on the 12AX7s are in series running off the -36VDC supply. The bias is clean you say, but it has an extra filter stage. Check the -36VDC supply for ripple. A little is OK, but gross amounts would mean C46 is bad.  
This amp has four grounds. Three of them are bonded together by a pair of 47 ohm resistors bottom center on the drawing. (the other bonding is at the input jack.) If either of these resistors is open, you surely will get hum. R61,62.  
I said the reverb was not involved, but there is one small possibility. If the 4558 driver chip is shorted, it could load down the -30VDC supply heavily, which could load down the -36VDC and thus increase ripple. Seems unlikely.  
Try those things.