Peavey Dual amp

[manuel]

Thank you very much!

I really appreciate it.

[Valvehead]

The reverb pot symptom was unusual. Pulling Q6 will tell us if that direction is fruitful of not. JFETs have no standard pinout, so it was possible the perfectly good replacement you installed has its legs in the wrong holes. SO by just eliminating the part, we find out if that circuit is causing the problem or not.

Had to walk away from that damn amp for a while.....

So tonight i removed Q6 . The hum is still there

[Enzo]

OK, so not coming in through Q6. But with Q6 out, does the reverb control still act in that odd way?


Your 12AX7s have heaters running on DC. V1, V2 are in series from +23v to ground and V3, V4 are in series from -23v to ground. But the power tube heaters run on 6VAC. But that 6VAC is still elevated +25v above ground. Down in the power supply, R81 connects to one side of the 6VAC. Is R81 OK? An easy test is to check at the heater fuse, the 8A F2. Is there +25VDC on that fuse? SHould be.

I don't recall getting an answer on how much ripple is on that +23v rail?

Hmmm, occurs to me now that you can't pull preamp tubes. The heaters are in series so pulling V3 or V4 makes both those tubes go dark, and same with the V1, V2 pair.

And if this approach is not paying off, try another. You say turning down the master kills the hum? Well, all the 12AX7 grids except the PI are at no DC on the3ir grids, so glip a wire to ground and try grounding each grid. WHich ones kill hum and which don't?

[Valvehead]

OK, so not coming in through Q6. But with Q6 out, does the reverb control still act in that odd way?


NO - THE POT DOESNT DO ANYTHING NOW.





Your 12AX7s have heaters running on DC. V1, V2 are in series from +23v to ground and V3, V4 are in series from -23v to ground. But the power tube heaters run on 6VAC. But that 6VAC is still elevated +25v above ground. Down in the power supply, R81 connects to one side of the 6VAC. Is R81 OK? An easy test is to check at the heater fuse, the 8A F2. Is there +25VDC on that fuse? SHould be.


YES 25V. AND R81 IS OK .




I don't recall getting an answer on how much ripple is on that +23v rail?


ACROSS IT THERE IS 3 VAC. FROM EACH SIDE THERE IS 55 VAC TO GROUND.




Hmmm, occurs to me now that you can't pull preamp tubes. The heaters are in series so pulling V3 or V4 makes both those tubes go dark, and same with the V1, V2 pair.

And if this approach is not paying off, try another. You say turning down the master kills the hum? Well, all the 12AX7 grids except the PI are at no DC on the3ir grids, so glip a wire to ground and try grounding each grid. WHich ones kill hum and which don't?


V1 , V2 NO EFFECT. GROUNDING GRIDS ON V3/V4 KILLS THE HUM . AND THE MASTER KILLS THE HUM


.

.......

[Valvehead]

Reply is above - thanks enzo

[Enzo]

don't recall getting an answer on how much ripple is on that +23v rail?


ACROSS IT THERE IS 3 VAC. FROM EACH SIDE THERE IS 55 VAC TO GROUND.

Let us clear up this point. There is ripple on the +23v rail of some amount. There are no sides to the 23v rail to have 55v from. If there is 3VAC sitting on the +23v power supply, then the filters are not doing their job. When you say across it, I am unsure what you mean now. I am looking for a reading from the +23v point to ground. if you are measuring across the dual resistors or something, that is not what I need.

So:
Look at the +/-25v supplies. C55 filters the +25v and C63 filters the -25v. Measure each of those for DC. They should be ABOUT 25v more or less, but they should be pretty much the same as each other, other than polarity. 24 or 26 is OK, but we want them to be more or less the same whatever it is.. Now assuming that, flip the meter to AC and measure the same points. There should be very little AC on each. But it should be about the same on both.

NOW go over to the 23v supplies - the far end of the dual resistors. C56 filters the +23 and C64 filters the -23. AGain, are both DC voltages about the same? And flipping to AC, do both have about the same level of AC on them?

[Valvehead]

Ok, everything you listed checks out.

EXCEPT when measuring the AC on C56 , the meter just fluctuates. I cant get a reading on that one.

Also, I still have Q6 removed . Should i be measuring this with it back in ?

[Enzo]

Nah, mute transistors and such are not needed to make sound, so leave them out until we find the hum.

Try a different cap at C56 then.

[Valvehead]

already have. I had caps in the parts bin , already replaced C55 , 56 , 63 , 64

[Enzo]

Hmmm....