Peavey XXX powers on but no sound

[Redline]

I just acquired a XXX head for stupid cheap. It wouldn’t power on but the ht fuse was shot. I changed it and installed new tubes. No it powers on but doesn’t produce any output. My meter crapped out on me so I’m kinda stalled out till tomorrow. I just thought I would reach out to the gurus on this forum for advice. There is nothing obvious like burnt components. I replaced all the fuses. Thanks in advance.

[tedmich]

Hey I bought a (not stupid) cheap XXX that was similarly quiet recently! Two of the 100 Ohm screen grid resistors was burned, check them for continuity (with everything unplugged and no tubes).

They are at least on a removable board and don't require drilling out rivets like on 5150 etc.

I always replace them with 750-1k 5W versions, lately this type


Ohmite 45F1K0E, 1k 5W, wirewound rated to 460V/275C, 1%, $1.73 at Mouser, I mount the up off the board to give them a chance to cool a little better
(CR100/103 can also benefit from some space from the board as they can burn a hole clear through when they go)

Good luck! If its not the SGR be sure the tubes are good, then we can run down voltages...

Once you have a meter!

[Enzo]

Just a note: on 5150 et al, I don't bother drilling out the rivets. I can measure those resistors from the solder side. ANy need replacing I just unsolder them and poke their wires on through the board. The part falls into the chassis where I can easily fish it out. I then mount the new part up on the solder side, all exposed and accessible. the amp doesn't care.

[Redline]

Update:
I decided to go ahead and upgrade all of the screen grids to 1k5 and put all new fuses in. Still no output. The tube are all new but I pulled some from one of my other amps and got the same result.

[The Dude]

Are all of the filaments lit?

[Enzo]

Amp makes no sound?

Are ALL the heaters lit?

Is there B+ voltage on both pins 3 and 4 of each power tube socket? And is there good bias voltage on the pins 5? (I don't know, maybe -45 to -60v)

[Redline]

All filaments are lit. I’ll check the B+

[The Dude]

Also, run a jumper from send to return to eliminate the possibility of a bad switching jack.

[Enzo]

And check that your speaker and cord are working.

[Redline]

I have no bias voltage

[tedmich]

well since filaments are lit the F3/F4 fuse pair and CR27/30/31/32 are likely good; see if any bias voltage at J59 coming off main PS board,

if no check all diodes upstream, then caps, then resistors


If correct VDC at J59, check PA board components, good luck!

[g1]

Are the tubes not red-plating with no bias voltage? You did not say whether you found B+ or not.

[Redline]

Update: I shelved this amp for a bit but decided to dive back in to it. I found that R64 was fried and replaced it with a ceramic equivalent. When taking the amp out of standby it split that resistor in half! Exciting!!!! I’m definitely getting closer.

[Enzo]

SOmething is shorting out your SCREEN node B+, most likely a tube, but also possible filter cap. Beyond that we have actual shorts between traces, solder blobs, that sort of thing.

[Redline]

So it can’t be the rectifier? I checked the caps that connect to that resistor. Sorry I don’t know the numbers. The caps check out fine. The tubes are brand new