SS amp symptom, no power to speaker

[sjhorvath]

Hi, I found a 1978 peavey backstage 30 for cheap on craigslist, and I'm afraid that I broke it. The symptom seems to be an blown OT.

It worked fine but with scratchy pots both when I tested it and also at home. However I wanted to deal with two things to improve the amp's usefulness:

1) to clean the pots, 2) to convert the hardwire speaker wire to a 1/4 inch jack. So in other words, like a Princeton I wanted a 1/4 jack on the back panel with a right angle switchcraft on the speaker wires going to the speakers. This way I can unplug the combo speaker and go to a bigger 8-ohm cab if I wanted to bring it to rehearsal.

So I opened it up, and clean the pots with Kester spray cleaner and gave each of the knobs many turns, though mainly the gain knob was scratchy.

Then, I drilled a hole in the back and installed a 1/4 jack for the speaker out, and put a right angle switchcraft (Chinese copy) on the speaker wire. The wires were blue and yellow, I forgot which went to which but I assumed the lighter color would be "red" and wired it such that the blue wire was on the sleeve and yellow on the tip. And to the speaker the yellow went to the painted dot which I believe marks the positive terminal.

I put it all back, and after power-up, I play and the sound comes out but is super low. Less than, say 100mW percieved volume, or perhaps described as the volume level of studio headphone cans lying on the table in front of you and not on your ear, if you know I mean.

Then the sound stops, dead... It has a preamp out and that works. Please don't tell me I blew the tranny. :jo I was careful to never play it without any speaker connection.

Even though it's a SS amp, I took care not to play it with out an attached speaker. Every amp I approach, I start with master on very low and guitar vol very low and make sure that sound comes out of the speaker before bashing away. I know this is less important with solid state amps, but I do the same practice to be extra careful.

I read on another forum that there could be something in the preamp area that causes this symptom. But it is odd that I tested the preamp and it works. Though I don't have a power amp, I went preamp out- to smokey amp - to speaker and it works.

Also I use the combo box speaker and the 1/4 plug with a low watt tube amp and it works just fine. The solder on the 1/4 jack appears visually stable as well.

Any help/ideas appreciated.

Thanks,
Steve

[rockon1]

SS amps generally dont use output transformers.They do use output transistors though. Also with SS amps it doesnt matter whether there is a speaker hooked up or not like tube amps. Hopefully the output didnt get shorted to ground or something funky like that when you installed the jack and damage the output transistors. Is there any protective fusing? Have you tried unmodding it? Bob

[Rosewood]

If it has any switching jacks like effects return or power amp in it could be the contacts need cleaning. If no switching jacks you could check the input jack solder connections. That's about all you're gonna be able to do without signal tracing the amp. One thing is make sure the speaker connections are not shorted to ground, that will take out the output transistors.

[donnyjaguar]

With older SS amps you'll find the heatsink heat-conductive compound tends to dry up. That teamed with the constant expansion/contraction of the output devices loosens and pulls them away from the heatsink itself. This causes the output devices to overheat and burn out. I always check this first on older SS amps, especially 30 year old ones. You've probably cooked yours. It can be repaired though and given its only 30 watts shouldn't be a bear of a job.

[sjhorvath]

Thanks guys for the help. Here are combined responses/questions to all. It seems I confused OT with the power transformer. Thanks for the education as to why there's only one transformer.

- rockon1 - fuse is intact, and preamp out seems to work. But I could always try removing the quarter inch jack.

- rosewood - when you say take out the output transistors, do you mean destroy them requiring repair/replacement. Or just block them from working until I fix the possible short?

- donnyjaguar- heatsink drying... interesting, but odd the timing had occured with me pulling out the chassis and tinkering. Could what I did accelerate that and make them come loose? Also the problem happend immediately after I powered up. Not say a after a couple hours of running it.

[Structo]

If you can find the transistors cheap enough, I would replace them all.

Use heat sink compound, they also may possibly have mica washers between them and the heat sink depending what kind of tranny they are.

[Structo]

Probably also replace the power supply electrolytic caps and any other e caps.

[Rosewood]

Thanks guys for the help. Here are combined responses/questions to all. It seems I confused OT with the power transformer. Thanks for the education as to why there's only one transformer.

- rockon1 - fuse is intact, and preamp out seems to work. But I could always try removing the quarter inch jack.

- rosewood - when you say take out the output transistors, do you mean destroy them requiring repair/replacement. Or just block them from working until I fix the possible short?

- donnyjaguar- heatsink drying... interesting, but odd the timing had occured with me pulling out the chassis and tinkering. Could what I did accelerate that and make them come loose? Also the problem happend immediately after I powered up. Not say a after a couple hours of running it.
What I meant by take out the Transistors was destroy, sorry should have been more clear. And like donnyjaguar said being a 30 watter the job will not be as big, or expensive for that matter.

[Fuchsaudio]

Quick suggestion: Many solid state Peaveys had a floating speaker output, meaning the positive and negatives of the speaker were not connected to the chassis at all. There's a whole lot of technoblabber you don't need to know, as far as why BUT If your new speaker jack grounded either the positive or negative line, the amp will not be happy, and maybe not work well, if at all.

You may want to start by removing the modification, and returning it to stock form. I encourage taking digital pictures of this kind of stuff, so you know where things came from before you started doing the work.

I would not start replacing all the parts you can find, before you figure out the problem. It's costly, many Peavey transistor part numbers are "house numbers" (meaning only Peavey knows wtf they are), and you could be opening a deep can of worms randomly changing anything you can solder...

[Structo]

Good point Andy.

They are probably PNP transistors. I haven't looked at a schematic but just like PNP based pedals, it would have a positive ground.
So that may be the reason for the isolated output jacks.

Check that before pulling transistors.

Just had a look at a Peavey Backstage 50 schem and it does use PNP transistors.

http://www.schematicheaven.com/newamps/peavey_backstage50.pdf

[Fuchsaudio]

Nah, they float the ground of the speaker on a small resistor and take feedback through the speaker coil to make the amp a little soggy (tube like sorta).....kills some of the damping factor. Instead of taking the FB off the + lead, it goes through the speaker coil and get's "roughed up" a little. When you ground the negative, the amp has fits...I learned it the hard way when I replaced a missing jack washer and nut without the insulating washer....years ago, in a galaxy far, far, away...

[mark norwine]

I learned it the hard way when I replaced a missing jack washer and nut without the insulating washer....years ago, in a galaxy far, far, away...

I've visited that galaxy, too....:crazy

[WaltC]

makes the speaker hookups (extension, impedance switching, etc.) look more like Star Trek than Fullerton though <G>...

[sjhorvath]

Nah, they float the ground of the speaker on a small resistor and take feedback through the speaker coil to make the amp a little soggy (tube like sorta).....kills some of the damping factor. Instead of taking the FB off the + lead, it goes through the speaker coil and get's "roughed up" a little. When you ground the negative, the amp has fits...I learned it the hard way when I replaced a missing jack washer and nut without the insulating washer....years ago, in a galaxy far, far, away...

Hi Andy, I bet this is it.

So my next step is to undo the mod and see if I zorched those components or if it is still working. Being that the sound faded away, I am not expecting it to work. Then if I want to do this speaker jack thing, I would need nylon washers on both sides?? The threaded shaft may still touch the side of the hole I drilled...

Peavey did email me a schematic, but I am not sure if it's OK to post it publicly.

[Fuchsaudio]

Float (insulate) your jack, and you should be okay. Hopefully you didn't hurt it. :(

[Tone_Terrific]

Float (insulate) your jack, and you should be okay. Hopefully you didn't hurt it. :(

Rewire to stock config to verify that you didn't fry it.
Then make your connection directly to the speaker leads OUTSIDE of the chassis.
Just mount a small metal plate to part of the cabinet and use it as your new jack mount point. A right angle bracket sort of thingie. Easy, less intrusive, guaranteed to work.

[sjhorvath]

TT, don't know why I didn't think of that.. :) I even have a few of those jack plates in the garage. The black ones with 3 mounting holes. Thanks.

[rockon1]

TT, don't know why I didn't think of that.. :) I even have a few of those jack plates in the garage. The black ones with 3 mounting holes. Thanks.

Before you go crazy with it I'd unhook the speaker output and wire it directly to a seaker or leave the jack dangling to see if the thing isnt toast first. Bob