Thanks gents. Yes, I tried whacking the top when playing and nothing happened. I tried wiggling the guitar chords, wiggled input jack but it seems to  be solid (I will do a continuity check). While doing all of that I played the bass and no problem.

But what I did discover was this: every time after I re-started amp the siren sound would start after anywhere from 10-14 seconds then stop and everything would  be fine. The siren sound would start and build quite rapidly then abruptly stop--this sound lasts approx.4 seconds every time. Then, the amp sounds fine and I play away no problem.

I repeated this  on/off process five times and got the same results: 10-14 seconds all good, nasty siren sound for four seconds, then all is fine.

Note: the above occurs without the bass plugged into amp.

Cap discharging/charging?

There is a 12AX7 tube in the pre-amp, so wondering if microphonic to a degree, but even now, amp  still running, no squeal...I'm sure it will show up again at some point...

Hello Folks,

This amp sounds great, except in the last 30 minutes it's made two high-pitched squealing sounds which sound like sirens. This siren-like sound last for 1-2 seconds then stops. Thoughts? Cap issue (s)?

Thanks.

Always more questions: so on the schematic it shows GN. Ground or Green?

I have two pedals that were supplied with the amp. Both Peaveys, one says Peavey Automixer and has three pushbuttons (Selector, Combiner, and Reverb) and has a green, black, red and ground wire. In this scenario there is no white wire which the Schematic calls for.

The other pedal says Peavey Automixer and has two pushbuttons (Selector and Combiner) and has a red,white, green and ground wire. In this scenario there is no black wire which the Schematic calls for.

According to the schematic we need a red, white, black and GN wire(ground or green?).

Each pedal appears to be missing the correct wire colour that matches the schematic. Wiring suggestions??

I see it. Thanks Enzo. :cheesy:

Anyone know the pin out configurations for the foot pedal that comes with the amp? It has an xlr end. Has a red, green, black and ground wire. Wires were ripped off the terminals and I'm not sure how to reconnect.

Thanks.

Okay, this amp works!!! Thanks Fahey, Enzo, and G1 for sticking with me on this. Learned a ton! Couldn't have done it without everyone's help. :tu: :tu:

Great info! Since only one is shorted can I replace just that one or should I replace both--i ask this because Peavey sells this particular transistor as a "matched pair" so I bought two....

By "those" you mean MJ15024G? Just need to make sure! Thanks for your help!! Appreciated!

2N3055's. My local electronics supply shop has these in stock....hmmm

They sent me 70484200/MJ15024G.  I have the Peavey cross reference chart which shows the replacement info you've given me.

On the phone, when I re-ordered 2x, I told them it's SJ6392. I also told them the replacement I see on my chart which is all over the internet and matches your info, and they say that this has been changed and they read a bunch of engineer's notes on the computer telling them to send me something else. I think I need to speak to a supervisor when I re-order for the 3rd time. Friendly, but confused.
If I can't get satisfaction from Peavey maybe I should follow your advice and pick up two MJ15003's--do they have to be a matched pair if only one transistor is shorted? Will one do the job?

Okay so Peavey sends me the new transistors and they are NPN but the woman on the phone had to do a lot of cross-referencing and I'm concerned that what she gave me may be incorrect...again!

Tested on my Peak Atlas DCA55:

Old Transistors (Ic 2.5ma test current), Hfe-27 VBe-.58 (Test current Ib-4.83mA, Leakage Current Ic=0
New Transistor (same test currents) Hfe=59!!! VBe-.62 everything else the same.

What am I doing with transistors with current gain of 59? Can these be used, or will the extra current cause problems?

QuoteThe amp will function with a transistor missing on each side.  I wouldn't expect it to go to full power, but you can play it at reasonable volume.  I also wouldn't play it for extended periods that way.

But the value of doing that is you have the opportunity to find out if other parts are bad that were not discovered while the amp was blowing fuses.  We don't want to order our one part only to find one more part reveals itself after we get the first new one.  You may find you need a few parts, and that helps put together a good order.

This was Enzo's response earlier on in this thread.  So remembering this and, after making sure the collectors weren't shorting to the heat sink: I ran the amp with variac at 49 volts, at low volume, light bulb not lighting up, and two less output transistors (still waiting for the correct transistors to arrive), plugged my guitar into the amp and: Sound!!! Yes, sound!!! So this looks very good. I remembered Enzo saying that these transistors turn on at lower voltages so that we can troubleshoot the amp to find out if anything else needs replacing.
Will follow up once I insert the new transistors! Fingers crossed!  :tu:

Thanks Enzo. I'm now getting 23.95 volts on all collectors at 77 volts from the variac.

Peavey sent me the wrong parts. They are going to send me the correct ones with no extra charge.

The original transistors in the amp only came with a shim per transistor and nothing else. Somehow,  the screws through the heat sink were not touching the heat sink when this amp was  built but when I replaced the transistors I was having a hell of a time getting them not to short against the chassis In fact, just as a quick fix, I've used electrical tape on the screw to see if that would work and it has. Crude yeah, but I was determined to see if that was the problem--it's been a rough ride!!

The shims appear to be in good shape.

The amp is unloaded.

So, wouldn't plastic screws be a good solution for these output transistors??

Thanks for all your help everyone. Is it too early to say I might be getting somewhere?

Thanks Phil!  So some good news. The light bulb is not lighting up!!!!!!!!!!!!!!!!!

70 volts variac, 20 Volts collectors of Transistors 6 and 7 (12 removed, -20 volts on emitter resistors of 10 and 11, 13 removed. -.57 on collectors of 10 and 11.

So, here's my question: Yes, I understand that the collectors can't short out to the heat sink but the heat sink is part of ground, and 6 and 7 aren't shorting and 11 and 10 collectors even if I remove the screws they short out (beep test, so I get a beeping sound). I removed the screws so that I wouldn't get the beeping sound and I didn't so I figured no shorts and decided to power up the amp for a short while to see what I could find. Are 11 and 10 collectors supposed to be shorted  ground??? Do my collector voltage readings on 10 and 11 make sense? they are connected to ground so it makes sense to me:tu: