I need some help ASAP from all of you amp mechanics out there.

I have a peavey 5150 combo that die on me tonight during our last practice before the big gig in a few days. I had it setup like usual, on the lead channel....about 6 on the pregain and 3 on the post.

Everything was normal then all of a sudden the amp got super loud and brittle sounding. I walked over and turned the post lead (volume) all the way down to 0 and it was still loud at hell. I had to turn the pre gain down to 0 before it would qiet down....and even then it didn't quiet down 100%.

It's like that on both channels. Even though the post gains for both channels are on 0....it's acting like that are all the way up. I tried turning the amp off for a minute and then back on........the controls all acted like normal for about 30 seconds...and then it got loud as hell and brittle sounding again. Both channels.

I looked at all the tubes (new 1 week ago) and they all looked fine. I then pulled the chassis out of the amp and checked everying. No burt smells of signs of anything melted or burnt.  Just to check....I put all of my old tubes back in the amp....and the same problem is there.  

Any ideas as to what the problem is? I hope I can fix it before the big show!

If there is any more info you need....or any tests I can do to narrow down the problem, please let me know.

Thanks

Find a music store with a tube tester before you try to power it up. A buddy of mine repairs and builds custom tube amps, he says that if you have problems, have the tubes tested. This will usually solve 70-75% of the problems. Also, Be prepares to buy new tubes.

cajunboy2k

Sounds like a pre-amp tube burn out.

<reminding self why I don't use tube amps live anymore>

I put the old set of tubes back in (that were working just fine) and I still have the problem.  Unlike makey tube amps, the Peavey 5150 combo does not have a bias adjustment.  It's set from the factory.  

Quoted: Check your bias voltages. If you don't know how to do it find someone who does.

+ check filament voltage

Quoted: Quoted: Sounds like a pre-amp tube burn out. +1 In my experience if it didn't pop followed by all kinds of neat fireworks and smoke, its as simple as replace a tube. Tj

+1

If a replacement tube set didn't fix the problem, start looking for charred resistors under the tube sockets. Then look for any discolored components, including capacitors. It does sound like the preamp circuit or a power supply problem since it is unrelated to the audio output circuits according to your description. Open filter capacitors in the power supply could put some distortion in the output circuits....

Good luck.

Sam  

If a replacement tube set didn't fix the problem, start looking for charred resistors under the tube sockets. Then look for any discolored components, including capacitors. It does sound like the preamp circuit or a power supply problem since it is unrelated to the audio output circuits according to your description. Open filter capacitors in the power supply could put some distortion in the output circuits....

Good luck.

Sam

On edit: Possible leaky filter capacitor since it lasted for 30 seconds before the hum started again. Note: I've never worked on an guitar amp but did work on tubed test equipment more than a few years ago... Ok, WAAAAY more than a few years ago.