Can you help me describe the problem I'm having so I can research it?

[Jamez0r]

Hey guys, I'm trying to figure out how to describe the problem I'm having with my amp so that I can start googling and searching for a solution.

I have a Peavey XXX. From what I recall it sounded fine until one day I brought it to my friends house to jam. I plugged it in and started playing and we noticed a burning smell. The burning smell went away after a minute or two and hasn't happened since.

From then on, whenever I play a note that results in a lot of bass frequencies, such as palm-muting an open power chord, the sound isn't smooth. Its like it is choppy. I don't really know how else to explain it. It doesn't cut completely in and out... more like it is oscillating from quieter to louder really quickly. Besides that, the amp sounds okay. No problems with volume level, etc. Also verified that it isn't the cab, by plugging another amp into it.

Like I said, I'm more looking for someone to tell me "oh, I would look up 'oscillating bass frequencies' ", than to actually try to tell me what is wrong with it (although any suggestions would be appreciated). I haven't begun researching this yet because I don't know what to look for!

Thanks guys!

EDIT: I just listened to it again and can verify that the volume when I palm mute and play open chord is OSCILLATING. Based on the burnt smell, and the oscillating power, could it possibly be a blown capacitor in a rectifying circuit? (BTW I know a decent amount about amp theory but haven't had that much hands-on, so I want to gather up as much information before I try to crack this guy open!)

[vavlehead]

"burning smell" = take it to a tech... I dunno what else to say.
If you want to "go for it" open it up and drain the caps and see if you can see the damages.... take pix and post here.
there are potentially "LETHAL" voltages in the chassis... drain the caps or find out how to and then drain the caps.

[Jamez0r]

Thanks for the reply vavlehead. If I'm unable to locate any blown/bulging caps I'll take a couple pics and post here.

Just throwing the question out there - could this problem be in any way due to one of the tubes being messed up? I can't really see that being the problem, but if there is any possibility of it, I should probably test that before taking the whole thing apart. Thanks guys!!!

[Jamez0r]

HAH, just randomly noticed you're from Charleston. I live in Goose Creek =)

Maybe if I'm unable to fix it you could recommend a good amp tech?

Thanks again!

[Euthymia]

Is your XXX a head or combo? If it is a combo, try plugging it into a separate speaker, and if a head, try isolating the amp from the speaker cab.

The idea here is to reduce the possibility that the chunkin' is vibrating a loose connection in the amp and making it cut out. Also, note whether it happens at any volume or just when the amp is turned up past a certain point.

Does the cutting in and out happen if you pound on the top of the amp with your fist?

You had a component burn, and this most often happens to components that are attached to tubes, like screen resistors, plate resistors, grid resistors and cathode resistors. Other components can burn, but it's most likely to be one connected to a tube, and due to the tube pulling too much current through it.

In any case, a component getting hot enough to smell is probably out of value if not completely dead. Your amp needs to go on the bench.

[Jamez0r]

Thanks for the response Euthymia-

The oscillating definitely tends to be worse the louder you turn it up, but doesn't completely go away if you turn it down. The amp is not a combo, and I plugged it into a separate cab that it was not resting on, and it had the same problem (aka not a vibrational thing). Thanks for your insight into where the burnt component could be located!

[vavlehead]

wow... small world.
If I remember correctly the peavey has a separate PCB for the tubes. There should be a ribbon cable connecting the board where the power tubes sit. Lift that cable and see if you burnt lugs or connections.

[Euthymia]

wow... small world...

If you think that's weird, I went to Summerville High School in my sophomore and junior years. Green Wave!

My first rock concert was Foghat and Head East at the Municipal Auditorium before it was the Gaillard.

[Jamez0r]

What a coincidence!

Ok, ill check behind the ribbon cable.

I really appreciate you guys spending the time to help!

[Ronsonic]

FFS,

I plugged it in and started playing and we noticed a burning smell.

Nothing else needs to be said or done until you get it open on a technicians bench.

What the hell else do you want? More stuff burned so it will be easier to find the crater where the original failed part was.

If you're wondering what to say to the tech try "there was a burning smell and now it sounds funny."

Now keep the power off and get it to a tech.

[Jamez0r]

FFS,

Nothing else needs to be said or done until you get it open on a technicians bench.

What the hell else do you want? More stuff burned so it will be easier to find the crater where the original failed part was.

If you're wondering what to say to the tech try "there was a burning smell and now it sounds funny."

Now keep the power off and get it to a tech.

Thanks so much for your insightful advice! Its not like I was trying to locate and fix the problem myself. Nah, I was just posting here for fun before I bring it to a technician.
What else do I want? Oh, just for you internet guys to completely solve my problem for me, without me doing any research at all.
I'm so glad you took the time to respond to my post, thank you Ron.

[Ronsonic]

Sorry bro', there is nothing to research. Often when something burns that burn will continue as long as power is applied. Continued testing just causes more destruction. Not always but often enough that when I smell the burn carrying it to my bench I open it before doing any testing.

[Steve Conner]

Sorry, but like Ron says, any further "research" would consist of opening it up and trying to spot burnt parts, then trying to figure out why they burned and correct it, so the replacement parts don't just burn again.

My guess would be that you gave it a jolt while moving it, this shorted one of the tubes, which then burnt out a screen resistor. With one tube out of action the output power is crippled, and that's why palm mutes sound bad: they need lots of power to make the solid chunking bass.

Unless you're looking to be talked through a repair, you'll have to take it to a tech anyway. And any good tech will ignore your personal hypothesis about what the fault is, they are only interested in your description of the symptoms.

[Prairie Dawg]

"Unless you're looking to be talked through a repair, you'll have to take it to a tech anyway. And any good tech will ignore your personal hypothesis about what the fault is, they are only interested in your description of the symptoms".

What you'll be providing is raw data, the significance of which you may not understand. It's sort of a universal theory of troubleshooting, and it was exactly the same in the aircraft engine business.

[Ronsonic]

And any good tech will ignore your personal hypothesis about what the fault is, they are only interested in your description of the symptoms.

This!

Musicians give great descriptions of symptoms, full of detail and often the emotions they felt when hearing it; many can do a very convincing verbal emulation of the problem, "Brrrpptttfftbbbtt." But they usually suck at diagnostics. And why would anyone expect otherwise.

Tell me what it did wrong and I'll know where to start. Give me your theories and I start from zero. It is very common for a tech to find more and/or different problems than what the customer noticed or thought was a problem. That is why it is very important to tell the tech what was bothering you.

Some guys get good and learn to break it down in a linear sensible way, that really helps to narrow down the problem. But running a smoking amp longer to do it, isn't part of that program.

The worst customer diagnosis I've met was a guy standing in the lobby with his Krell power amp and he's holding a 2W flamepruf resistor that fell out of it saying "this transistor went bad." And wondering / demanding to know why we can't just replace that transistor while he waits.

Actually worse than that was the amp that came in with the complaint "Keeps blowing fuses - even bigger ones." Yep, charcoal inside.