Peavey 5150 and a burning smell.

[Dukoduko]

So here's the deal.

I have a Peavey 5150 combo converted into a head, 3 days ago I got it retubed - 2x6l6gc and 5x12ax7. Fired it up yesterday and after 10-15 minutes I started noticing kind of a burning smell when I got close to the amp. I know this sometimes tends to be the case with new tubes (the logo or just dust/oil/grease on them burning up) but I've never had this happen to me before. Nor did the amp show any signs of this before retubing it.
I tried literally sniffing out where the smell was coming from and stuck my head to the back of the amp (where the tubes are usually located) and the smell wasn't really coming from there, it was definitely present coming from the grills on top of the amp towards the right side, where the power section would be, also it's really warm to the touch on that side, no smoke or any audible noises, works just fine.

Anybody got an idea what this might be ? I realise a burning smell is never a good thing in an amp. I put in a pic just so you can see the are where I can smell the scent coming from.

[Enzo]

I see the vents on the top. Are there vents on the bottom? How can fresh air get into the cab?

[Chuck H]

1) Was it retubed and biased at a professional shop?

2) Did you have it retubed because it lacked performance or sounded bad? And if yes, was anything else done to service the amp when it was retubed.

3) Take the amp into a dark space and fire it up in play mode. Is there any sign of "red plating" or screens glowing? That would be anything other than the filaments in the tubes.

If you had the amp serviced by a shop it's possible some cleaning was done to pot's or other non soldered connections. You may be smelling the last vestiges of whatever product was used evaporating when the amp is up to operating temperature. But...

In my experience when it smells like something is burning it means something is burning.

[Enzo]

This is a combo made into a head. The combo has no bias adjustment.

You might be smelling a cabinet getting hot. Take the chassis out of the cab and run it. Does it still smell burning?

[Chuck H]

Some history might be good. Has the amp been run as a hacked combo without issue for some time? I got the impression it has.?.

All of us always appreciate Enzo's familiarity with these amps. Best man to have on this thread.

[Enzo]

I am just another Bozo on this bus.

[Dukoduko]

I see the vents on the top. Are there vents on the bottom? How can fresh air get into the cab?

Yes , the whole back plate is "net like" and there's way for fresh air to come in. The room in which the amp was is fairly cold and well vented.

[Dukoduko]

1) Was it retubed and biased at a professional shop?

2) Did you have it retubed because it lacked performance or sounded bad? And if yes, was anything else done to service the amp when it was retubed.

3) Take the amp into a dark space and fire it up in play mode. Is there any sign of "red plating" or screens glowing? That would be anything other than the filaments in the tubes.

If you had the amp serviced by a shop it's possible some cleaning was done to pot's or other non soldered connections. You may be smelling the last vestiges of whatever product was used evaporating when the amp is up to operating temperature. But...

In my experience when it smells like something is burning it means something is burning.

1.Yes, I gave it to a licensed technician from a local shop.
2.I got it retubed cause I realized I've got a microphonic tube, and another one very close to it. Nothing else done besides just retubing and that's pretty much it, he checked the bias and said it's a bit hotter ( what is usually the case with 5150 combos and 5150ii) and said that he'll just leave it be.
3.No sign of red plating, besides the filament in the tubes.
4. I talked to the guy just 20 minutes ago, and he said it's most probably because the entire set of tubes is new and it just takes some time, if there's literally no smoke or anything coming out, which would definitely be the case if something was burning or melting , he said to wait it out a bit and if the problem persists, I should have him check it out.
And to answer your other question, yes, the amp played perfectly without these "symptoms" for years, maybe just a bit of noise and hiss when a tube went microphonic.

[Dukoduko]

This is a combo made into a head. The combo has no bias adjustment.

You might be smelling a cabinet getting hot. Take the chassis out of the cab and run it. Does it still smell burning?

The bias hasn't been tampered with and it has never before given any of these symptoms, so I kind of doubt it's the chasis.

[Chuck H]

Well I only have one observation at this point. Maybe Enzo can chime in on the particulars if I'm wrong.

"I" (personally) don't have a familiarity with these amps (though I did own one for about a week in my youth). But my general understanding of high gain preamp/master volume designs is not that the power tubes would be biased hot. If the amp IS biased hot AND the bias was not adjusted, as observed by your repair tech, it's possible that the amp is biased hotter than before the tube change because the new tubes may be drawing more current than the old ones. This may be cooking and browning the logo paint on the tubes. I did have a Marshall that didn't red plate but did brown the logos on a set of Groove Tubes anyway. I'll just say that since designs like the 5150 are usually run below their maximum volume that the overdrive characteristics of the power tubes as affected by the bias is rarely an issue. Unless you actually are cranking the amp up to get some power tube overdrive the hotter bias shouldn't be affecting the tone much. And even then a hot bias isn't necessarily better sounding. I'd suggest having the bias adjusted so it's not "a bit hotter". Your tubes will last longer and the amp will run cooler.

[eschertron]

(snip) I'd suggest having the bias adjusted so it's not "a bit hotter". Your tubes will last longer and the amp will run cooler.

Quite the windup for your pitch, there, Chuck!

[Chuck H]

Quite the windup for your pitch, there, Chuck!

I know. I just wanted to be clear about my reasoning because there's a lot of info on line about running amps hot. Sometimes arbitrarily as would be the case for a 5150 if it's never pumped up to clip the power tubes. Since my suggestion may be contrary to some internet lore I felt it deserved an explanation.?.

[Leo_Gnardo]

This is a combo made into a head. The combo has no bias adjustment.

You might be smelling a cabinet getting hot. Take the chassis out of the cab and run it. Does it still smell burning?

I like this theory best. Hot air rising from the bank of output tubes could be baking the wood in the new cab's "ceiling" above the tubes. You might consider stapling a square of aluminum flashing onto that area overhead the output tubes. Also - handy rectangles of aluminum are available at hobby shops, and I've noticed them for sale at Home Despot as well.

If there are stickers on your output tube bases, showing the vendor's logo and/or test parameters, these may emit a stink as their stickum warms up.

[Dukoduko]

Just an update, after closer inspection ( I brough the amp into a completely dark room ) seems like both the power tubes are redplating, I guess the bias is too high for them, gonna get it to my amp tech asap.
Thanks for all the help guys! Especially Chuck H for proposing me to check it.

[The Dude]

The 5150's, like many other Peavey amps, are biased on the cold side. The sound/distortion/gain on these amp comes from the preamp. Peavey doesn't normally drive the power amp into distortion.