Peavey 5150 combo

[SteveJ]

Hi all,

A guy brought over this amp and it had a noisy preamp tube in V1 so I swapped it out and played it for a few minutes and all was okay. I ordered 5 tubes to replace the originals per his request. Plugged them in tonight and turned on the amp and there was no sound. About 30-45 seconds later a capacitor popped and we had smoke. I had previously pulled the chassis to look it over. I did notice the 470 ohm 10W resistor (R86) had heated up the power connector next to it where it was starting to melt.

What I found was the 470 ohm (R86) had blown apart, C48 100uf 50V popped and the protection diodes appear to be bad. I'm just wondering what the heck happened. When I put the preamp tubes in I did reach under the chassis and put them in by feel. I didn't have any bent pins after I checked and all tubes test good including the power tubes. I check them on a Hickok after the failure. I'm almost positive I had the speaker plug inserted good but maybe it wasn't all the way in. I may have got in a rush and made a stupid mistake.

Just trying to figure out what happened. Anyone run into anything similar before? Maybe it was going to fail anyway. Any advice is appreciated.

Thanks,
Steve

[Jazz P Bass]

Sure would like to help here.
Can you post a schematic so we know what your talking about?
I do not know R86 from C48.
What circuit are they in?
Even with the schematic, it simply is not fair to ask anyone to search through the entire schematic looking for R86 & C48.

[SteveJ]

Absolutely. It's the small board that the power tubes are mounted on. This is the output section of the amp. Might also add that no fuses were blown.

[Jazz P Bass]

It appears that one or both 6L6 output tubes are bad.
R86 is the shared screen resistor.
C48 is attached to the heater circuit.
There is no telling exactly what went wrong.
My guess is the plate voltage arced internally.
I may be mistaken, but I believe the output board is held on with pop-rivets.
They will have to be drilled out to remove the board.
Or you can remove the components on the underside & reinstall good parts on the upper side.

[Chuck H]

+++ It's a cpoincidence that a power tube happened to fail. Perhaps a result of the testing procedure. If you cranked it up, as you may when testing, this could have done in a marginal tube. The owner may never do that.

I didn't look at the schem but I can't think of any reason preamp tube replacement should cause a power tube to short to the heater. Even going through some of the odd DC preamp ideas that get used it doesn't make sense.

[SteveJ]

Thanks guys. That makes since. I'll get some new 6L6's and drop them in. Jazz you are right about the board being riveted. I drilled it out last night and cleaned up the mess. I was able to get the resistor and cap locally but couldn't get the diodes. I guess I could series a couple of UF4007's. Think that would be adequate?

[Jazz P Bass]

It's not so much the fast recovery, as it is the high voltage that the diode can take.
I think the diode peavey uses is good for 3000V.
I have seen repairs come in , from previous techs, that have three 1N4007 didodes in series.

[Enzo]

One way a power tube "shorts" to the heater is when the center post breaks off and someone installs the tube anyway, and gets it in the wrong position. The heater is a dead short from pin 2 to pin 7, depending on what position the tube goes in it shorts various things together.

[SteveJ]

One way a power tube "shorts" to the heater is when the center post breaks off and someone installs the tube anyway, and gets it in the wrong position. The heater is a dead short from pin 2 to pin 7, depending on what position the tube goes in it shorts various things together.

Hi Enzo,

Thanks for the response. I did check for that. That's the first thing I thought of was that I somehow cross some pins. The guides are there. I also checked all of the tubes for shorts in my Hickok 600 and they checked okay. I'm going to check those 6L6's again. I know the Hickok don't put much voltage on them but I figured it would pickup a short that bad. If they test good I'm going to drop them into a single ended test amp I built to see what they do. Regardless I'll have a new set of tubes tomorrow to drop in. The whole thing kinda of puzzled me when it happened. I had to post in here for a reality check. I'm thinking at this point it may be just a coincidental tube failure but my first reaction was WTH did I do. I'm going to get back on it tomorrow, the grand babies kept me busy after work today.

It's not so much the fast recovery, as it is the high voltage that the diode can take.
I think the diode peavey uses is good for 3000V.
I have seen repairs come in , from previous techs, that have three 1N4007 didodes in series.

Yeah I agree with that Jazz. Trainwrecks also use three in series and it seems to work well. I'll go with that. -Thanks

[SteveJ]

I just had a chance to recheck the 6L6's. I tested them again in the tube tester and then I tried them in an SE amp. Both tubes are okay. Also checked the primary of the output transformer by testing each lead to ground for shorts. None present.

[SteveJ]

Just an update. I replaced the faulty components and reused the power tubes after testing them again and it works like a champ. Only played about a half an hour on it, I'll kick it a bit harder tomorrow. I not sure what caused the problem but I think I'm good for now. Thanks for the help.

[Chuck H]

Sometimes that can happen too. More often there is an underlying cause but sometimes a marginal component just fails and takes a couple of others with it. My only diagnosis was a power tube failure. If the power tubes are working then I'm wrong but can't figure what else might cause the problem besides a gross short. You'll know soon enough if it fails again.

[SteveJ]

You're absolutely right Chuck. I'll crank it up good tomorrow and let it burn for a couple of hours. I've got extra parts and plenty of pop rivets just in case.

[cornchild]

FIRST LET ME SAY THIS IS A GREAT SITE , MY FIRST TIME HERE. I also have a 5150 combo, I bought it used about 3 years ago and do not know the background of it. i had used it for about 20 minutes since i bought it and it sounded ok then it started humming pretty bad. I shut it down and that was about 2 years ago. I have 4 other amps. last week I bought new tubes. 4 electro harmonix 12ax7 and 1 groove tube gold series gt-12ax7R and two groove tubes ST-6L6C silver series. I replaced all the tubes being careful of pin position and tried it again. I first plugged into the lead channel turned it on and played for about 5 to 10 minutes. it sounded pretty good but a bit of a hum when guitar volume turned low. Then i shut it off, plugged into the rythm channel and played for about 5 minutes, it sounded good. then all of a sudden the amp started humming super loud and a burning smell came out of it and i shut it off. I would be grateful for any suggestions and comments. I am not an electronics expert and was wondering if someone could suggest how to learn about diagnostic and repair. I didn't just fall off banana truck I have a lifelong mechanics, metal fabrication and expert boat and fabrication and outboard rebuild and repair background.

[Chuck H]

Well, most of passive electronics is very mechanical in nature. I'm sure you'll be able to pick up on it, but... There's an awful lot to know and even more to experience, likely years, before developing diagnosis and repair skills. A common mistake many novices make when repairing amps is to simply replace the burned up parts. As it happens with most amps, parts usually burn because other parts went bad. This kind of mistake can become very time consuming and costly since replacing a burned part without fixing the root problem can often cause additional failures. In other words...

If you'd like to learn something about electronics NOW because you'd like to fix this amp NOW, that ain't happenin'. But, the problem may be simple enough that with some investigation and inspection you can fix the amp. Possibly with help here. And the electronics bug may bite you. By your resume I'm thinking it will. So...

Open it up and drain the power supply. Search here and abroad about this and other safety measures that pertain to electronic repair and tube amplifiers. This is important because high voltage and current capacity, especially in tube amplifiers, can kill you deader than shit. In fact I think the 5150 power supply drains itself, but not all amps do and it's an important ritual to follow EVERY TIME YOU OPEN A TUBE AMP.

After draining the power supply, inspect the guts for obviously burned components. Try to locate where the bad smell originated. (hint 1) Some of those small and large plastic coated aluminum cans (electrolytic capacitors) are what I'm suspecting from your symptoms. Inspect them and the circuit board around them for anything that looks unusual, ie: bulging ends, fluid leaking out, charring or browning on the capacitors or circuit board, etc. And repost back here. (hint 2) Groove Tubes uses a special paint to mark their tubes. If they get too hot the red color will turn brown. It won't always mean that the tube has gone bad, but it is an indication that the tubes are getting too hot. So inspect this too. I suspect your power supply or bias supply (or both) are in need of new capacitors. Electrolytic capacitors have a life of 10 to 20 years on average and this can be shortened if the amp is not in use.

PS, To get the most views, and therefore help with your problem, it's best to start a new thread rather than piggyback an older one that no one has posted on for five months.