Tube head, blown fuse, why?

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chamelious

www.thesunexplodes.com
Last night at practise my head (Peavey 3120) blew its fuse. About 30 seconds before i noticed the volume had gone right down, when the band stopped i heard it was quieter and sort of crackly, then the power cut out. When i came over to it, the tubes were glowing red hot, and there was a plasticy burning smell. The fuse had blown, and everything was fine when it was replaced. All settings were as they have been for the year i've had the amp, the impedance was set right.

Could this be a power surge or something or could i be looking at a bigger problem?

Thanks for any insight.
 
Was just one tube glowing red, or all four? Were they EL34's or 6L6's? Chances are it's a tube issue, but there are a few other slim chances it's something else. All four tubes glowing red is weird, and likely a bias issue. You may have temporarily lost your bias voltage, from say a bad solder connection that the heat build up inside the head made worse and caused this. One tube glowing red means just that one tube is going bad, and when you let it cool down it seems OK again for now. EL34's seem to have a more spotty QC than 6L6's, and Sovtek seems to be worse than average.
If you have a good tech, have him check all four tubes, and then check the idle Plate current for all four tubes. The bias may be set too hot overall, or one tube may be way too hot when the other three are set just fine. Wouldn't be the 912th time I've seen that, either.
 
I don't actually remember how many tubes were glowing, was a bit of a panic at the time. They are EL34's though. Trying to find someone to take it to although there seems to be no techs in my area, Thanks.
 
ranjam has it right. One tube could be just that one is bad. All 4 ..... almost gotta be some problem with the bias. However, it's conceivable you could have a tube shorting out when it heats up which could affect everything in there including the other tubes.
And something like that is gonna show up again. It really doesn't sound like anomaly to me.
I'm betting it does it again.
 
Did you ever get this fixed? Mine did the same thing pretty much yesterday. I was playing and all was well, then a loud pop followed by all 4 tubes redplating. I instantly turned it off. I really would like to know what is wrong with my amp. There is no tech within 2 hours of where I live so I would like to know what is wrong before driving so far.
 
You may have lost your bias voltage. It's weird to see all four tubes glowing red. That 'pop' could be the bias filter capacitor. That's a long shot, but why else would all four tubes go at once?
 
I know right? Thanks for the response. Ive heard from a few different sources now something is wrong with the bias. Ive heard maybe a bias resistor or something of that nature. Is this a simple/cheap fix for a good tech.
 
It is a simple fix, but many 'techs' aren't efficient at taking a Marshall PC board out, so the time adds up. No biggie. Even if it is a cap, it should still be no more than 1.5 to 2 hours for labor. And that's being generous. If it is just tubes, and no board work is involved, say an hour, especially if the tech has a minimum bench charge. I doubt it is a resistor. They rarely go.
If you can use a DVM, you can troubleshoot it to a point without taking the chassis out. All it might cost you is some fuses. Then you go to your tech knowing what it likely is, and save him time and you money.
 
Were these matched tubes or just random tubes put into the amp? The reason i ask is because every time you change power tubes, the amp has to be re-biased if it's a fixed bias and i believe the Peavey 3120 is a fixed bias (adjustable bias). I know some players like to run the bias pretty hot to achieve their tone but that comes with some drawbacks like shortening the life of the tubes. From what you described, it seems like one of the Power tubes was arching or failed thus the redplating.
 
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