Peavey Classic 20 Thermal fuse?

[LarBal]

Had a Classic 20 come in the other day with an awful buzz, the OT protection diodes were blown, and owner had been running an 8ohm speaker on the 16ohm OT forever. Related? Possibly. Nonetheless I put 3 1n4007 in series and the amp was fine.
Owner picked it up, emailed me hours later thanking me for the work, and how his amp sounded awesome(I also replaced those JJ EL844 which I think sound horrible anyways, with el84s), and that he had just been playing it "for hours".

He then texted an hour later and said it didnt work, piloty light powers but no tube glow.

So I got it back here, and confirmed all internal fuses good, 120VAC into power Xformer good, with secondaries disconnected from board I got nothing. Then I read about the thermal fuse in the transformer. So whats the best course of action to get this running again? If I replace the fuse , wont the same thing happen ?

Edit, right now with the primaries disconected, I'm getting open reading across them, but I'm not sure how the thermal fuse is arranged in there

[nosaj]

Being a customers the only course of action is a replacement transformer. liability( if you replace the fuse and the amp burns down the house guess whose possibly gonna be on the hook. (I only recommend changing the fuse on my own stuff)But something else is wrong. was he still using same speaker setup?

nosaj

[LarBal]

Being a customers the only course of action is a replacement transformer. liability( if you replace the fuse and the amp burns down the house guess whose possibly gonna be on the hook. (I only recommend changing the fuse on my own stuff)But something else is wrong. was he still using same speaker setup?

nosaj

Right. I figured there had to have been something else that developed that caused the thermal to open, but then I read that sometimes they'll open after being on "for too long" but that sounds silly.
I strongly advised when I gave it back to him the first time, to put a 16ohm speaker in there, and he did.

I guess I don't understand why the thermal fuse is there. I mean, I understand what it does, but wouldn't mains fuse protect the building it's operating in, if whatever failure mode opened the thermal fuse happened without one?

Next question, how would I go about figuring out what else is wrong, safely, that opened the thermal fuse, without said thermal fuse protection? Barring an obvious visible fault ...

[nosaj]

May have opened by being previously stressed.
nosaj

[LarBal]

May have opened by being previously stressed.
nosaj

So what is my course of action here? Quote him a new PT ?

[nosaj]

I would and then I would burn it in for several hrs.

nosaj

[J M Fahey]

I guess I don't understand why the thermal fuse is there. I mean, I understand what it does, but wouldn't mains fuse protect the building it's operating in, if whatever failure mode opened the thermal fuse happened without one?

Not the same thing or action.
Plain fuse is sensible only to *current* , sometimes (as when just *one* secondary is horted) main fusen wonŽt blow, but part of the transformer will overheat, melt and catch fire.
To boot, mains fuse is normally user replaceable, and we allknow what that means.
Thousands of horror stories about car/bike oversized fuses or plain wrapped cigarette paper, rating 1000A.

Thermal fuse is strictly thermal, and better matches to plastic bobbin melting or copper wire insulation breaking down.
They do have a maximum current rating, typically 10A which is a lot, actual current breaking is left to the other one.

Next question, how would I go about figuring out what else is wrong, safely, that opened the thermal fuse, without said thermal fuse protection? Barring an obvious visible fault ...

After PT replacement I would turn amp on and *carefully* watch it for an hour or two, keeping an eye open to tubes redplating, main caps overheating (burning fingers temperature), PT overheating, OT and choke (if present) overheating, etc.

IF something is abnormal, post it here ; you canŽt just replace the PT and pray or cross fingers.

[Enzo]

Why would something have TWO protective devices? Why would a car have air bags, when it already has seat belts?

The two kinds of fuse do different jobs.

[LarBal]

Ok thanks a bunch for the explanation guys. Owner decided to cut his losses. He also tells me that the guy he bought it from had some crazy configuration where he was running 3 or 4 different speaker cabs with this amp, simultaneously, somehow, and was very proud about it .
So who knows what other horrors this was subjected to....

For the sake of discussion, why don't all PTs have a thermal fuse?

[Helmholtz]

A thermal fuse is required when a PT cannot safely handle any overload/fault condition of a given amp.
A bigger PT or one using higher temp rating wire insulation would have more safety headroom and might get safety approval without one.

[Enzo]

And many many DO have them, you just are not aware of them.

[LarBal]

Ok great, thanks. So basically a cost cutting device

[J M Fahey]

Not at all, a SAFETY device.

It saves you not one cent, in fact itself costs at least a buck or two, in bulk, maybe more over the counter, definitely WAY more expensive than a regular (current) fuse.

YouŽll find them hidden inside MANY transformers, *definitely* inside microwave transformers, also inside many electric motors, plus anything which works by heating, such as sandwich toasters, grills, coffee machines, electric blankets, heating pads, t shirt thermal transfer presses, hot plates, cloth pressing irons, etc.

https://fuse-china.en.made-in-china....rmal-Fuse.html

[LarBal]

My bad, I didn't mean to diminish the value of the fuses as far as safety goes, I was just suggesting that from a manufacturing standpoint, if the transformer itself had been overbuilt, perhaps there would be less of a need for the thermal fuse...
But I see now, I suppose any transformer, no matter how well built, can get so hot that it causes a safety hazard. Thanks!

[Mick Bailey]

Don't assume that because the primary is open that the thermal fuse has blown. I've had a few transformers where the fuse has been OK and the winding is open elsewhere and this can give a clue to the failure mode.