Peavey Classic 30-Power transformer thermal fuse/limiter?

[1ampman]

Greetings to All, My friends amp died in the middle of a gig on a hot day. I have ACV going to the primary, but zero out of the secondary. I've read about thermal fuses being wound into the pwr xmer. Are they easy to find? Has anyone dug one these out and by-passed it? Aren't these supposed to reset them selves? The regular fuses are good. The power lite indicator come on, but no voltage anywhere on the amp. So I'm asking if I should look for thermal fuse? Is it possible to jumper? Or do I need a new pwr xmer?....

[Enzo]

First, go right to the primary wires of that transformer. Measure resistance. Is the primary showing open?

Thermal fuses do not reset. They are just fuses.

The thermal fuse is inside the transformer, buried in the windings. Sometimes we can dig them out and replace them. You can bypass them like any other fuse, as long as you no longer want the protection the fuse provides.

[1ampman]

BINGO! Found it ! And yes it was bad. Here's some pics.... Huh, my files are too big?

[1ampman]

Thankyou Enzo.............I was lucky as I saw a bulge in wrapping tap and decided to go for it....As you can see it jumpers the winding...Should I just jump it or put pigtail fuse in? It has a fuse on the AC side. (Whole amp).

[glebert]

It has a fuse on the AC side. (Whole amp).

The thermal fuse on the transformer has a somewhat different job than mains power fuse. For me I am OK with jumping the thermal fuse in order to troubleshoot the amp and see what else might be wrong before spending the money on a new power transformer. I wouldn't sell an amp that had the thermal fuse bypassed though, too much liability for me.

[drewl]

Thermal fuses are available to restore the safety they provide.
Another reason I was pissed that Radio Shack went out of business because they always had them in stock for quick repairs.
I bought like a dozen before the local one closed.

Fixed the coffee maker for my boss which needed one.

[J M Fahey]

Glad you found it.

Your pictures show many things at once.

1) yes, that´s the thermal fuse.

2) they made it *easy* to replace, they put it outside the winding (it would work better if it were smack in the middle of it) and to boot even provided its own handy terminals.
Both of course addressing their own ease of manufacturing, not your convenience.

3) I am surprised at the amateurish bulk winding, with some wires crisscrossing others.
This wastes strictly limited window space so it forces you to use thinner wire gauge than optimum, both signs of cheap manufacturing.
Would never had expected that in a Peavey, but something like this instead:


4) I am even more worried because they used the dreaded "self stripping/self soldering" wire.
Its enamel *evaporates with no residue left" at 130C leaving shiny copper behind.
Fine in a $2 wall wart because it saves precious seconds in assembly, you don´t need to strip wire, just apply molten solder, it has NO business whatsoever inside a Tube guitar amp ... yet I have seen many (including Messy Bugger) do the same.
Marshall was plagued by a series of shorted output transformers while older (60/70/early 80's) ones are still alive, surviving many "no speaker connected" stress situations.

5) which means you *do* need to replace the proper thermal fuse, exact same rating.
Picture if possible or anyway write here the rating printed on one side; I would expect something like 100C or so, in no case higher than 130C ; if it had a higher rated one "downgrade" to 130C tops for safety (which is sizzling hot in any case, water boils at 100C).
You can get them at Mouser.

6) a pigtail fuse won´t do, current is not the problem here (they are usually rated to 10A) but *temperature* ... which is printed right on the body.

This one for example is 184C :

[Enzo]

Last I looked, they had them at Mouser and similar.

ANy number printed on the thing? As you might suspect, they are rated for degrees rather than amperes.

[1ampman]

Thankyou everyone.... This is uncommon for me. I have heard about them and read some info, but this is a first......Cheers!

[Enzo]

Well, y'know, everything is new the first time you see it.

[Tom Phillips]

Your other required task is to determine if the Transformer itself is good. If a winding shorted and caused the Transformer to overheat then a new thermal fuse won't fix the Transformer itself. This is a test you can do without reinstalling the transformer in the amp. If it were on my bench I would temporarily jumper the thermal fuse and apply voltage with a variac slowly turning it up from 0V and watching to determine if the Transformer drawers excess current or overheats. If the Transformer passes that test then you can proceed with purchasing and installing a new thermal fuse. Otherwise it's toast and you can go straight to the task of buying a replacement power transformer.

If you don't have a variac then you can install a standard fuse in series with the primary before you power the Transformer. Just use the same size fuse that the amp uses. If the fuse blows when you apply power then it indicates that the Transformer has failed.

For both these tests all the secondary windings should be disconnected.

[pdf64]

Or power up via a light bulb limiter.

[1ampman]

I checked everything with my ohm meter and all is good , so I jumped it and the amp works fine. I beleive this is a newer version of the Classic 30. My buddy loves this amp w/ a foot switchable boost...I feel a clip on fan would be good for the whole amp as they run HOT....

[nevetslab]

A very lucky bit of surgery. All too often, that thermal fuse is buried below the top layer or two. The better transformers have resettable thermal breakers, which I always had specified in OEM transformer designs. Nothing more irritating than having a transformer overheat from being pushed hard, and NOT recover when it normally would.

I had set aside a power transformer from a Fender Deluxe Reverb, with the intent to pull the end bells and see if perhaps they too had placed it under the top layers, under the wrappings.

BTW, those thermal fuses are the same culprits that cause your coffee maker to fail. I've kept many a cheap coffee maker going for years, dealing with those thermal fuses and hi temp silicon rubber hoses that crack and flood your kitchen counter.

[J M Fahey]

I checked everything with my ohm meter and all is good , so I jumped it and the amp works fine. I beleive this is a newer version of the Classic 30. My buddy loves this amp w/ a foot switchable boost...I feel a clip on fan would be good for the whole amp as they run HOT....

Cool, but given the low temperature wire it uses, please replace with same temperature thermal fuse.
Your transformer is working *only* because that thermal opened ... next time it will not toast or ooze varnish (by the way, *what* varnish?) but plain turn into a solid block of copper joining Hot to Neutral, with ... um .... "interesting" Fireworks.