Power Transformers With Internal Thermal Protection Fuse

[TomCarlos]

Thanks Everyone....
I truly appreciate the great replies here- absolutely informative.

As for the old PT, I will keep that for "project / testing" purposes. One last questions... since we are talking AC, does it matter which way you connect the thermal device?

Tom

[R.G.]

since we are talking AC, does it matter which way you connect the thermal device?

Not at all.

[jpfamps]

Sorry to revive such an ancient thread, however I've just taken a Fender Blues Junior in for repair where the thermal fuse has blown.

The owner of this amp left it on overnight in his house.

In my view the thermal fuse has done its job, ie prevented a potentially lethal fire.

Incidentally, all the new(ish) Fender amps I see in the UK are wired to the incorrect voltage, which means they all run on the toasty side.

[nashvillebill]

Sorry to revive such an ancient thread, however I've just taken a Fender Blues Junior in for repair where the thermal fuse has blown.

The owner of this amp left it on overnight in his house.

In my view the thermal fuse has done its job, ie prevented a potentially lethal fire.

Incidentally, all the new(ish) Fender amps I see in the UK are wired to the incorrect voltage, which means they all run on the toasty side.

It does seem like a very poor design if an amp cannot be safely left on overnight.

[nashvillebill]

Sorry to revive such an ancient thread, however I've just taken a Fender Blues Junior in for repair where the thermal fuse has blown.

The owner of this amp left it on overnight in his house.

In my view the thermal fuse has done its job, ie prevented a potentially lethal fire.

Incidentally, all the new(ish) Fender amps I see in the UK are wired to the incorrect voltage, which means they all run on the toasty side.

It does seem like a very poor design if an amp cannot be safely left on overnight.

[R.G.]

Sorry to revive such an ancient thread, however I've just taken a Fender Blues Junior in for repair where the thermal fuse has blown.
The owner of this amp left it on overnight in his house.
In my view the thermal fuse has done its job, ie prevented a potentially lethal fire.

Your view is correct.

It does seem like a very poor design if an amp cannot be safely left on overnight.

Your view is correct.

[olddawg]

I would never jumper a thermal fuse in a transformer for a customer repair. But I've done it many times for my own uses and resurrected a piece that would otherwise not be economical to repair and got years more use out of it. Generally I just solder in a pigtail fuse and cover it with heat shrink tubing. I believe a lot of these thermal fuses fail randomly or have a funky duty cycle. They seem to fail for no apparent reason sometimes. In any case. I have never had a problem with a "repaired" transformer. That said, it would not be good practice to do it for a shop situation IMHO.

[Chuck H]

+++1

Can't say why. But internal fuses seem to drift from spec regularly. Failing prematurely or failing to fail. What could the harm be in putting another fuse in parallel!?! If there is a legitimate problem the new fuse will fail. If the OEM fuse was our of spec such that the new fuse holds, you get your amp back!

[jpfamps]

It does seem like a very poor design if an amp cannot be safely left on overnight.

Indeed.

I would speculate that the transformer may well have survived if the amp had been wired to 240VAC rather than 230VAC.

Despite EU harmonization of mains, UK voltage is still 240VAC, a fact acknowledged by Fender on there schematics (although in my experience they fail to wire export amps to "230UK/240V"). This would suggest that the mains transformer is running very close to saturation.

Worse, mains voltage tends to creep up overnight, 250VAC is not uncommon in my workshop.

I always rewire Fender amps that come into the workshop to 240VAC, although I don't leave them on overnight to check whether the transformer will survive..........

[J M Fahey]

Thermal fuses do not burn on current but on temperature, so they are very different from a regular fuse and are meant to protect from another problem.
They do not replace each other.
They are a small metallic tube with a spring loaded pair of contacts held together with a special kind of solder which has a low melting temperature; say, 100ºC (compared to regular lead-tin 270ºC) ; when it reaches that temperature, it opens.
I guess the real question is what happened to that Blues Junior to overheat that much, even *without* playing.
I guess bad/underbiased tubes , or some similar problem.
Those little Fenders are cheap but not known to self destruct, it's not a "typical design problem" for them, the culprit must be found or next time it may catch fire.
Wouldn't be surprised at all if on examination one or both power tubes were redplating.

[Enzo]

Because an amp is left on overnight AND it fails does not mean the amp failed because it was left on. If it cannot idle for 8 hours, then it cannot play for 8 hours.

[bsco]

Very,very good discussion....However, I think that if I have a failed power transformer I will be replacing it with the exact replacement from the manufacturer. And if one can't be found, then I will tell the owner what could possibly happen if an exact replacement in unavailable and you try to either repair or replace with an after-market one...I for one do not need any legal issues at all...so if a direct replacement is not available I will not fix it...If they want it fixed that bad they will have to take it somewhere else...