Peavey Classic 30 II 2018

[Enzo]

I think the thermal fuse has a amperage rating so you know how much current it can conduct safely.. Thermal fuses don't blow from current, they blow from temperature. So the mains fuse amp rating and the thermal fuse amp ratings is apples and oranges.

[Chuck H]

I think you are saying a 1A fuse will blow before a 5A which makes the 5A sorta useless in protecting the transformer?

The mains fuse is never there to protect the transformer. It's there to prevent situations dangerous to people. Many transformers go bad in amps that don't blow the mains fuse. As Enzo noted, the mechanistic action of the thermal fuse is, well, thermal. Making the current rating for the part moot provided it's not under rated for the circuit. This is much more likely to save a transformer.

[g1]

I think the thermal fuse has a amperage rating so you know how much current it can conduct safely.. Thermal fuses don't blow from current, they blow from temperature. So the mains fuse amp rating and the thermal fuse amp ratings is apples and oranges.

But at some point above their 'safe' rating, they are going to blow aren't they? In this case, installing a 1A thermal fuse in the PT primary of a unit that is running at more than 1 amp is going to be a problem I would think. For all the trouble of changing out the thermal in a PT, I would want to be pretty sure it is not going to open for any reason other than over-temp ?

[g1]

Making the current rating for the part moot provided it's not under rated for the circuit. This is much more likely to save a transformer.

My take is that a 1A part is quite under-rated for the circuit.

[The Dude]

My understanding is that the current rating is more or less a "self-heat" rating. In other words, if the current through the device is at or near the max device rating, it will heat by itself without influence from the part it's protecting- motor, tranformer, etc. It is heat that makes the thermal cutoff go open, but too low of a current rating will add additional heat, which may or may not be part of the design. In this case, I'd have no problem using a higher current rated part. If the transformer gets too hot, the thermal fuse will still open. There is an additional mains fuse to take care of overcurrent.

Edit: The only thing you might want to check is the physical size of the higher current rated cutoff to make sure it will fit. I have not checked, but look at the datasheets.

[Chuck H]

but too low of a current rating will add additional heat, which may or may not be part of the design.

True! And a smart observation I hadn't considered. And as you said, likely based on the temp rating on the thermal fuse in question, the current rating is probably not part of the heating considerations for the thermal fuse in this case.

[Oldmactech]

Advice please?

Ironically the replacement thermal fuses I bought ( 250v, 1A, axial, 145degree C)
Panasonic EYP- 1BF145.) arrived on the same truck as the replacement transformer I ordered from Peavey.
Original fuse was 134degrees but is obsolete where I looked so 145 C was recommended.

I’m about a C- in my understanding of this but i can handle soldering in the thermal fuse. I’m mostly curious to see if I can’t “fix it” with the fuse but simply putting in the new transformer is probably the simplest solution.

Is there a downside I am not considering to replacing the fuse? Do I risk additional damage to the circuit by doing so?

Thanks.

[Helmholtz]

The new fuse would allow for a 11°C higher winding temperature. That might be too much for the wire insulation used. Consequence could be winding shorts and smoke in case of an amp failure that causes high transformer current.

In other words the PT/ amp will probably no longer meet safety requirements.

[Oldmactech]

Thanks, so means I risk damage to Transformer and or fire? Or other parts of the circuit also?

[Chuck H]

Thanks, so means I risk damage to Transformer and or fire? Or other parts of the circuit also?

Well, not as a direct result of the higher temp thermal fuse. But in the event that some fault happens in the amp that causes the transformer to heat, then maybe. I don't like thermal fuses in transformers. More often than not they blow before the transformer is jeopardized and then you have to add replacing a transformer to what might otherwise have been a less expensive repair.

You could always use the new transformer for the repair just so you know you practiced vigilance. Then repair the other transformer for use in a personal project.?.

[Helmholtz]

I'm sure no one likes thermal fuses in transformers, including transformer and amp manufacturers- as they increase cost.
Rather they are forced by product safety standards.
There is no guarantee that a different replacement transformer with or without a thermal fuse complies with safety regulations in a given amp.

[Enzo]

A thermal fuse might open before a transformer suffers damage, but if that same transformer had no such fuse, the failure mode that heated the thing may well have gone unchecked and the transformer would have then burned up.

[Chuck H]

A thermal fuse might open before a transformer suffers damage, but if that same transformer had no such fuse, the failure mode that heated the thing may well have gone unchecked and the transformer would have then burned up.

Or not. For the purposes of repair your replacing the transformer either way. Less often the case if no thermal fuse was stuck in there. But as Helmholtz mentioned, it's a probable design/liability matter and the manufacturers don't like it any better than we do.

[The Dude]

I don't usually even suggest taking apart a transformer to replace the thermal fuse. That's not to say I'm suggesting you shouldn't do it either, but at your own risk. I've told the story here before, but a tech who is a friend of mine once bypassed a PT thermal fuse on a home receiver. Shortly after, the customer's apartment burned as a result. The tech was held liable for damages. Of course, bypassing the fuse is different than replacing it, so I'm not sure about the liability in that case. I suspect, however, that it wouldn't matter if it's seen that the transformer was cut open in any way. There's also the possibility that a fire would destroy the evidence. At the very least, I wouldn't detail that sort of repair on any service invoice.

[Jazz P Bass]

Most thermal fuses are not solder connected.
They are crimped.