JMP mod 1987, component between power tubes

[AndersHall]

Hi,

I have an '83 JMP mod 1987, SEMKO version. It has a component between the power tubes I cannot identify - neither through searching here nor elsewhere, or trying to understand what it actually would do.

See images below, and:
- Mounted between the power tubes
- Seems stock, based on the look/age of the mounting and the drilling of the chassis hole
- Connected in parallel with fuse F2, see schematic
- I couldn't fins any useful information based on searching for the codes on the component
- If I removed the fuse, the resistance between the ends of the fuse holder was still 0 (if I remember correctly)

What is it? A small speaker or fireworks module notifying the user if the fuse F2 is blown?

Maybe it's not stock (but I still cannot understand what it would be).

 

[dtier]

It appears to be, and the location suggests a thermistor. They can vary in resistance upward or downward with temperature. It may work with the second mains fuse to allow initial transformer surge currrent to flow around it (smaller fuse) when the tubes are cold. When the tubes are hot the resistance would go high to allow the smaller fuse to protect. This is why they make time delay fuses but this circuit would act faster.

 

[Jon Snell]

It is a thermal fuse that breaks contact at the allotted temperature then resets when the temperature goes back down.
Usually 100°C or thereabouts. The value is stamped on the fuse.
Not shown on your diagramme. Possibly an afterthought ... who knows.

 

[dtier]

That makes sense. The F2 fuse would have to be removed then to make the switch functional.

 

[AndersHall]

Thanks, that was solved quickly! It indeed looks a lot like an older variant of e.g. https://www.sensata.com/sites/defau...s-fixed temperature thermostats-datasheet.pdf

Got curious and just had to open the amp up to check if there’s a temperature marking I had missed previously - and sure, I could see a “L85C”, so that’s it!

Still not sure why there’s a normal fuse in there, as the thermal fuse then wouldn’t work. It really looks like it left the factory in this way - maybe someone later also placed a fuse in the holder. Tomorrow I’ll test to remove the normal fuse and play the amp.

 

[AndersHall]

Just to get back (if anyone later searches for this) and close the thread: I tried the amp without the F2 fuse inserted - and it worked just fine, also cranked. I’ll probably just leave it without that fuse, to allow the thermal fuse to actually work if needed.