1977 Peavey Series 400 Bass Amp Thermal Breaker Question

[twilightofthedogs]

Hi All. I have a few questions about the thermal breaker that is in line with the fuse on this amp. I purchased this amp as non-functional and it gets no power. The thermal breaker (part # 70900087) that is mounted to the heat sink with the power transistors reads open. I assume this is supposed to read as a short unless its tripped, otherwise I don't understand how it would work. To me that indicates that it should be replaced. The thermal breaker is a Elmwood Sensors brand unit with the following numbers on it: 74/A L185 87-100. I couldn't find a direct replacement online, so I am curious if someone could help me understand what those numbers mean so I can find a compatible replacement. Thanks!

[twilightofthedogs]

Additionally, this is an ancient rough and ready amp without a lot of monetary value. how bad would it be to bypass the thermal breaker?

[g1]

Yes, it's a thermal breaker that should be normally closed, in series with the AC fuse (schematic attached).
I would guess it to be 185 deg.C, maybe 7A current rating. No comment on the other question.
(edit, should be deg.F, not C)

[twilightofthedogs]

Yes, it's a thermal breaker that should be normally closed, in series with the AC fuse (schematic attached).
I would guess it to be 185 deg.C, maybe 7A current rating. No comment on the other question.

Thanks G1! And not to be reckless or do things half-ass, I am just not familiar with this safety feature as I have worked mostly with tube amps. Is it a belt and suspenders thing with the fuse? Is it preventing a fire if the transistors over heat? Is it just going to wreck the amp if its not there are is it an actual safety feature? Thanks!

[g1]

Is it preventing a fire if the transistors over heat? Is it just going to wreck the amp if its not there are is it an actual safety feature? Thanks!

I think it mostly stops the power amp from damage if the transistors overheat, but possibly fire prevention as well.

[J M Fahey]

187C is WAY too hot.
Proper is 75C to 85C tops.

Still not sure whether it's a bimetallic one which self resets on cooling or a "one shot" one.
A closeup breaker picture would help.

[twilightofthedogs]

I think its supposed to be 185 F. Pic attached. Considering it appears to be blown I might guess a one shot deal. Trying to post a picture...

[g1]

Yes, 185 degrees F, not C. Sorry about that, edited previous post.
That appears to be the self-resetting type.

[The Dude]

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