Peavey Special™ 112 Solo™ Series

[JC@]

I found something rare to me.
On this amp the thermal braker placed by the power transistor's is connected in a way that it's action is directly on the mains circuit. That is, when hot cuts the mains.

The fact is that if you tap on it, its resistance stops being zero so there's a horrible scartchy noise on the speaker. Maybe I should just bypass it. Too wrong? Or should I do the right thing and find one that is a bit more stiff and less sensible to vibrations?

[Rcubed]

A lot of earlier (~1980) PV amplifiers used that connection. Those thermal breakers only have one contact so it's easier to just pull the pin on the mains to cool things down and cope with various faults.

I'm not aware of getting crackles when you bash them, although it depends how hard you bash them. They use a bi-metal plate which pushes a set of contacts so it is possible that a hair-triggered unit might disconnect the power. Perhaps you need to replace it.

[Enzo]

First, that is a safety feature and not a good idea to remove, though during testing you certainly can bypass it. It prevents fires and also protects the power transistors from self-destructing.

Keep in mind that when you tap it, the vibrations also affect other parts in the area. Dismount the thermal and suspend it by its wires. Leave it connected. NOW tap on it - be aware of the voltage please - and see if it was really noisy or not. And ALSO tap on the chassis where the thing used to be mounted just to make sure nothing else was sensitive.

[JC@]

Originally Posted by Enzo

And ALSO tap on the chassis where the thing used to be mounted just to make sure nothing else was sensitive.

It may have something else but that's more or less how I tested it.
With the amp unpluged and the DMM set to ohms and connected to both terminals of the sensor I tapped on it with a wooden tool and the reading went from zero to something else for a moment then back to zero. The DMM is a fast fluke.

I will test it with the sensor bypassed to see if there is any other sensitive part on the board, that's a good tip.