Ampeg B2R Protection Engaging

[lowell]

This amp came in. After about 10 minutes at 50 watts power the speaker relay shuts off the output. I checked the MOSFETs bias. I got ~5mv with no signal and no load. I adjusted this per the schematic to ~25mv across the source resistors.

My question. Does a cold biased amp cause overheating because of crossover distortion? Is it safe to assume this was the issue?

[lowell]

Ok so it's still shutting off signal via the relay even after rebiasing the MOSFETs. The heatsinks are getting to ~60deg C. Is that normal?

[52 Bill]

Ok so it's still shutting off signal via the relay even after rebiasing the MOSFETs. The heatsinks are getting to ~60deg C. Is that normal?

You need to figure out what is causing the shutdown, voltage on the speaker bus or temperature.

If the amp was sounding good at the lower idle bias, I'd want to go back and reset to a lower figure.

[Jazz P Bass]

Q23's job is to turn the relay On.

Everything else in that circuit is trying to turn it off.

Page 3 E/F- 5/6

http://music-electronics-forum.com/a...8-ampeg-b2.pdf

[lowell]

Ok sounds good. I see how the protection circuitry works...LM35 is the temp sensor and I see the DC feed that turns Q21 on/off. I'll monitor them. Thanks!

[lowell]

Ok so I began by seeing why the fan wasn't coming on. The thing is shutting down, and is quite hot, so the fan should be working. I checked the voltages on the fan. Someone installed the fan clip backwards. Flipped it and now it's on and working...ferociously. Gonna let the thing cool down, then bring it back up and see if it still shuts down.

[lowell]

Ran it at about 75watts into 4ohms for 30min. No issues. Thanks all!