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These integrated amps of the early to mid 1970s can sound very good in class A. There is a switch on the front to change from class A to "Normal" (class A/b). The owners manual says it's ok to operate the switch with the amp in use.
It's not okay. Flipping the switch changes the rail and bias voltages. It's analogous to driving on a highway at 60 mph and downshifting from 4th to second. You probably won't blow the engine the first time, but do it often enough and you will.
We've had about 20 of these in our shop, and most come in blown because people did exactly what the owner's manual says is okay. We've never had one fail in our shop.
You can flip from class A to class A/B as many times as you want. Just turn off the amp, wait 30 seconds, flip the switch, and turn it back on.
Follow Ups:
It's probably due to aged components, maybe capacitors that have lost value and are no longer able to "buffer" the switching transients. It's also possible the units you're seeing were already defective, and the customer didn't realize it until after they moved the switch. I performed Yamaha warranty service for years in the '70s and '80s. We never saw a unit fail for this reason.
I had a few of those amps back in the 90's. I never played with that switch because I never felt a need to use it. From what I remember they were quite good and had a great sound. Nothing stood out but nothing was lacking either.
nada aqui
nt
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