Peavey CS800 Power Amp

Peavey CS800 Power Amp

Just in case you thought we weren’t an equal opportunity repair place we fix a lot of solid-state gear as well! I happen to think these are great amps. They are workhorses, take loads of abuse, hold up really well and sport huge beefy power transformer.

Typically one side is out. Usually shorted output transistor related and I suspect usually because they become so caked with a mixture of dust and soot that they go into “thermal runaway” because the transistor device can’t dissipate its heat. This can be one, two, or all of the output devices. Usually we’ll pull them all out and start looking at the driver section, sometimes they’re shot too. Transistors fail in microseconds so it is not uncommon to see what is known as a “catastrophic” failure, meaning that something of a domino effect took place. 

One shorted output took out 7 others, the entire driver section, two opamps, a handful of diodes and a couple of emitter resistors. Maybe a temp sensor and a Triac as well!


In all fairness to Peavey; this is not a CS 800 thing.
This could be any direct-coupled solid-state power amp. 

Here’s another prime example:

The circuit topology is basically the same in all of them and the failure results are often the same in all of them. Regardless of how much nifty protection circuitry they put in there! We prefer to replace all of the output transistors when one has failed, as it is impossible to predict when the next one will go and do all that damage again! Also, it can be difficult to detect leakage (before it’s already taken it’s toll) under loaded conditions, which can also be disastrous.