Power amplifiers with selector Class of operation?

mijostyn beat me to it.  I found the lower bias setting on the JC-1s good for background listening and high bias (25 watts) better for critical listening.  A toggle switch on the back of each chassis so very convenient.

Most plinius power amplifiers have a toggle that lets you select class a.

most amplifiers that have had a switch in the bias eventually go full nuclear and fail.

Probably the worst place in an audio circuit to put a switch.

It might take 10 years or more for it to happen and usually does take 10+ years....but consider it a done deal.

Tick tick tick......

You can’t afford to have those contacts in those particular switches to ever suffer any lack of contact, but..eventually they do.

It’s like all switches. They get dirty, and have contact issues, eventually. No exceptions that I’ve ever found. So, logically...

I'm not saying it's a deal breaker, as things should be good for quite a while.... 

This is typically a feature that allows amplifiers which are designed for a high bias current (i.e., mostly Class A amplifiers) to be run at a lower bias, as an energy saving feature.  The low bias setting typically decreases the power consumption and heat output, with the penalty being a decreased output in Class A (before switching to Class AB to provide the rated total power output), and some associated level of sonic degradation (i.e., increased distortion).  In addition to the amplifiers mentioned, some (but not all) Clayton Class A amplifiers include high/low bias switches.