SOLDERLESS OPTION - PC boards come fully assembled and tested. just screw it together and be playing music in super quick time..... Free delivery within Australia.

Technical Blog


Circlotron Amplifier

For years I've been mucking around with valve amplifiers, much to the amusement of my family and friends. My first attempt was a version of what's called the Murray amplifier; a 20 watt per channel design by Australian Cyril Murray in the 1960's, which used 6CM5 television line-output valves. I used PL509 valves for a bit more power. Cyril Murray went on to build amplifiers for the Sydney Opera House and his work was very well respected. 

Later I made a 60 Watt 'circlotron' design using four triode connected KT120's in each channel. The circlotron is a fully balanced cross-coupled power supply design which has the advantage of a very low output impedance; enough valves and enough negative feedback and it will drive a loudspeaker directly without an output transformer. I used a fairly low turns ratio output transformer mounted at the loudspeakers - some voltage isolation for my expensive VAF-i66 speakers. Using an output transformer also requires less output valves, less or no negative-feedback, and no need for expensive speaker cables. See the photo below. 

I still use the circlotron amplifier when I need some power, but it's a real room warmer, so for everyday listening I built the RedRoo 5 Watt Class-A design. It's a great little amp, and I actually prefer the sound to anything else I have for casual listening. 


60 Watt per Channel Circlotron Amp.

60 Watt per Channel Circlotron Amp.
The Redroo 5+5 Watt Amp.

The Redroo 5+5 Watt Amp.


This product has been added to your cart