Electro-Voice A20-C
"Circlotron" Integrated Amplifier
 

These rare units have been on & off my work bench for the last several years pending restoration of both units. I completed one of 'em well over a year ago, the second one has taken back seat to my other projects and obligations. I completed the project as of the posting date of this web page.

These amplifiers are mono-block units that employ a unique push-pull, class B output stage. E-V refers to this as the "Wiggins Circlotron".  There are several circuits that differ significantly from the typical Push-Pull Williamson circuit. The most obvious; individual rectifier circuits for B+, one supplies B+ to one half of the output stage, the other supplies the other half. This allows a smooth transition between phase changing by not over-taxing the supply voltage during heavy B+ demand. Another unique circuit; the output transformer has no applied B+ voltage, the transformer is in the cathode circuit of the output tubes. The transformer also supplies damping control feedback to another unique circuit.

E-V claims this circuit allows smoother, cooler operation and less distortion in the output circuit all while providing a full 20 watts of power!

My Cosmetic Improvement

Since first seeing these amps, I had the thought that the acrylic control panel would look nicer is were illuminated. I was advised not to do so as such modifications might reduce the value if I were to sell. So after comparing the sound to my LK-150, I decided to go through with my plan to illuminate the panels. I did so in such a way that it could be un-done for a purest. Details can be provided upon request. Here's picture of the pair powered up in my low lighted Radio Room.

click/tap to enlarge