Circlotron - Basic Transformerless Schematic

Basic Transformerless Schematic

The electrical bridge of a circlotron is formed by a matched pair of triodes (V1, V2) and two floating power supplies ('B batteries'), B1+ and B2+. Grids of each triode are driven in opposite phases with a balanced, symmetrical input signal; differential current flows through the loudspeaker load and a simple, relatively high impedance, resistor network that ties floating supplies to the ground. Tubes are usually fixed biased with an external negative power supply ('C battery'); each side normally has independent bias adjustment to compensate for minor tube mismatch.

Output impedance Z of a transformerless circlotron where each stage is a single triode with plate impedance of R_p and voltage gain of μ is defined by the formula

The tubes best fitting the circlotron concept belong to the regulator pass-through type: 6AS7, 6C33C, 6C19P. An amplifier with one 6AS7 dual triode per channel (R_p = 600 ohms, μ = 2) will have a Z of 150 ohms—sufficient to drive high-impedance headphones. Driving loudspeakers requires paralleling output tubes, and in practice, V1 and V2 are not single triodes, but massive banks of paralleled triodes. Simple OTL amplifiers, for example Atma-Sphere M-60, employ 8 double triodes of 6AS7 type per channel; each continuously dissipates an average of 30 watts. Without feedback, this arrangement results in an output impedance of 6–8 ohms. It can be further lowered by applying global negative feedback or paralleling more output tubes. On the extreme end, Atma-Sphere MA-2 amplifier uses 20 6AS7 tubes per channel, with a continuous power consumption of 600 watts per channel (800 watts at maximum rated output of 220 watts, or a 27% maximum efficiency).