The voltage swing at the primary output transformer coils is fed back into the anode. Triodes have an internal negative feedback loop which is described by the Triode Equation Ra(plate resistance) * gm(transconductance) = µ (gain).Gain µ is given by the quotient from the output and the input signal byµ = - Vout / VinThe minus sign indicates the 180° phase shift between input and output signal. If the input signal is moving Vinpositive, the plate current increases and Vout decreases due to the increased voltage drop across the plate load. As a consequence the decreased Vout will try to reduce the plate current. If the input signal is taking Vin more negative the plate current decreases and the rising Vout will try to increase plate current. Vin and Vout are pulling
Fig. 1: SuperTriode concept. Simplified the circuit acts like a servo steering where the tube (the driver)controls the signal and the transistor (the servo)delivers the power.
The rating plates of my Newtronics Temperance speakers recommend at least 20W into 4Ohm and thus rule out most power triodes such as 2A3, 300B and even the legendary 6c33c unless you are willing to parallel tubes which may generate more problems than it solves. On the other hand in the absence of strong global negative feedback harmonic spectra at large audio signal amplitudes seem to be more favorable in tubes rather than in solid state devices.
Figure 1 shows the basic concept of a tube / solid state hybrid SE amplifier which inspired me for my own project. The circuit was published in the TubeCAD Journal articles “SuperTriodes” (2009) and „more SuperTriodes” (2015). The triode works as a cathode follower driving a power solid state device (BJT or MosFet).
SE stands for Single-Ended, mostly referring to Single Ended power amplifiers. At first glance SE could also stand for Small Efficiency, Super Expensive or may be even Stupid Engineering. However, even respected audio designers seriously consider SE designs and evidently they have some basic advantages in managing the audio signal (Nelson Pass, Peter Qvortrup, Pavel Macura, Andrea Ciuffoli and others). But is their real life performance really superior to that of conventional push-pull designs? To find out it might be a good idea to just build one and listen.
in opposite directions and reduce the change in plate current inducing the specific internal negative feedback of a triode. In conclusion the power triode controls the signal through local feedback while the transistor multiplies the current by its given current gain and virtually makes the triode a SuperTriode.
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