Nashville SE 20 - Introduction
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 R a  (plate resistance) * g m  (transconductance) = µ (gain). Gain µ is given by the quotient from the output and the input signal by  µ =  - V out  / V in   The minus sign indicates the 180° phase shift between input and output signal. If the input signal is moving V in   positive, the plate current increases and V out  decreases due to the increased voltage drop across the plate load. As a consequence the decreased V out  will try to reduce the plate current. If the input signal is taking V in  more negative the plate current decreases and the rising V out   will try to increase plate current. V in  and V out  are pulling
Fig. 2: 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 distortion spectra at large audio signal amplitudes seem to be more favorable in tubes rather than in solid state devices.
Figure 2 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 decive (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 (see Nelson Pass's article, also 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. Within SE designs the amplifying output device may be combined with either a load resistor, a current source or an inductor (fig.1). Combinations with a load resistor show the poorest efficiency. Constant current sources as active loads perform better but an inductor provides the best efficiency for it truly stores and releases energy. Once the decision is made in favor of an inductor it seems only consequent to choose an output transformer to avoid a big electrolyte capacitor in the signal path plus to allow impedance matching between the output devices and speakers.
Fig.1: Single ended designs with load resistor (theoretical efficiency 12.5%), current source (theoretical           efficiency 25%) and inductor (theoretical efficiency 50%).
in opposite directions and reduce the change in plate current which describes 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 gain and makes the triode a SuperTriode.