gofar99 wrote:
Has anyone seen anything like it? Or have some thoughts on it.
Yes, I have seen this configuration. It is simply the use of an active load. However taking the output off a series fed transformer is a different kind of twist.
The goal in this situation is to define a stage with a much higher AC load without the attendant voltage drop in the load. This is generally done to increase amplification and reduce distortion. The upper tube is biased such that the DC current matches the desired lower tube's bias point, but the DC drop across the load triode can be made small. However, the AC resistance consists of the upper biasing resistance in series with the AC plate resistance of the upper tube. In this way, the AC load can be made very high and the amplification of the lower stage approaches the amplification factor µ very closely.
However, in general one uses a pentode for the upper tube because it's natural Rp is extremely high. You do however need to be careful because the inter-electrode capacitances of the tube can cause rolloff in the upper audio frequencies. This configuration is normally used when a high sensitivity and low distortion servo amplifier is required. It's usually not seen in audio amplifiers. due to bandwidth limitations.
By putting an unmonitored load in the B+ path, this is not a shunt regulated amplifier but simply a semi active load on the amplifier tube. In this configuration it is doubtful that this would provide any benefit. Also, I would think that putting this active load in series with the transformer would act as an AC voltage divider and seriously reduce the AC signal into the transformer. In general, a transformer load accomplishes the same thing as the active load in that it exhibits a very small DC load but a much larger AC load. Hence the reason that, on power stage load line designs, we usually only plot the AC load line and just assume the DC load line is perfectly vertical.
Overall, I'm not sure why one would take this approach. Were you trying to solve a particular problem or just tyring out new configuration?