[Synchrodyne]

In a qualitative sense, given that mutual conductance is usually expressed in mA/V, and that the anode voltage swing of the lower valve is thought to be quite small, does that not imply a fairly large gain from the upper valve (a small voltage swing into a large mA swing), and a fairly small gain from the lower? If one doubled the gm of the upper valve, then given that the mean current through both valves does not change, would that mean that an even lower voltage swing (half as much?) would be required at the lower valve anode to produce the same output swing into anode load of V2. In turn the lower valve would then be operating at half the gain.

Another way of looking at this is, since the current through both valves is the same, to neglect for the time being the non-vacuum interface between the two and to see the grid of the upper valve as an earthed screen (and one that does not partition the electron flow) between the lower valve grid and the upper valve anode. Now we have a pentode-like situation where the output swing is determined by the combination of the mutual conductance of the lower valve and the load resistance of the upper valve.

From either of these simple and simplistic analyses, it would appear that the gain is primarily determined by the mutual conductance of the lower valve, with the mutual conductance of the upper valve determining the gain distribution between the two valves but having little effect on the overall gain.

No doubt such a simple approach fails in detail, but it does seem to help create a mental picture as to what is going on in a macro-sense.


Cheers,