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I'll try to explain it as well as I can.

That's the same circuit that I put in my post above. Look at the AC signals indicated on the diagram. A 2VAC input signal results in 1VAC at the shared cathode connection. That's how a LTP works when one grid is driven and the other is grounded (either connected directly to ground as in this case or AC grounded through a capacitor.)

If you recognize and accept the AC signals at the points shown on the diagram - both their magnitude and phase as depicted by the sine wave inside the circles - then look carefully at the signal between grid and cathode of each tube. I don't need to remind anyone that that is the signal that actually gets amplified. For V1, Vgk is 1/2 the input signal. The 1V at the cathode effectively is subtracted from the 2V at the grid. It sees only 1/2 the input signal and consequently only 1/2 the output signal appears at its plate. In other words, the circuit has only 1/2 of the voltage gain that the same tube would have if it were in an ordinary grounded cathode stage (same operating point, etc.)

In the drawing above both V1 and V2 have voltage gains of 20; 1VAC between grid and cathode results in 20VAC at the plates. But, the circuit, with only one grid driven, has a voltage gain of only 20/2 = 10.

-- Dave

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