you need a resistor in cathode for power tube if you want it run in class A1. In this case, tube run also in auto bias.
Some people call this circuit "cathode choke for driver". My friend has success with this circuit.
You are talking about the second schematic? It runs with grid current even at idle as the grids are positive. With all three grids (or even just the screen grid) at the same potential as the cathode, the tube would be in cutoff. So all three grids tied together make a higher mu triode than just one grid being driven but now it requires grid current and positive bias. I got the idea for the second schematic from reading the Tubelab pages. Tubelab
SE outputs here: "When the grid is negative it is a high impedance, when the grid goes positive it becomes a low impedance. There is a large abrupt change in grid impedance when the grid transitions into the positive region. If your driver stage is not designed for this it will distort badly." and "Since it only needs a few volts of drive the grid is always positive and the transition never happens. The driver tube must be capable of driving a low impedance. Everyone seems to need a name for their unique tube circuit, so we call this A3 operation since grid current is drawn for 360 degrees." I am curious about it and posted both schematics to share the ideas. To try both would be easy enough by switching a couple wires.
The first one might could be called "plate choke for driver" and would run in a1 99% of the time, but could maybe push the grid a little positive on a loud drum sound or something. Thank for the replies!
