# Vacuum tube circuit behavior

I’m the following picture, how does the circuit work between points A and B? I’m confused about the polarity of the cathode in front of the heater element. Is the cathode positive?

Image source: The Real McTube II by Fred Nachbaur, Dogstar Music

The pictured tube is indirectly heated. The heater has its own separate circuit where current flows in a loop through the 'A' battery and heater. Negligible current flows between this circuit and the actual signal path in the tube; the only purpose of this circuit is to get the cathode warm.

There is no way to meaningfully discuss the cathode's polarity with respect to the heater.

However, we can still discuss the polarity of the cathode with respect to the rest of the tube - if the tube is to conduct then the plate must be positive with respect to the cathode, so that electrons emitted thermionically from the cathode can actually reach the plate (which is cold and does not emit electrons).

The heater element supply in an "indirect" tube is isolated from the cathode, which is shown in your diagram by battery "A" not being referenced to the common ground of batteries "B" and "C". In many mains powered systems the filaments are commonly powered by AC from an isolating transformer.

The grid in turn is biased negatively to the cathode, to provide control of the cathode/anode current flow.

All voltages must have a reference point. There's no ground shown, so let's add one:

Now we can say the cathode is grounded (0V), the grid is negative with respect to ground, and the plate is positive with respect to ground.

The heater powered by the 'A' cell is floating and does not enter into the calculation of plate current (its job is just to heat the cathode sufficiently).