Phase Inverter Quest.....

[g1]

With the shared cathode resistance forming a short circuit, the voltage on the triodes' cathodes are the same. This voltage goes up if the tube connected to the preamp input conducts more current through the resistor, making the other (grounded cathode) tube conduct less current by decreasing its grid to cathode voltage, because its cathode voltage went up.

I'm really struggling with this explanation. Where you said (grounded cathode), did you mean (grounded grid)?
And doesn't decreasing the grid to cathode voltage make it conduct more, not less?

[Malcolm Irving]

It's easier to understand (I think) with some actual numbers. Let's say there is 100V across the tail (including the cathode resistor). At quiescent conditions both triodes have +100V on their cathode and lets say +98V on their grids. The left-hand triode now has an input signal which puts its grid voltage up to +99V. The right-hand triode's grid is held at +98V by the cap to ground.

What happens to the cathode voltage (which is the same for both triodes)? It goes up to +100.5V
Overall effect is that the left triode has changed its grid-to-cathode voltage from -2V to -1.5V so it conducts more. The right triode has changed its grid-to-cathode voltage from -2V to -2.5V so it conducts less.

The voltage across the tail has only changed by 0.5% so the current through the tail has remained approximately constant.

[Tooboob]

I'm really struggling with this explanation. Where you said (grounded cathode), did you mean (grounded grid)?
And doesn't decreasing the grid to cathode voltage make it conduct more, not less?

That's funny. I only commented because I though everybody else was confusing the poor guy. Yes, I meant grounded grid. The grid to cathode voltage is usually (hopefully) negative, and if you increase the cathode voltage with respect to the grid, it gets more negative. Does that increase or decrease it? I guess that one's semantics or math or something.

Unfortunately, I've come to realize that I've been gifted with excellent self-comprehension. What I write makes much more sense to me than it does to anyone else. It's a curse.

I did read it a couple of times to make sure it was perfect. It seemed like trem didn't really see the two triodes talking to each other through the cathode circuit. Then folks started talking about constant current sinks, Kirchoff's voltage law etc...