I'm thinking about building a single-ended tube amp that'll use a 7591A power tube (because I already have a half-dozen, and an amp using a pair of them configured push-pull) (and I have a different single-ended 6V6 amp already). My question is about the difference between the pinout of the 7591A and the 6CA7(EL34). On a 7591A (and for that matter a 6L6), the G3 and K internal connections share a pin. On a 6CA7, the G3 connection and K connection are separate pins. Looking at the ur-text of SE amp circuits, (not actually, since the ur-text would use octal preamp tubes etc), the 5F1, I only see (non-heater) connections to the anode, two (2) grids, and the cathode. The 6V6 and 6L6 have an identical pinout, and while the 7591A does not match 1:1, it shares the internal connection between G3 and the cathode in common with those tubes. The 6CA7, alternatively, uses pin 1 as the connection to the third grid, and pin 8 as the connection to the cathode. Obviously, you can not plug a 6L6, 7591, or 6CA7 into a socket wired for one of those tubes specifically. My question is whether changing a single-ended circuit designed for a 6CA7 to use a 7591A involves accounting for the G3, K difference specifically, outside of the pinout differences.