could i use 9 pin 12AX7 tubes with this? and if so, what do i do with the extra pin?
12AX7 are high mu, whereas the tubes I used (6111 on the breadboard, 7327 in the build) are medium mu tubes, closer to 12AU7. I'm experimenting with some 6112 subminis at the moment, which are high mu, so pretty similar to 12AX7. With these high mu tubes I'm finding that it really a case of taming the gain, which means lower value plate resistors and lower value grid leak resistors.
This shouldn't really matter with stages 2 and 3 of the Vintage Vibe, as those two stages are cathodyne phase inverters, so I think the volume at the plate should be the same as the volume at the cathode. You will need to reduce those resistor values I mentioned for stages 1 and 4 though, or the circuit will distort. Try 100K grid leak resistors instead of 1M.
12**7 tubes have an extra pin which is a centre tap for the heater/filament. If you use 12**7 tubes for this circuit, connect pin 5 to 12 volts and pin 4 to ground. Leave pin 9 (the centre tap) unconnected. Each 12**7 tube will draw 150ma across pins 4 and 5.
Be sure to compare the data sheets of your 12AX7 to the 6111 data sheets, as the pinouts do differ.