In my experience with the Crack/SB and driver tubes, I have stuck to pretty much 4 different types:
1. 12AU7
2. 6SN7
3. 12BH7
4. E80CC
Although the E80CC is my favorite (and then 12BH7A), they are only very slightly different than the 6SN7 or 12AU7. In fact, all of them are more alike than different IMO.
I have 470 ohm resistors installed permanently on the Speedball boards. I changed the transistors permanently to the more robust ON MJE5731AG to safely use the 12BH7A.
I just solder in parallel resistors depending on tube type.
With 12AU7 and 6SN7GT I solder in a parallel 470 ohm resistor to end up at 235 ohm
With 12BH7 I solder in a parallel 150 ohm resistor to end up at 113 ohm
With E80CC it's just the 470 ohm resistor with no other ones needed.
Plate voltages end up in the 70 to 80 volt range.
Pictured is a shot of 470ohm with 150ohm for 113 ohm, 12BH7A operation. Beefier transistors are shown as well. If I want to switch back to the E80CC, I just de-solder one leg and lift it out of the way.
Many people have installed switches too, but I usually just stay with a tube type for a longer period so the soldering is no bother.
