- 2x 12AX7
- 1x 6AQ5A
- 1x 12BH7
The 12BH7 is less commonly seen in musical instrument use. It looks from one of the datasheets like it was originally designed for use in old TVs. It's a dual triode pin compatible with the 12A_7 series, and seem to see most of its use as output tubes in very low-wattage single end amps, or as output tubes for headphone amps. It's kind of like a 12AU7 on steroids - low ra, and high current handling capability, which means it should be an excellent choice as a cathode follower driving low impedance loads. And, in fact, it's shown in the LA-2A schematic as a cathode follower in a " totem pole" configuration that I hadn't seen before.. turns out it's called a "White cathode follower", and it essentially behaves like a push-pull cathode follower - which should indeed be good for driving an output stage. I've never seen one in a guitar amp, so I'll have to do some LTSpice simulations to familiarize myself with it and see what advantages it offers. Anyway, I didn't have any of these on hand, so I picked up a couple of NOS units on fleabay. One is a Philco branded-one, and the other appears to be a 1960's grey-plate GE. Both look pretty good, but you can't be sure with fleabay purchases until trying. Fortunately, there is also current production of this tube from Electro Harmonix, so I'll pick up a new one too, and do some tube rolling when the time comes.
Finally, the 6AQ5A looks to be a really interesting tube. I'd never heard about it before, but it turns out to be basically a 6V6 tube, but in a 7-pin package and with lower overall ratings. The curves are essentially identical to a 6V6, so for lower voltage amps that won't overstress it, these tubes should sound glorious. It's no longer in current production, but the good news is that it still seems to be broadly available as NOS way cheaper than the equivalent 6V6. I was able to pick up a lot of 9 1960's era RCA tubes (which are usually pretty good sounding) for a reasonable price. I'll be happy to get three or four good ones out of the lot, and again, some tube rolling seems to be the order of the day. In the worst case scenario -- if all the available 6AQ5As turn out to be crap -- it should be a simple matter to convert the unit to work with 6V6s, which are naturally still in current production. In the LA-2A, the 6AQ5A is used as the driver tube for the electroluminescent panel. One thing that is not at all clear to me is why so much power is needed to drive the panel....I'm sure there's a good reason, but it seems like overkill to me.