Although the amp, most likely would work, with 5 of either type, it would be best, to run the amp, with the tube complement, that it was designed for. The gain, of a 12AX7, is higher, than that of a 12AU7. Most likely, the gain stages, depend on the tubes, to be a certain gain, to produce the intended sonics. It shouldn't hurt to try, but most often, it will sound better, and, as the designer intended, if you use the tubes, it was designed with.
Tubes: 12AU7A or 12AX7 or can one mix them?
I just acquired a 55 watt per channel tube power amplifier with eight EL34's (four per channel configured in push-pull) for power tubes. There appears to be five input/driver tubes (lined in a row in the front of the amp) three of which are 12AU7A's and two of which are 12AX7's. My question is how interchangable are these two types of tubes or should I be running 5 of the same type for optimal performance purposes? Any ideas about this would be greatly appreciated. The amp, in case it matters, also has 2 RU4G tubes as rectifiers and one each of the following tubes that round out the tube compliment: one OA3, one GAS7, and one 6SJ7.
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