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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.
ered66

Showing 2 responses by ered66

ered66 OP
4 posts
 
Thanks for the response Fletchj. Unfortunately the amp is a custom job so I can't know for sure. I just purchased some additional 12AU7A's so at least I can try all the same type. While I'm at it, how interchangable are 5U4GB's and 5U4G's (I know the amp originally had 2 5U4GB's as rectifiers but I have been advised that 5U4G's are fine to use as well).
Thanks again,
Eric
ered66 OP
4 posts
 
Thanks for the input (especially on the mixing of the 12's), you saved me from making a mistake!
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