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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
7 responses Add your response
fletchj
91 posts
 
A 12AU7, and a 12AX7, both uses the same filament(heater) voltage, 12.6 volts. Any heater that is designed for a 12AU7, will do just fine, for a 12AX7. Yes, you might get distortion, or a unbalanced gain, but you will not fry a 12AU7, or 12AX7, in a 12.6 volt filament.
ered66 OP
4 posts
 
Thanks for the input (especially on the mixing of the 12's), you saved me from making a mistake!
ed_sawyer
588 posts
 
5u4G and 5u4GB should be fine to interchange. Those are also interchangable/compatible with some other rectifiers if needed. Agreed, do not swap the Au7s and AX7s - not the same tube in many ways, unpredictable results if you do so. OA3 is a cold cathode voltage regulator tube. (neat colored glow on those. ;> )

-Ed
buckingham
333 posts
 
Not a good idea, I accidentally switched the positions for my 12au7 and 12ax7 and I had some MAJOR distortion coming from my 120W monobloc.
gs5556
1,209 posts
 
It would not be a good idea to mix - the 12AU7 operates at a higher plate current than the 12AX7. The Ia/Vp curves are therefore different and without the techno-gobble, they will not perform their function in each other's socket. The 12AU7's can also fry if you put them in a 12AX7 socket whose heaters are series connected without current limiting on startup. To answer your question, they are not interchangeable.
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
fletchj
91 posts
 
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.
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