12ax7 to 12au7??

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Jouni

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Tha-Taa everyone!

The same geezer that provided me with some 12ax7-tubes a while ago, Came up with a 12AU7WA tube today, saying he's got a few if I'd need them.

Can a 12au7 be used in place of a 12ax7?? perks? Cons?
Less gain, I'm sure, but maybe less hiss?? microphonics??
 
Jouni said:
Tha-Taa everyone!

The same geezer that provided me with some 12ax7-tubes a while ago, Came up with a 12AU7WA tube today, saying he's got a few if I'd need them.

Can a 12au7 be used in place of a 12ax7?? perks? Cons?
Less gain, I'm sure, but maybe less hiss?? microphonics??
Click to expand...

A 12AU7 in the first or second pre-amp stage will improve your clean headroom and reduce the amp's output and distortion. You can try it with a simple swap. It will not harm the amp. I don't recommend using a 12AU7 for the reverb circuit.

If you have microphonics they are probably caused by a bad or loose tube. If you have hiss it is probably a filter cap. Changing tube types will make very little difference. Some of the tube manuals say the 12AU7 is more prone to microphonics but I haven't found that to be the case myself.

I am no expert on tube amps so take the above with a grain of salt.
 
I just received a HUGE list from the man, containing all sorts of tube types he's got around....

12BH7A, EL95, EL41, ECC40 E188CC..... it goes on and on...

Any Ideas where I could check the list for audio amp tubes??

These are all NOS, mil-spec OR premium industrial grade.
 
12AU7 is a lot lower gain than 12AX7, so if you want the first tube stage to overdrive the second, it probably isn't what you want. If you want a little less drive, try 12AT7.
 
FWIW I built a pre-amp designed for a 12AX7. I put twice as much voltage on teh plate as the design called for (although the magazine article suggested it was fine to do so) but it distorts if you even think about picking. (If I want that sound it's fine, the fuzz is reminiscent of teh first Van Halen album.) I swap in a 12AU7 when I want it to be a little less growly. I don't know what impact this will have on tube life however.

I'd check the RCA manual.
 
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