2. amps-preamps
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Anyone try using a 5751 in place of a 12AT7?

I'm running a Jolida Fusion 3502P amp, with a pair of 12ax7s in the preamp stage, a pair of 12at7s in the power driver stage, and 4 KT88s in the output stage.

I've been rolling the smaller tubes obsessively (as one does), and while I've settled on my favorite 12ax7s, I've been experimenting with 12at7s and their variants (6201s, 12BH7s, 12av7s, etc), and it never occurred to me until recently that while most substitutes for 12at7s have nearly the gain of a 12at7, it suddenly occurred to me that a 5751 is the only 12xxx tube that has 70% gain (as opposed to the 12at7s 60%). Thought if it didn't sound distorted, it might just add that little tiny bit of punch this amp lacks.

And it did!

So now, of course, I need to stock up on even more tubes.

Oh well.
jsbw

Showing 1 response by tomcy6

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tomcy6
2,623 posts
 
The 5751 is considered a substitute for the 12AX7 not the 12AT7.

Here is what tube dealer Brent Jessee says about it:

5751: This version was usually the one made for the US military. These have a lower gain than the 12AX7, but are otherwise totally compatible. Great hi-fi tubes, as the lower gain reduces noise and microphonics.

I’d be interested to know what atmasphere (Ralph) thinks of this opinion. Ralph knows more about tubes and their uses than the rest of us combined. He has been designing and building tube amps and preamps for quite a while.

As Ralph said, you need to be careful about substituting tubes. There may be consequences, like damaged gear, beyond the immediate change in sound.
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