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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
atmasphere's avatar
atmasphere
11,756 posts
 
For starters, a 12BH7 might sub a 12AU7 pretty well, if the circuit can support the additional filament current, but its not a sub at all for a 12AT7!

The difference in mu is significant with any tube. The circuit has to be optimized to take advantage of the tube's characteristics. You might be able to get the circuit to work with a tube of the same pin base, but that's not the same as the circuit working correctly.


If I were you, I'd stick to 12AT7s (also 6201 and ECC81). There are so many different brands- and you're far more likely to find what you're looking for. But keep in mind that the voltage amplifier and driver might not be the issue- the match to the speaker is likely more important.
jsbw OP
14 posts
 
The 12bh7 was just an experiment, without impressive results (but not unimpressive...just, meh).

No, the main question here is, has anyone used a 5751 in place of a 12at7, especially in a power driver stage? 


tomcy6's avatar
tomcy6
2,582 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.
facten
2,863 posts
 
I use the 5751 in place of 12ax7 in my Line Magnetic 518IA and the lower gain hit the mark for me in that amp. Agree with others I wouldn’t substitute it for the 12AT7 unless the amp designer stated that it was okay to do so with his circuit
jsbw OP
14 posts
 
The amp’s a Jolida Fusion 3502p running in ultralinear, with KT88s. All power tubes are still within bias spec with 5751s as power drivers.

Seriously, the 5751 seems to add a bit of detail. Especially in the higher frequencies.

(Tbh, the difference may be negligible)

But as we all know, it’s the combination of tubes that matters.

I’m working on a spreadsheet with different combos.

I’m just surprised no one else has tried this. Seriously, am I the first guy to sub a 5751 for a 12at7?
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