Anyone try using a 5751 in place of a 12AT7?

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.

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? 

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.

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

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?

@jsbw  The 12AT7 can handle a lot more current than the 5751 can. So you might run into saturation and/or a short lifespan with the latter.

Another issue is bandwidth, and this plays a lot on how the feedback in the amp will behave. Feedback relies on Gain Bandwidth Product, which put simply is the amount of gain and the amount of bandwidth. When you reduce the bandwidth, the amp will have less feedback at higher frequencies; the same thing will happen if the amp has less gain.

When the feedback is decreased at higher frequencies the distortion rises (this is why power amps are usually measured at 100Hz instead of 1KHz as the distortion at the latter frequency might not look so good). The ear perceives distortion at higher frequencies as brightness, just so you know.


If you really want to see how the amp is affected, run it up to full power with a sine wave and see if it can make rated power, see how its bandwidth is affected and finally see how the gain is affected. Measure its distortion at 100Hz, 1KHz and 10KHz. These things interact- you may not be able to drive the power tubes to saturation with a driver tube that has a significantly higher plate resistance (which a 5751 has)...

I've been accused of making some of the tweakiest amps made- all triode zero feedback OTLs that are fully differential with one stage of gain. With an amp like that you can imagine that there are all sorts of audible changes you can make. But the simple fact is that you have to take the measurements seriously! What you are looking for is for it to measure well and sound good at the same time. You're also looking for a good distortion signature that is innocuous (you can't get rid of distortion; that's always going to be a thing).


So yes, you might be the first to try doing a sub like that.

I always stick with the correct tube.  I am positive this has contributed to the long life of any tube gear I have owned.   My amps can take any EL34 or 6L6 variant but I only use 12ax7 input tubes, 6922 drivers.

Frank Van Alstine uses 5751's in place of 12AX7's in his re-engineering of the Dynaco PAS pre-amp. Lower gain, lower noise.. 

I bet he's set the circuit up to optimize it for the 5751 too.