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Old 14th May 2007, 05:18 PM   #1
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Default 5751 and 12ax7

Is it OK to substitute in a 5751 for a 12ax7. I know that the 5751 has a lower mu, but other than that the two look pretty similar.

The reason I ask is that I have heard guys putting in the 5751 and claiming that they offer sonic improvement over the 12ax7.....


Any comments?

Fran
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Old 14th May 2007, 05:21 PM   #2
SY is offline SY  United States
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It won't cause a mushroom cloud or anything, but it's unlikely to make a circuit work better unless you optimize the surrounding components for a different operating point and the lower mu and rp.

BTW, treat claims like that with great skepticism. I saw some guy claim that substituting a 12AX7 for an ECC88 (not a typo!) made a sonic improvement. Uh, yeah, sure.
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Old 14th May 2007, 05:40 PM   #3
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Default 5751 substitution

Yes, the 12AX7 in an ECC88 socket is really bad news! (at the minimum, the heater is running at half the voltage - in other words, barely on!).

On the other hand, the 5751 is quite close to the 12AX7. The 5751 was designed to have nearly the same characteristics as a 6SL7GT (mu = 70, gm - 1.6 ma/V @ 1.5mA). The 12AX7 has a mu of 100 and a gm of 1.6mA/V at 1.0mA. In most circuits with cathode bias, the 5751 should work OK, with just a slight reduction in gain.

I've had reports from friends that tried the 5751 substitution for a 12AX7 that ranged from mildly better to far better (the latter in an EAR phono preamp). So, it is worth a try. As they say "your mileage may vary".

One other minor difference: the heater current for the 5751 is 0.35A/0.175A (for 6.3 and 12.6V connections) vs 0.3A/0.15A for the 12AX7. This might cause slight problems in circuits where the heaters are wired in series, such as in the Dynaco PAS-2 and PAS-3 preamps.

- John Atwood
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Old 14th May 2007, 05:46 PM   #4
SY is offline SY  United States
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In a Dyna PAS, there actually could be a mushroom cloud- the power transformer is marginal, so the extra heater current draw could be fatal.
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Old 14th May 2007, 08:37 PM   #5
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Hi,

John Atwood hit the nail on the head.

Subbing it for a 12AX7, nothing needs doing, even in relatively critical apps (I have a lab power supply that uses 12AX7 for error amps and I subbed 5751 because I had no 12AX7 at the time and it worked fine).

Personally, I have dropped 12AX7 for 5751 for all new designs unless it's requested

Cheers!
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Old 14th May 2007, 10:10 PM   #6
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well there were no mushroom clouds. I had the amp on for a few hours and all looks stable, the PTx only got to body temp, and maybe less.

Now I have to listen and see!

(They are sovtek 5751 and a EH 12au7 replacing EI 12ax7/12au7)



Fran
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Old 15th May 2007, 01:06 AM   #7
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Quote:
Originally posted by SY
I saw some guy claim that substituting a 12AX7 for an ECC88 (not a typo!) made a sonic improvement. Uh, yeah, sure.
Just skimming around here on final approach to bed. This made me think of the occasion long ago when someone announced that he switched a 12AX7 for an ECC83, and got a substantial improvement ....
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Old 15th May 2007, 01:45 AM   #8
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My experience is more limited than most of you, but this is one instance where I have at least some. My sense is, that there is a great deal of overlap in actual performance. Depending on the manufacturer, you may find that some 'x7's have lower gain than some 5751, etc.. I would bet the same applies to heater current.

Sheldon
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