5751 and 12ax7

woodturner-fran · 14th May 2007, 05:18 PM

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

SY · 14th May 2007, 05:21 PM

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.

JohnAtwood · 14th May 2007, 05:40 PM

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

SY · 14th May 2007, 05:46 PM

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.

Geek · 14th May 2007, 08:37 PM

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!

woodturner-fran · 14th May 2007, 10:10 PM

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

Johan Potgieter · 15th May 2007, 01:06 AM

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 ....

Sheldon · 15th May 2007, 01:45 AM

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

14th May 2007, 05:18 PM	#1
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	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

14th May 2007, 05:21 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ "The way to deal with superstition is not to be polite to it, but to tackle it with all arms, and so rout it, cripple it, and make it forever infamous and ridiculous."- H. L. Mencken

14th May 2007, 05:40 PM	#3
JohnAtwood diyAudio Member Join Date: May 2006 Location: Rural Nevada	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

14th May 2007, 05:46 PM	#4
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ "The way to deal with superstition is not to be polite to it, but to tackle it with all arms, and so rout it, cripple it, and make it forever infamous and ridiculous."- H. L. Mencken

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14th May 2007, 08:37 PM	#5
Geek diyAudio Member Join Date: Sep 2004	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!

14th May 2007, 10:10 PM	#6
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	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

15th May 2007, 01:45 AM	#8
Sheldon diyAudio Member Join Date: Dec 2003 Location: San Diego	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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