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 On Hiatus Join Date: Oct 2002 Location: Chicagoland	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. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."

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 On Hiatus Join Date: Oct 2002 Location: Chicagoland	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. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."

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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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