[Steve]

Hi ZLS,

     If the circuit is designed for the 12AX7s, just dropping in 12AT7s usually increases the distortion by alot, and I mean alot. Bandwidth will increase and gain will decrease, although not alot.

Take care.

   I don't know if this is the right thread to ask this question, if not I apologize. 
    I have a rebuilt Eico preamplifier which line stage consists of two 12AX7 tubes.  I have two questions,
1. Can I drop in two 12AT7's?
2. What would I gain or lose? 

                                 Thank You

[1000a]

Hi gentlemen,

     I have to respectfully disagree on the 12AT7 being more linear, esp against the 12AX7, 12AU7, and even the 6dj8/6922/E88cc tubes in lower signal service. In my measurements, and inspecting the curves, years ago, the 12AT7 had very high distortion. I found it very unacceptable.

hey Steve

in my situation do you recomend me using the JJ12ax7 instead of the JJ12at7 (or a JJ12au7).   I am hoping the new JJ12at7 is similar to the JJ12ax7.

Maybe I should just order a set of JJ12ax7 also and compare em in that postion, they are 10. before matching fees, any advice-perspective would be appreciatted,

thanks in advance (ps the eruo #s confuse me I am tube stupid)   

[JoshK]

Hi gentlemen,

     I have to respectfully disagree on the 12AT7 being more linear, esp against the 12AX7, 12AU7, and even the 6dj8/6922/E88cc tubes in lower signal service. In my measurements, and inspecting the curves, years ago, the 12AT7 had very high distortion. I found it very unacceptable.

Here is a picture of the curves. Notice the odd spacing.
http://www.mif.pg.gda.pl/homepages/frank/sheets/087/1/12AT7WA.pdf

Now a typical 6dj8 (not JJ curves which is a more linear tube).
http://www.mclink.it/com/audiomatica/tubes/6dj8.htm

Now a typical 6sn7wtb tube.
http://www.nj7p.org/Tube4.php?tube=6SN7GTB

Notice the 12AT7 curves are no where near the other tubes. (For newbies, check the spacing between the lines. The spacing varies.)

At lower signal levels (say 5 volts rms) the JJ E88cc (european number) is much much lower in distortion than any NOS 6dj8/12AX7, 12AU7 and 6SN7 tube made.

Anyway, if JJ did to the 12AT7 what they did to their E88cc, it is probably the lowest distortion 12AT7 made in any era.

Take care.

Interesting and certainly from this data I would agree with you.   I have seen a couple curves of the 12AT7, but that may have been the JJ come to think of it.  I have also seen a number of tube builders who do their own measuring remark at its linearity.  I guess there could possibly be great variation in production?  Who knows. 

[Steve]

Hi Josh,

     Could very well be Josh. The measurements and curves I  tested years ago were for NOS 12AT7s. I know that when I measured the JJ E88ccs and NOS 6dj8s (Amperex Bugleboys Holland for example) for distortion, each under optimum operating conditions, the JJs were some 16 db less. The JJ brand 12AT7s could very well be Low distortion as well. I like seeing a company improve things. At least they are trying.

Take care Josh.


"Interesting and certainly from this data I would agree with you.   I have seen a couple curves of the 12AT7, but that may have been the JJ come to think of it.  I have also seen a number of tube builders who do their own measuring remark at its linearity.  I guess there could possibly be great variation in production?  Who knows." 

[Steve]

Hi 1000a,

     I would use the tube type (not necessarily the same brand) that was originally in that particular socket.

     As an example of what not to do; if one were to install a 12AU7 in place of the 12AX7, not only would the gain be dramatically reduced, but the distortion would rise dramatically; in fact would be astronomical in this particular situation because of the vast differences in Mu and Gm between the tubes. Same if going the opposite direction with these tubes.

Installing a 12AX7 in place of a 12AT7 would also cause an increase in distortion, especially if large signal levels are involved. One could try to design such that one could use either 12AX7 or 12AT7 in a particular tube socket/stage, but one would be compromising some for both tubes in that situation. One would have to decide if the results merit such a design.

I would tube roll using the same type tube, but different brands for optimum sonics. Otherwise, a complex redesign would be necessary.

Hope this helps.


hey Steve

in my situation do you recomend me using the JJ12ax7 instead of the JJ12at7 (or a JJ12au7).   I am hoping the new JJ12at7 is similar to the JJ12ax7.

Maybe I should just order a set of JJ12ax7 also and compare em in that postion, they are 10. before matching fees, any advice-perspective would be appreciatted,

thanks in advance (ps the eruo #s confuse me I am tube stupid)   
[/quote]

[1000a]

Thanks Steve

I already (ordered) JJ12at7 to try in place of my EH12ax7s (input tubes).
Unless I am confused Bill (Response Audio) said this was an OK move and he has been modding Jolidas for many years.  I believe he prefers them in that gear over the 12ax7,
I do not know if he has heard the JJ 12ax7, the JJ stuff is getting many good comments from people.

I also ordered JJ12at7 to try in place of my EH12at7s (driver tubes).

Considering the price & praise of the JJ tubes and your recommend on keeping the (input tubes) as 12ax7s, I am going to order 2 matched JJ 12ax7.   Then I can compare with my ear, its cheap enough.

Thanks for jumping in, I do appreciate your feedback, it will be well worth my 30. to try the JJ12ax7 also.  My only reference in this part of my amp was Chinese tubes and the EH stomped them, I am getting the impression the EH are detailed but not necessarily graceful.  I have restrained high hopes for the JJ 12ax7s and the 12at7s.   

[Steve]

Hi 1000a,

     Didn't mean to step on Bill (Response Audio). Please accept my apologies Bill. Maybe I am a little picky or that particular design is such that the distortion isn't that much different. I do get picky sometimes.

I am looking forward to see how the JJs sound in your rig.

Take care.

Thanks Steve

I already (ordered) JJ12at7 to try in place of my EH12ax7s (input tubes).
Unless I am confused Bill (Response Audio) said this was an OK move and he has been modding Jolidas for many years.  I believe he prefers them in that gear over the 12ax7,
I do not know if he has heard the JJ 12ax7, the JJ stuff is getting many good comments from people.

I also ordered JJ12at7 to try in place of my EH12at7s (driver tubes).

Considering the price & praise of the JJ tubes and your recommend on keeping the (input tubes) as 12ax7s, I am going to order 2 matched JJ 12ax7.   Then I can compare with my ear, its cheap enough.

Thanks for jumping in, I do appreciate your feedback, it will be well worth my 30. to try the JJ12ax7 also.  My only reference in this part of my amp was Chinese tubes and the EH stomped them, I am getting the impression the EH are detailed but not necessarily graceful.  I have restrained high hopes for the JJ 12ax7s and the 12at7s.   

[Bill Baker]

No apologies needed Steve. It looks like there may have been some misunderstanding of my words.

As you can see from my previous post (below), I did state that the 12AU7 would cause increased distortion but would not actually damage the amp. I hope nobody took this as being an "optimal" sustitution. This is why I wrote: "....While you can sometimes use the 12AU7 in place of the AX7....."

You can use variations of the 12AX7 for the input tubes right down to the 12AU7. The problem here is that the amps circuit is not optomized for the 12AU7 and causes increased distortion when pushed too hard.

 We do use the 12AU7 in the new MK II version of our 3205 amplifiers but the circuit has been redesigned and optomized for this tube. I felt the original circuit had much more gain that actually needed and therefore redesigned the circuit for use of the 12AU7. In a circuit optomized for the 12AU7, you CANNOT install anything other than the 12AU7, due to voltage and current differences, as I also mentioned in my previous post (quoted below)


 

The circuit for the JD1000 is the same as the 302/502 amps with only additional circuitry for the extra output tubes. I do prefer the 12AU7 over the AX7. This is why I redesigned the circuit of our Bella Extreme 3205 MK II amplifiers (based on the Jolida "A" series 302/502 chassis) and optomized it for the 12AU7. While you can sometimes use the 12AU7 in place of the AX7, you CANNOT substitute the AX7 for the AU7 due to differences in opperating points.

 As I mentioned below, I do often run the 12AU7 in th eoriginal circuit. No, the circuit is not optomized for this tube but again, it will not damage the amp and does not cause as much audible distortion as one might suspect but the output level is deminished. In the case of the 12AT7 driver tube, I would not use any other tube type here. This part of the circuit is more critical as it is also the phase splitter.

 

I do run one of my original 3205 amps, before the MK II circuit change, with 12AU7 input tubes as I still prefer the sound.

 Moving on..... the JJ's are very good tubes and you will find that the input and driver tubes will make a sonic difference. Sometimes even more-so than the output tubes. There are also some inexpensive NOS tubes that will really make you rethink this affordable amplifier

[1000a]

Thanks Bill

For taking the time to reiterate everything so its perfectly clear (for me and any other Jolida owners who may study this thread), I look forward to the JJs.  I am still getting quite a bit of pleasure from my amp.  Am quite happy with the SEDs in the power tube section and am excited the input and driver tubes can sometimes have an effect greater than the power tubes, (same thing Herbie told me).

It is fortunite the driver and input tubes are inexpensive and I can also look forward to playing around with some NOS tubes without spending crazy money on them.  Very good and great fun.