12AT7 swapping

[aussievintage]

A valve that does not get as many mentions as it's cousins 12AX7 and 12AU7.  The 12AT7 is midway in gain between the 2 and in other specifications as well.

 

I needed a tad more gain in my line preamp section, which was running a 12AU7 gain stage and a cathode follower.  No point in changing the CF but the gain stage will readily respond the the various 12A*7 variants, so I decided to try a few 12AT7s. The three I chose to test against each other were, in order of testing,

 

1.   Siemens ECC81

2. GE   6201 / JAN-12AT7WC

3.  Magnavox 12AT7WA

 

Turns out that was also my order of preference.   The Siemens and the GE were close and had improved high end over the 12AU7 I removed for this test.   I had a second example of the GE valve.  So I tried it as well.  This time I thought it was just as good in the high end and maybe more rounded, i.e. the middle frequencies were fuller and better matched to the top end.  The Magnavox was a bit flat sounding.

 

After a bit of listening, I discovered the Siemens seemed a bit mismatched between it's two triodes.  The left hand channel seemed to have less gain.  So I am now happily running the second of the two GE 6201s

 

 

[Mr.Anderson]

Thanks @aussievintage for the bit of knowledge I managed to gain from your post which is that you can swap 12AU7 with 12AT7.

 

Can the 12AT7 be a suitable substitute for a 12AX7 as well ?

Are 12AX7 & 12AU7 interchangeable ?

 

Cheers

Edited May 20, 2020 by Mr.Anderson

[aussievintage]

10 hours ago, Mr.Anderson said:

Thanks @aussievintage for the bit of knowledge I managed to gain from your post which is that you can swap 12AU7 with 12AT7.

 

Can the 12AT7 be a suitable substitute for a 12AX7 as well ?

Are 12AX7 & 12AU7 interchangeable ?

 

Cheers

 

You can swap one for the other, provided you don't exceed some of the specifications that differ between the types.  Some examples below.  You can see the max power for a 12AX7, Pa,  is 1 watt vs 2.5 and 2.75 for the others.  The cathode current Ik is likewise much less.   In a cathode follower circuit you need to be careful of heater to cathode voltage Vhk and that varies, this time the 12AT7 is the weakest.

 

In your typical gain stage designed for a 12AX7,  none of these come into play, and so the you can usually swap them and the main difference you will notice is a change in gain.

 

12AX7 Ratings
Vh    Ih    VaMax    VhkMax    PaMax    IkMax
6.3    0.3    300            100               1          8
12.6    0.15    300           100             1          8
 

12AT7 Ratings
Vh    Ih    VaMax    VhkMax    PaMax    IkMax
6.3    0.3    300          90                    2.5    15
12.6    0.15    300        90                   2.5    15
 

12AU7 Ratings
Vh    Ih    VaMax    VhkMax    PaMax    IkMax
6.3    0.3    300          180                2.75    20
12.6    0.15    300        180               2.75    20
 

 

Edited May 20, 2020 by aussievintage

[Mr.Anderson]

Thanks for the description. So Just to double confirm,

 

Its next to no issues if I'm putting a 12AU7 into a 12AX7 socket (Cuz the gain will never be exceeded beyond the limit and the heater won't burn out)

 

but have to be careful when putting a 12AX7 into a 12AU7 socket.

 

Correct ?

 

Thank you

[aussievintage]

41 minutes ago, Mr.Anderson said:

Thanks for the description. So Just to double confirm,

 

Its next to no issues if I'm putting a 12AU7 into a 12AX7 socket (Cuz the gain will never be exceeded beyond the limit and the heater won't burn out)

 

but have to be careful when putting a 12AX7 into a 12AU7 socket.

 

Correct ?

 

Thank you

 

Yes, broadly speaking that is correct  -  for the usual gain stage you will find in a simple valve amplifier -- the common cathode gain stage.  There are other ways to use a triode, so be a little careful

 

 12AU7 in 12AX7 socket - heaters the same, the rest of the limits are exceeded by the 12AU7 so it's safe, except gain is a LOT less - you'll have maybe a fifth of the gain.

 

12AX7 in 12AU7 socket - heaters the same, but cathode current could be exceeded, plate dissipation  could be exceeded, and max heater to cathode voltage could be exceeded - the last especially in cathode follower circuits often found as the last "driving" stage of a preamp circuit.

 

I know people swap them willy nilly, especially guitarists,  and by and large they get away with it.   It really pays to know the circuit before you do it though.

[Mr.Anderson]

Thanks. Do you happen to know much about the Prima Luna Dialogue Integrated amplifiers ?

 

https://www.primaluna-usa.com/dialogue-premium-integrated (This link has a pic to the guts of the amp but it doesn't have a circuit diagram)

 

This amp has 6 x 12AU7 in the pre stage where I've seen some say that the front 2 x 12AU7s being the most important to changes in sound when u roll tubes. I have a few TFK 12AX7 which I Love and was wondering whether or not I could use them without damage cuz those damn things are rare and cost me quite the penny when I bought them.

 

Thank you

[aussievintage]

6 minutes ago, Mr.Anderson said:

Thanks. Do you happen to know much about the Prima Luna Dialogue Integrated amplifiers ?

 

https://www.primaluna-usa.com/dialogue-premium-integrated (This link has a pic to the guts of the amp but it doesn't have a circuit diagram)

 

This amp has 6 x 12AU7 in the pre stage where I've seen some say that the front 2 x 12AU7s being the most important to changes in sound when u roll tubes. I have a few TFK 12AX7 which I Love and was wondering whether or not I could use them without damage cuz those damn things are rare and cost me quite the penny when I bought them.

 

Thank you

 

No I don't know the amp, and as much as I stare at the picture I can't make out the circuit for sure.   I suspect you'll end up with way to much gain, and even if you don't overload a subsequent stage, you'll have the volume control way down near zero.