Question about 12AT7/E180CC tubes!

[primosounds]

The A2900 was a designation only used by G.E.C/M.O.V and is indeed their industrial version of the 12AT7, with very similar specs. No other companies ever produced this reference, all my exemplars have the green G.E.C decals just like on the famous KT66/88. A wonderful tube but good luck finding one at a decent price.

Datas were published and can be found here:

http://tdsl.duncanamps.com/pdf/a2900.pdf

If you notice during the introduction of the tube characteristics they mention tube will maintain its emission capabilities after long periods of operation under cut off condiitons. Which is what happens in computer circuits.

 

[Tubologic]

If you notice during the introduction of the tube characteristics they mention tube will maintain its emission capabilities after long periods of operation under cut off condiitons. Which is what happens in computer circuits.

... and also (at the end): "the A2900 is not intended for applications which are critical with regard to microphony", though I used it in some audio circuits and found it no more or less microphonic than any good quality 12AT7 family tubes (which were not intended for audio applications to start with)
G.E.C doesn't state the recommended applications but from the characteristics it looks it would made a good tube for computing/counting circuits or industriall use. It's also a high reliability/long life (10.000 hours) tube with stabilized characteristics.

 

[primosounds]

My A2900 were not in the least bit microphonic. Perhaps the lawyers made them put in that statement to cover their arses.

 

[GordonW]

I drew the load lines for both 12AX7 ( top) and 12AT7 (bottom).

View attachment 1083661

1) I drew RL=100K for 12AX7, I make the operating point at 200V( typical). You can see the region between A and B is very linear, that is the spacing between the plate curves is very even.

2) I drew RL= 22K (red) and RL=100K(green) for the 12AT7. You can see the spacing of the plate curves at operating point of 200V is very uneven. The spacing on the left side of the loadline at 200V is much wider than the right side. There is severe distortion.

Yes, miller effect can be less for 12AT7 ONLY IF you change the plate and cathode resistor to 22K and 330ohm as in my drawing. If you just replace the 12AX7 without changing the resistors as what OP want to do, it will not help at all.

If you look at the uneven spacing of the plate curves of 12AT7, I don't think there is a value of plate resistor that can produce low distortion like 12AX7.

Actually, I am using 6SN7 for my preamp tube instead of X7 or T7. It is more linear, just all around better tube. Problem is it will not fit in the small socket.

Well, naturally, I was optimizing the plate and cathode resistors for each tube in my sims. I wound up with the plate idling at somewhere around 100v or so. 200v on the 12AT7 plate was not really optimal at all, from what I remember of my early sims. And yes, the 12AT7 wound up with slightly less gain (but not as much as it might intuitively seem from comparing the max gain of both tubes, though- the higher transconductance of the 12AT7 made up for some of that, under load conditions). In my case, the 12AT7 had enough gain...

If you want the 6SN7 parameters in a mini bottle- the 6CG7/6FQ7 is exactly that. Shares the same plate curves as the 6SN7. It's lower gain than the 12AT7 or 12AX7, of course...

Regards,
Gordon.

 

[Alan0354]

Well, naturally, I was optimizing the plate and cathode resistors for each tube in my sims. I wound up with the plate idling at somewhere around 100v or so. 200v on the 12AT7 plate was not really optimal at all, from what I remember of my early sims. And yes, the 12AT7 wound up with slightly less gain (but not as much as it might intuitively seem from comparing the max gain of both tubes, though- the higher transconductance of the 12AT7 made up for some of that, under load conditions). In my case, the 12AT7 had enough gain...

If you want the 6SN7 parameters in a mini bottle- the 6CG7/6FQ7 is exactly that. Shares the same plate curves as the 6SN7. It's lower gain than the 12AT7 or 12AX7, of course...

Regards,
Gordon.

Yes, from looking at the plate curve, if you lower the plate voltage to about 130V, use lower plate resistor, the curves should be more even. But the output swing is very limited. So that's another draw back.

Thanks, I'll look into 6CG7/6FQ7, I don't like the big socket. More importantly, OP here should look into this. 6FQ7 has no problem like the 12AT7 and it can swing big voltage.


Just have to change the resistors, not just roll it in.

 

[GordonW]

Well- the 6CG7 will require a heater re-wire on the socket, compared to the 12AX7. It's a 6v tube on pins 4 and 5, while the 12AT7 is 12V between pins 4 and 5- you have to parallel 4-5 for one side of the heater, and use pin 9 for the other side, to get 6v heater operation.

Regards,
Gordon.

 

[Alan0354]

Well- the 6CG7 will require a heater re-wire on the socket, compared to the 12AX7. It's a 6v tube on pins 4 and 5, while the 12AT7 is 12V between pins 4 and 5- you have to parallel 4-5 for one side of the heater, and use pin 9 for the other side, to get 6v heater operation.

Regards,
Gordon.

Ha ha, he should be thankful he got one that is definitely an improvement. Changing a few things is expected. A lot more meaningful than roll the 12AT7 in. Hey, it's a lot better than drilling the chassis to fit a 6SN7!!!