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OK someone Please enlighten me about the old GT12Ax7M

salsage

Member
Messages
529
I have an old Groove Tube 12Ax7M and have had it for 5 years give or take. I recently read a post here about some of those tubes overheating and causing problems in some amps? The tube has been ok for me nothing stellar.
After reading about the potential problems, I immediately pulled it.

Whats the word on these tubes?

 

Erik

Member
Messages
1,682
IIRC, they were hit or miss. (The good ones sound really good IMHO.) I've got a few that I'm not using because they're noisy compared the the tubes I am using. (Bogner XTC)
 

Pentode

Silver Supporting Member
Messages
353
I have used bunches of them with no problems. They sound really good in vintage Marshall circuits.
 

salsage

Member
Messages
529
Thanks Guys.
I read a post here recently about some of them causing damage to amps. Thats my concern.
I'm not going to risk any potential damage.
The tube sounded decent.
 

phyrexia

Member
Messages
507
Haven't heard anything about damage to amps.

I bought 5 used ones from a good friend, 3 of them are microphonic in certain situations.

i stuck a real, 50 year old Mullard in my amp and it has held up spectacularly so far.
 

HeeBGB

Member
Messages
610
The scoop on the GT 12AX7's is that they were hyped to no end and "looked" very promising. Problem was that a large percentage of them would develop noise like "Snap, Crackle, Pop" after a few hours use. The ones that didn't develop this noise were pretty good.

Never heard or read anything about them causing damage to any amps so I won't comment on that.

It would be VERY cool if Fender decided to revisit this project and fix whatever caused the noise. It was a pretty good tube when it worked.
 

Amp_Addicted

Member
Messages
775
I bought 4 based on Pittman's hype, and 2 busted within a week of limited usage, so I took them back to the store for a refund. This series of tubes kind of destroyed Groove Tubes reputation. I never bought from them again, and for sure would not buy any of their products under the current regime. Kind of shame as the tubes did sound good while they worked.
 

salsage

Member
Messages
529
Thanks again Guys:aok

Dont want to take anything out of context but here is what I read that made me cautious. And its written by Myles!

It would have been nice if the 12AX7M would have worked out. At times they were great but they were never consistent. As time went on they got worse in reliability, microphonics and especially performance. The last run before the sale of GT to Fender had the tubes having gain between a 12AY7 and 12AT7 at best.

But ... if you found a good one that was half the battle. The other half was having it remain good for more than a few weeks. The older the tube the more the odds are with you that it may continue to perform properly.

The last year or even two years of production showed test results of very low plate resistance, in many cases less than half of design spec. This had to main effects. One was the gain of the tube was very low and in fact it was the lowest gain 12AX7 produced by anybody, past or present at the time. The other effect was two fold; crosstalk between the A and B sides of the tube and very high current draw in the amp in the position of the tube. The high current could cause other problems associated with heat and reliabilty of not only the tube but also the amp itself.

Bottom line ... happy playing with the GT 12AX7Ms you have and like and good luck in the future.

Myles
IDK exactly when mine was made but I'm certainly thinking better safe than sorry.
side note, an an amp I sold last summer had that tube in it for about a year. When I sold it, I put the stock tubes back in and the buyer complained of burst capacitor's within a week of his purchase. So you can understand why the hairs on the back of my neck stood up when I read Myles statement. Did that tube cause the amps failure? Probably not . Is it curious? I'd say so.
 

charveldan

Senior Member
Messages
3,374
The "M" stands for Mullard, as in reissue, as in new sensor, [russian]. Unless those are the one's they supposedly make in california.
 

zombiwoof

Silver Supporting Member
Messages
5,900
The "M" stands for Mullard, as in reissue, as in new sensor, [russian]. Unless those are the one's they supposedly make in california.
No, the GT12AX7M was not a New Sensor tube, it was made in China to GT's specs. Many people loved them, but as Myles pointed out, the production quality was not consistent, and the gain of the tubes varied quite a bit. Some people raved about them, others hated them, probably because of just what Myles reported. Yes, the "M" was a nod to Mullard, it was supposed to be a recreation of a Mullard 12AX7.

Al
 




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