12AX7a vs 12AX7

[erichayes]

Hi All,

A 12AX7A is a 12AX7 with a controlled heater warm-up. For some reason, only the American companies made the "A" version. You might have noticed the European 'AX7s light up like a flashbulb when they're first turned on. That's what the "A" version got rid of. Otherwise, they're the same--on paper. When I get the sockets for the Tek 570 finished, I'll start posting curve families of various "12AX7"s.

[Shannon Parks]

I always thought the 'A' meant that it was more suited for audio applications (less hum/microphonic?). Googling the Usenet, it does seem the 'flash' is all there is to it.
BTW, the Keystone CL90 makes this flash disappear with an old Tungsol I have which is like a photoflash without it. :idea:

Shannon

[erichayes]

Hi All,

Actually, Shannon, we're both correct for the same reason. The first description of the 'A stated that it was audio-oriented due to its controlled hum and microphonic characteristics. The original 'AX7, introduced in 1947, was considered an all purpose dual triode to be used in AF amplification, multivibrator (clock) and oscillator circuitry. When the Hi-Fi manufacturers adopted it as their quintessential voltage amp. tube in the '50s, RCA changed the heater design to lower the hum and noise (it runs at a lower temperature) and taunted it as the audio 12AX7 in 1960. When they discontinued the 'AX7 a couple of years later, the 'A was referred to as strictly an audio tube, and the 7025 (introduced at the same time as the 'A) became the upgrade of the 'A.

I swear, the 12AX7 and all of its mutant cousins probably has more mystique in its history than any other low level audio tube.

As I said, I'll be posting EpIp curve families for as many different 12AX7 brands as I can in the near (I hope) future, and the brethren can draw their own conclusions on sonic vs electrical characteristics. Should be fun.

[EWBrown]

Slightly OT, perhaps, but I found that some of the Made In China 12AX7As and other variants, have very skinny pins, as compared to US made NOS and most European and Russian made tubes. The pins are so skinny, that in some cases, the tubes can simply be dropped into the tube socket without having to press them in. Needless to say, this is not a good design feature. If you are using these (Shuguang and others) you wll need brand new "virgin" sockets or else be able to pinch the indivicual
receptacles on the socket to be a tight fit.

I recently found this when trying to use a couple of Shuguangs in a recently acquired Dynaco SCA-35. Sovteks and JJs (as well as US NOS) were OK, just the Shuguangs were way too loose.

/ed brown in NH