tubes: 12ax7 vs 12ax7a

[Firedome]

Electrically they're the same as far as I can see...so why was a new variant introduced... lower noise?
I thought 7025 was introduced to serve that purpose, or was that just GE?
So many of the high quality stuff used Telefunkens for that reason, maybe
the US makers were trying to compete with them?
I have 3 RCA 12AX7-A's but no RCA 12AX7 non-A's.

[electra225]

The only theory I ever read on the "A" versions vs the "non-A" versions is the 6GH8. The "A" version of that tube is alleged to have additional bandwidth to better work with the color demodulation circuits in color TV sets. I guess I never paid any attention to the 12AX7. That tube is used as a cathode follower, so maybe that application has something to do with it.

[Firedome]

Based on more research, in RCA's own words in their 1960 Industry Announcement for the new RCA 12AX7A:

Dated 9/21/60: RCA stated that this new type 12AX7A utilizes a "double helical heater to minimize magnetic hum" and a "short, sturdy cage to minimize microphonic noise". This is the well-known RCA "short gray plate"

Additionally it was "designed to function with controlled heater warm-up for series filament application" (in TV ?).

Interested due mainly to having ZERO US made NOS 12AX7s, but 3 NIB NOS RCA identical 12AX7A "short gray plate" tubes.
Almost every console, amp and preamp I've ever had uses these, one way or another, in their circuit.

1 new in a "RCA" old style logo box
1 new in a blue/white "GE Mobile Communications" box
1 new in a green/black "Zalytron" box
all are identical internally, test & look NOS.

All in original boxes they came in, that's known because they were all obtained new by my uncle in the early '60s.
He had over 1000 tubes when he gave them up w/ the metal storage cabinet for them about 15 yrs ago.
Down to about 250 now.

The concern now is "new production" 12AX7s from JJ, Russia, or China are harder to get and getting very expensive.
I'm now looking into alternatives as I use them a lot.

[Firedome]

"Almost every console, amp and preamp I've ever had uses these, one way or another, in their circuit."

I should have been more specific, by this I meant 12AX7 tubes in general, not RCA 12AX7s in particular.

It seems to be one tube almost everything Post-War II uses in some way: Hi-Fi, Stereo, Radio, PA, organ, and guitar/musical amps.

it's alleged to be the only small signal tube in continuous production since its introduction, in this case in 1947.

[electra225]

I don't believe I have anything that specs a 12AX7A tube. I guess I never paid that much attention. I may have put "A" tubes in and not know the difference.

[macgolf]

I wish I could help with the newer designated 12AX7A's vs the old. I have 3 = 12AX7A's in my collection. My Philco pre specs a 12AX7A for its only tube. I plucked a Matsushita from the TV tuner section of my Mom's non-functioning Astro. The other(RCA) was in one of the PI spots in my Fisher integrated when I bought it at a yard sale almost 7 years ago. I suspect it wasn't happy there based on the blown 2A slow blow fuse. 1.5A is specified. The RCA did work very well as the single tone/balance control tube of the Fisher, though the gain was slightly lower.

The only new production 12AX7's I've purchased were a pair of Electro-Harmonix, shortly after acquiring the Fisher. They've held up very well in the PI spots for that unit.

Mark

[electra225]

Then if your Philco specs a 12AX7A in the pre amp, may we assume it is there for additional gain, perhaps? Is that in the Multiplex section of the tuner?

[macgolf]

Then if your Philco specs a 12AX7A in the pre amp, may we assume it is there for additional gain, perhaps? Is that in the Multiplex section of the tuner?

I think Philco calls it a Control Center. It's the only separate pre I have in my collection. Just a single 12AX7A/7025. The amp is 3 channel 7189/7199. I wish I had the knowledge to know why a 12AX7A. Not sure if it could be related to the electrostatic tweeters in them? Here's a front and rear photo.

Mark

[electra225]

Do you have the schematic you could post or link? That might give us a better idea of what that tube does.