GE 12AX7 vs GE 12AX7A
- Thread starter 2string1
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Or any 12AX7/12AX7A
What does the A at the end signify?
(don't mean to jump on your thread but I have wondered that as well.)
I see, thanks knockbill.
Don't know if that answers your question as well 2string1.
Don't know if that answers your question as well 2string1.
tubeampking
Chronic dabbler
I don't know about the spiral filament but the A versions are premium low noise Variants. Military grade are usually designated with a "W" in the suffix...like 12AX7WA or 6L6WGC etc. Military grade are ruggidized versions expected to last 10,000 hours.
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sKiZo
Hates received: 92644 43.20°N 85.50°W
^ ^ THIS ^ ^
Only true spiral wound I could find that would be available in current production are the 12AX7LPS and 7025WA. Just went through all that looking for some low noise tubes for a Eico HF12A, an amp noted for their high sensitivity to noise.
The "A" sometimes gets confused with low noise because tube production did get better over time when compared to the original 12AX7's. That no longer holds true, so what goes around, comes around ...
Only true spiral wound I could find that would be available in current production are the 12AX7LPS and 7025WA. Just went through all that looking for some low noise tubes for a Eico HF12A, an amp noted for their high sensitivity to noise.
The "A" sometimes gets confused with low noise because tube production did get better over time when compared to the original 12AX7's. That no longer holds true, so what goes around, comes around ...
2string1
Super Member
Thanks...read this some where...Fender specifies 7025 tubes in their later amps. I have seen tubes labeled both 12AX7A/7025 on the same tube. The 7025 and 12AX7A data sheets both say: ...For high-fidelity audio-amplifier applications critical as to noise and hum... The 12AX7 data sheet simply says "For use in mobile communications equipment" the 7025 was a special 12AX7A for instrumentation, having low microphonics and noise characteristics. The 12AX7A was an 'improved' 12AX7. Although commonly known in Europe by its Mullard-Philips tube designation of ECC83, other European variations also existed including the low-noise versions 12AX7A, 12AD7, 6681, 7025, and 7729; European versions B339, B759, CV492, CV4004, CV8156, CV8222, ECC803, ECC803S, E2164, and M8137; and the lower-gain low-noise versions 5751 and 6851, intended for avionics equipment.
Kilowatt915
Active Member
Thanks for the info. Was curious myself.
jaymanaa
RIP 1961-2018
That's what I always thought, because the A stood for controlled warm up time (11 seconds). :scratch2:
kvflyer
Super Member
One has to check the tube manual because there is no "universal" application to the dash A designation to a vacuum tube. As noted, some are for controlled warm up, some are for higher plate or screen voltage, some are for microphonics. You just have to read what the manufacturer says.
2string1
Super Member
So if the manufacturer says 12AX7 period we should not substitute?
kvflyer
Super Member
The HH Scott 299 amplifier (Not 299C/D etc.) show the use of the 12AX7 in the HW Sams Photofact as being different with a different location in the circuit. The 12AX7A is listed in the AF amplifier section, right before the AF Amp/phase-inverter ECF80. The 12AX7 listed in the pre-amplifier section. Mind you, the tubes in the pre-amp section are wired filaments in series!
I don't see that distinction between the 12AX7 and 12AX7A in the copy of the Scott generated schematic. My amps are in FL and I am presently in MD so I can't walk over to them and look, sorry.
kvflyer
Super Member
I would like to comment but as I have said above ^^^ all of my amps are in FL and I am in MD. Surely, it will not do any harm to substitute. I have the RCA tube manual in FL also and I could look it up.
Maybe one of our members has a GE tube manual. I guess the point I was making is that there is no definite designation for the "A". You gotta look it up. If I were in your shoes, I would use what I have. I don't think that the Telefunken ECC83/12AX7 tubes had an "A" version. But I yield to others who have access to their stuff right now.
kvflyer
Super Member
I did just find one of my RCA 12AX7A tubes. It has a spiral wound filament. So, that to me would be for hum reduction, not controlled warm-up. Funny how the HW Sams schematic has the plain 12AX7 in the pre-amp and the 12AX7A in the AF amp before the driver tube. But Sams was known to often have mistakes.
Mind you, I checked an RCA tube. GE may be different...
Mind you, I checked an RCA tube. GE may be different...
wa2ise
Super Member
If you use DC on the heaters, then this issue goes away.
I've gotten away with using non A versions on series string heater circuits. Things settle down after initial warm up.
kvflyer
Super Member
If you use DC on the heaters, then this issue goes away.
I've gotten away with using non A versions on series string heater circuits. Things settle down after initial warm up.
Absolutely!
