GE 12AX7

[Structo]

Howdy guys,

I bought a couple GE 12AX7's from somebody and I was wondering if
these are the type that you guys are liking in your ODS amps?

I don't know much about these other than they appear to be short grey plate with a halo getter.
The 12ax7 designation is also etched onto the glass.

So did I get the good ones?

Haven't tried them yet, probably tomorrow when the missus is out.

[boldaslove6789]

Yeppers thems be the right ones.

There the same Ge's in #183.

To be more specific they should be:

Ge 12ax7wa's with short grey plates

[ToneMerc]

Here's the GE's that a few us have used in #183 builds.

https://www.kcanostubes.com/content/nos-jan-ge-12ax7wa

I have this GE 7025 in my 100W 2nd Gen hybrid.

https://www.tubeworld.com/12ax7age70.jpg

TM

[cbass]

I have quite a few of those.I like them in fenders.Good gain.Fairly bright

[alvarezh]

Here is a post in which Dogears discusses the GE's (with pictures):

http://ampgarage.com/forum/viewtopic.ph ... l&start=15

Happy New Year!

[Structo]

What is the difference between the ones I have and the green printed ones?

Mine have 12AX7 etched on them and then some dots, but the ink print obscures how many dots there are.

I'm just glad I didn't have to pay $65 each for these....

[boldaslove6789]

What is the difference between the ones I have and the green printed ones?

Mine have 12AX7 etched on them and then some dots, but the ink print obscures how many dots there are.

I'm just glad I didn't have to pay $65 each for these....

The green labeled Ge's are for JAN (Joint Army Navy) surplus, that's all. They're the same.

[ToneMerc]

What is the difference between the ones I have and the green printed ones?

Mine have 12AX7 etched on them and then some dots, but the ink print obscures how many dots there are.

I'm just glad I didn't have to pay $65 each for these....

12AX7: the original version of this tube. This can only be used in parallel filament circuits. This is not a big deal as virtually all audio equipment is of a parallel filament design. These often have large rectangular plates with several horizontal ribs. The older versions have blackplates, which are often preferred by audiophiles.

12AX7A: This version can be used in series or parallel filament circuits. These usually date to the 1960s and have greyplates. Vintage versions of these are about the most sought after tubes of any type today. Often RCA and GE made these for electronic organ manufacturers, and have the organ brand name on the label. These are usually specially selected tubes, and are a great buy---when available! Sometimes, 12AX7A tubes made for the US Military are labeled 12AX7WA, and I have seen WB and WC versions. The W is the military type code, A,B, and C are progressively later productions. These are nice military spec tubes. DO NOT confuse these with current production Russian or Chinese crap with the suffix WX, WB, or WC! These are not military tubes and are not NOS tubes at all!

TM

[martin manning]

Hey TM, I got curious about the difference between the AX7 vs AX7-A, especially the series-parallel filament stuff, and started Googling around. Your description in the post above is verbatim from here: http://www.audiotubes.com/12ax7.htm and has been repeated multiple times elsewhere (without citation, of course). Best to identify sources if possible, but who knows where that guy got it from.

The -A suffix is certainly a redesigned version of the original, and I've seen statements that it is low-noise. Indeed comparing the GE 12AX7 (1951) and 12AX7-A (1961) datasheets there is a statement saying that the -A is "advantagous in applications that require low hum and microphonic output." A total noise figure is given on that sheet where there is none on the older one. Both show the same cnter-tapped filament for series or parallel operation, but the quoted statement might be referring to wiring multiple tubes in series vs. parallel, and perhaps with the earlier version filament-induced hum was a problem with multiple tubes wired in series. Wikipedia has a short article on the 12AX7, which also identifies low-noise versions 12AX7A, 12AD7, 6681, 7025, and 7729, but doesn't shed any light on the series-parallel thing.

[butwhatif]

i have been using these for 40 years

[CHIP]

I've used both NOS GE 12ax7 and 12ax7a. I notice no difference. Maybe the 12ax7wa is better since it's the military version. Never tried one of those.
Brent does know his tubes.

[Structo]

I think my GE tubes are not 12AX7A or 12AX7WA.

Just plain old GE 12AX7 preamp tubes.

Yes I have bought tubes from Jesse before and read a lot of stuff on his site.

Very informative.

He has a video there as well where he describes and shows sample of different tube types and designations.

[David Root]

Tom, judging by the letter date codes, those are from the late '60s or early '70s and will be 12AX7A type.

UPDATE: I checked what little I have on these GE letter codes and C as the first letter is January. CE is Jan '71, CF is Jan '72 & CH is Jan '73. But several letters towards the end of the alphabet were not used apparently, so it's tough to figure where CW would be. Logically, probably more like late '70s. Anyone remember when the Kentucky plant (188-5) closed?

These are good tubes for Dumbles and generally too. I like a nicely balanced long plate GE in the PI in a D-type build and have both long and short plates in my Bluesmaster and the JAN GE 12AX7WA in my 183. Generally people seem to like the short plate in V2 (less microphonic tendency for the distortion stage) and the long plates in V1 and V3.

The GE 12AX7s are generally good gain. I have a few '51 blackplates that run up close to 110 on my tester, where 100 is a full spec gain reading. One of 'em is microphonic in triode 2 but I use it in a cathodyne PI so that doesn't matter.

[ToneMerc]

Anyone remember when the Kentucky plant (188-5) closed?

GE sold the facility in 1/87 to MPD which interesting enough continued to produce the 12AX7WA for the military.

TM

[renshen1957]

Hey TM, I got curious about the difference between the AX7 vs AX7-A, especially the series-parallel filament stuff, and started Googling around. Your description in the post above is verbatim from here: http://www.audiotubes.com/12ax7.htm and has been repeated multiple times elsewhere (without citation, of course). Best to identify sources if possible, but who knows where that guy got it from.

The -A suffix is certainly a redesigned version of the original, and I've seen statements that it is low-noise. Indeed comparing the GE 12AX7 (1951) and 12AX7-A (1961) datasheets there is a statement saying that the -A is "advantagous in applications that require low hum and microphonic output." A total noise figure is given on that sheet where there is none on the older one. Both show the same cnter-tapped filament for series or parallel operation, but the quoted statement might be referring to wiring multiple tubes in series vs. parallel, and perhaps with the earlier version filament-induced hum was a problem with multiple tubes wired in series. Wikipedia has a short article on the 12AX7, which also identifies low-noise versions 12AX7A, 12AD7, 6681, 7025, and 7729, but doesn't shed any light on the series-parallel thing.

The 12ax7 is a 12.6 volt tube (series or paralleled heaters) that has a tap so it can be wired parallel 6.3 volts.

In TV sets and the All American 5 Radios, Tubes with the same current draw (150 mA for example) would be wired in a series string and the voltage would be dropped by the Tubes volt amount.

A 50C5+50C5+12AX7 combination would be possible was produced, and would require 112Volts AC for the heater series chain and with the 110 mains would be in the 10% plus/minus variance built into tubes heater voltage tolerance.

Most AA5 Radios used a half wave rectifier at 35 Volts, Single Ended Power tube 50 volts, and other 12 volt tubes (seven pin sockets) and voltage dropping resistor to equal mains voltage. The home radio had a slight audible hum (from the Output Transformer as well as the AC series heater string and since most radio owners didn't dime the volume it wasn't objectionable. Design followed intended use and target market.

Other Tubes designed to be run in parallel can be run in series provided you ensure their current demands are met correctly, although series heater chains are not recommended for audio because of the hum issue.

Tubes with non matching current draw could be used in series. For example, an EL84 and 12AX7 could be run in series from a 18-20V supply. An EL84 0.76A wired in series 12AX7 0.15A required a resistor placed in parallel with the 12AX7 to pass the additional current without damaging the Tube.

Series AC heater chains don't benefit from a grounded center tap, but could benefit from an elevated center tap, to reduce noise. DC filament would reduce the noise in a series heater. But that would cost money in a value driving design.

Unless you want to build a clone of a 1960's budget/student model by a no name brand (originally without a power transformer; build it with an isolation transformer so no one risks shock or electrocution) there really isn't much point to series chains heaters for a new design

The earliest data sheet (RCA invented the tube in 1946) I can find in my reference source is Tong Sols, 1957. The TS data sheet mentions a younger brother, the 6ax7, which has a 0.6 amp heater current for use in series string TV sets (with other tubes that draw 0.6 heater current) for controlled warm up. A similar baby brother existed for the 50C5, for the same reason.

The 12AX7 design purpose was to replace the 6SL7 (and/or its cousin 6SC7) as the go to audio tube by RCA with a smaller, cheaper cheaper to manufacture tube. (RCA didn't invent planned obsolescence, but perfected it).

Other than the the following Power tubes 50L5 (octal socket), 50B5 (7 pin sockets) and the 50C5 (7 pin sockets) with 110V anodes, which only provide about 2 Watts of power, there aren't any power tubes that would be of interest to Guitar amps that have the same current draw as the 12AX7 to my knowledge.

Why weren't there 12ax7/50C5 amps produced? Socket inventory prices, you would have to keep nine and seven pin sockets in stock.

Actually Garnet Amp did produce a few series heater amps in their Stencil line/budget line for the Canadian Dept Store brand name Guitar amp market (similar to Sears and Monkey Wards selling their own house brand under Silvertone and Airline) a 2w SE, a parallel 4W SE, and a 4W PP amp.

Best Regards,

Steve