The ’12AX7′ Tube
The 12AX7 is a miniature high-mu twin triode each section of which has an individual cathode connection. This tube is especially suited for use in resistance-coupled voltage amplifiers, phase inverters, multivibrators and numerous industrial control circuits where high voltage gain is desired. A centre tapped heater permits operation of the tube from a 6.3V or a 12.6V heater supply. The 12AX7 is typically used as a class-A amplifier in the hi-fi, guitar and professional line and microphone preamps.
12AX7 tube equivalents
source material by Brent Jessee Recording & Supply, Inc.
- 12AX7: the original version of this tube. This can only be used in parallel filament circuits. This is not usually a problem as most audio gear utilizes parallel filament design. These tubes often have large rectangular plates with several horizontal ribs. The older versions have blackplates, being preferred by audiophiles.
- 12AX7A: This version can be used in series or parallel filament circuits. These date to the 1960s and have greyplates. Vintage versions are 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, if you can find them. Sometimes, 12AX7A tubes made for the US Military are labeled 12AX7WA, where the W denotes the military type code – A, B, and C being progressively later productions. These are high quality military spec tubes, not to be confused with current production Russian or Chinese tubes with the suffix WX, WB, or WC, which are not military tubes and are not NOS tubes at all!
- ECC83: This is the identical European equivalent of the 12AX7. ECC83 tubes manufactured by Mullard, Telefunken, Amperex and Siemens are sought after. Telefunken tubes have a diamond shape molded into the glass on the bottom center of the tube. The most desirable of the Telefunken ECC83 tubes is the “smoothplate” or “flatplate” versions. Amperex and Mullard tubes have a pair of alpha-numeric date codes printed in dark grey near the bottom of the tube. The older Mullard logo that looks like a shield, especially with the letters “BVA” below it. Look out for for the Mullard “10M” series of ECC83 tubes in the distinctive royal purple and gold boxes. These sweet tubes were factory screened for a 10,000 hour heater life, matching internal triodes, and low noise, rather like the Telefunken ECC803S tubes. The 10M Master Series has gold plated pins, and the 10M Concert Series has standard pins. Demand and scarcity has driven the price of these tubes to record heights. The 1950s versions of Mullard and Amperex (and some rare Siemens) tubes often had long narrow plates, with D or square getters, and are the scarcest and finest sounding of these two brands.
- 7025: This is the low noise, “industrial” version of the 12AX7. This has a spiral wound filament which reduces hum and noise. These are usually the best choice for demanding hi-fi applications, although the premium brands and blackplate types also have very low noise.
- ECC803S, E83CC, E803CC: This is a premium European version of the ECC83. The “S” indicates Special, and these tubes were screened for low noise and matching internal triode elements. These are rare in the USA. Telefunken and Siemens are the brands most often spotted, although several of the Mullard factories made these for various labels. Most of the Telefunken and Siemens tubes had gold plated pins. Sometimes the E83CC also carried the 12AX7WA military type label, even though it is a European made tube.
- B759: This is an ultra-low noise version of the 12AX7, with matched triode elements. To my knowledge, only Genalex (Marconi-Osram Valve Co.) made these in England for their “Gold Lion” series. Extremely rare in the USA.
- 6681: This is listed in some tube manuals as a “special 12AX7″, otherwise the electrical specs are the same as the standard version. Being an industrial type it most likely has a ruggedized filament, and is built to withstand many on-off cycles. I believe only RCA made these, regardless of brand on the label. UPDATE: One astute customer has informed me that GE offered 6681 tubes with the typical top seams, date codes, and “Gt. Britain” marking that indicates English Mullard manufacture. Indeed, some GE 12AX7 and 7025 tubes from the 1970s were also Mullard made, so they must have made some 6681s for GE as well. Overall, the 6681 is an excellent step up in quality from the standard 12AX7, and is a bargain at current prices.
- 5751: This version was usually the one made for the US military. These have identical plate resistance to the 12AX7, but the gain is 70 compared to 100 for the 12AX7 vacuum tube. Great hi-fi tubes, as the lower gain reduces noise and microphonics. These are all ruggedized and most can withstand a drop on the floor. The internal triode elements are matched as well. The early versions are more sought after as they usually have extra support rods and an extra mica spacer on top of the plate structure (“Triple Mica”), as well as blackplates, all said to contribute to low microphonics. Don’t overlook the greyplate versions, they are also excellent tubes, and many of these have the triple mica as well. The GE non-military version is usually the “five star” series, intended for broadcast use. The RCA broadcast type is the “Command” series. Both the RCA and GE broadcast types are scarce, as are any of the triple mica types.
- 12AD7: Non-microphonic/low hum version of the 12AX7 vacuum tube made by Sylvania. Great for audio applications that uses the 12AX7 vacuum tube.