The 12AX7 is the most famous vacuum tube in the world . There are over 2 million units in production per year. Two Russian factories, some eastern Europeans and the Chinese still make this tube. It is a dual triode tube, used mainly in guitar and mic preamps. Everybody seems to love that valve sound. The tube consists of two triodes in a glass vacuum.
Its story starts somewhere in World War Two. RCA originally developed the tube for low cost amplifier needs in radio equipment. RCA originally intended for the tube to serve as the two halves of a push-pull amp. In 1947, it was assigned the “12AX7” identifier for sale to the public. Many derivatives of this tube have been made, including lower gain models, low noise models, JAN spec (military) versions, and some very high-quality models for microphone preamps. Over the years, this tube has found its way into guitar amps by Carvin, Marshall, Fender and a host of other makes. This tube is the only decent choice for microphone preamps, and is used by over 10 companies currently making tube preamps.
Vacuum tubes are tricky to use, and tricky to test. They don’t burn out like light bulbs, but they do fail over time. They can lose their vacuum, develop internal shorts, or just plain degrade, as the plate ionizes electrons. The best tubes were made decades ago by western European companies like Telefunken and Mullard. None of the current makers institute rigorous quality control anymore. Tubes made by the aforementioned companies can sell for as high as $300 for an unused matched pair. I have a set of unused Mullards that are worth around $200. The quality problems come from two sources: First, a drop in demand due to the invention of the transistor and solid state diode has eliminated some makers. Tubes aren’t mission critical devices anymore. Second, the old makers have all either moved on or gone out of business and sold tooling. While some of the original tooling is still being used, it isn’t being maintained to tight tolerances. Brand new tubes can sound just fine, but the old tubes were just better made.
Notes on different makes:
I have a small collection of working 12Ax7s and variants with different gain (12At7, 12Au7, etc. No z tubes though) and I will be posting results as I test them and use them in a preamp I am building.
Mullard Tubes are considered to be top notch.
Amperex was a well know UK maker.
Notes on the name
“12AX7” is the American designation. The American designation can be broken down as follows:
12-plate voltage (assuming the plates are wired in series. Each plate draws 6.3 volts.)
A-physical size designation
7-Number of internal elements (anodes, cathodes, grids, heaters.)
The European equivalent it is known as an ECC83.
E-plate voltage (6.3 volts)
CC-indicating two triodes
83-base type and sequence
The US military designation is 5751, as Wiccanpiper reminds me. I haven't a clue on this numbering system.
See Vacuum tube numbering for further information.
RCA Receiving Tube Manual 1970
Materials and Techniques for ELECTRON TUBES, Walter H. Kohl 1960
First in a larger series about vacuum tubes, mic preamps, and tube testing adventures, and other things from an independent study. Node your homework. /msg me comments, etc.