12AX7’s are one of the most commonly used audio tubes. These were made by the millions in the golden years of tubes, and are still manufactured today in very large numbers. We will cover some of the older 12AX7 types in this article.
The tube roughly appears in the mid 50’s and is a high mu ( high amplification factor of 100) dual triode, meaning that each glass envelope contains two distinct and separate triode sections. The tube also sort of marks the demarcation of batteries from 6 volt to 12 volt as the tube can be configured in a 6 or 12 volt heater (OK, 6.3 volt and 12.6 volt heaters). For those old enough, 50’s cars sported 6 volt batteries and later models and current ones all use 12 volt batteries.
The history of the Mullard tubes starts off with the long plate series, where the plate structure is about 17 mm long. Later tubes are shorter and measure about 15 mm long. From 1954, Mullard made a long plate with the acid etching code of mC1. In 1957-58, they issue a variant with the etched code of f91. This is followed by the f92 and then the short plate series I61 and I63 and I believe an I65
The short plates were available in a Gold Lion version with gold plated pins, but these are 1960+ production tubes.
According to most sources, Mullard shut down manufacture in 1985. It should be noted that Mullard sold much of their tooling to Matsushita ( yep, Panasonic), so many Japanese 12AX7’s bear more than a simple resemblence to the later Mullards
Mullard made a special military 12AX7 called a CV4004. This one is called a box plate (Brimar also made one but not quite as boxy but with superb top end), and the plate structure consists of a large rectangular box. Build quality is excellent and the box plates have the best and cleanest bass of any 12AX7 tube.
Vacuum Tube Valley Magazine liked these box plates, but warned readers that there were unobtainable. With the fall of the USSR in 1991, NATO tube stocks were eliminated in about 1993-95 and many of these tubes then entered the surplus market. While the surplus stock is long gone, ADL still has a few of these rarities available.
In dissecting the CV4004 tube, the grid wires are of an extremely fine gauge, so small you can barely see it with the naked eye. While most grids are wound with often visible spacing, the box plate grids are so close, shifting the grid post wires creates an iridescent effect similar to a diffraction grating. This is extremely difficult to manufacture because the grid wires are wound onto a serrated post and the serrations are then burnished down on the wires in order to secure them. They are a purely mechanical assembly, not welded.
In the US, 12AX7’s were made by many companies: Raytheon, RCA, Sylvania, and GE and others, although these were the big boys in the industry. Most early US production feature a black plate construction where the large visible metal plates were carbonized and appear as a shiny black metal. Later production use a gray plate with a sand blasted finish.
Many of the early and more desirable tubes feature a triple mica construction. There are three sheets of mica in the glass envelope. Two are the standard mica sheets necessary for spacing the elements of the tube structure and centering them in the glass, top and bottom, and the third is a mica sheet inserted on the top of the structure. The third mica sheet is designed to prevent the getter material (the silver flash on the tube top) from contaminating the cathodes of the tube triodes. Later European makes as well as few American types use a sort of sprung mica sheet to cover the cathodes for the same purpose
In general, the black plates have a slightly richer midrange and the third mica extends the high frequency response.
ADL’s “bogey” 12AX7 is the GE 5751. This is the standard 12AX7 from which we make comparisons to all others. The 5751 is a military tube with a slightly lower amplification factor of 70 versus the 100 of a 12AX7. The change in ultimate gain is rarely noticeable, though, in most circuits. It should be noted that in our experience, the 5751 exerts a tighter control over the music giving a more detailed sound, in general ( there are some exceptions).
The GE, like the later Sylvania’s, sport an unusual construction. The plates in the glass envelope are turned 90 degrees and the grid posts face each other. This actually throws most of the heat generated on the plates away from the other triode and and since the grid posts actually create an electronic “shadow”, the shadow zones actually face each other for less crosstalk, in my opinion.
The Sylvania and GE have very similar sound as they share similar construction. Both are very extended and very even, although some may consider them a bit on the leaner side. We consider them to be very neutral.
It should be noted that early Sylvanias have a construction more like RCA types ( parallel plates with the edges pinched and stapled). Early GE have an unusual long plate with a sort of oval cross sectional area. They are more midrangy and usually quite dynamic (I believe these are 50’s vintages)
We’ll follow up with more about this versatile and common tube later.