The ’12AT7′ Tube
The 12AT7 is a miniature, high-mu twin triode designed for use as a grounded-grid radio-frequency amplifier or as a combined oscillator and mixer at frequencies below approx 300MHz. This tube has a mu factor of 60, as compared to the higher gain 12AX7 with a factor of 100, and the 12AU7, with a mu of only 20. The 12AT7 is found in many high fidelity applications, as each triode section in normal use operates as a class A amplifier. They also turn up in line and microphone preamplifiers, musical instrument amps, and vacuum tube equipped recording equipment. By their design, they are inherently low noise, making them a good choice for these applications. The tube requires a noval nine-contact socket and may be mounted in any position.
|DOUBLE TRIODE (separate cathodes)|
12AT7 tube equivalents
Special thanks to Brent Jessee Recording & Supply, Inc.for kindly granting Effectrode permission to reproduce the following material below.
- 12AT7: This USA manufactured tube has wing shaped plates. The early versions have blackplates (later versions have greyplates) with a square or D shaped top getter at the top of the envelope. All are great tubes, and the finest (like the RCA black-plates) are on par with the military and industrial versions. Sometimes “selected” versions of this tube turn up. Often made by large manufacturers like RCA, they are usually labelled with the brand of the product they were selected for. Most often electronic organ brands are found on the label like Baldwin, Wurlitzer, and others. Sometimes hi-fi and musicial instrument amps had select tubes made for them, and labels like McIntosh and Fender turn up. The tips of these tubes are usually painted a color, indicating they have been screened for low noise.
- 12AT7WA: This is a military version of this tube, and may also be listed as a 6201. Some tubes have both type numbers on the label. These usually have thick mica spacers. The RCA versions have extra support posts and tiny “12AX7″ type plates, nearly always black. This tube is usually very low in microphonics due to it’s rugged construction.
- 6201: Frequently, this number appeared along with 12AT7WA (military version) on the same tube. However, some brands made the 6201 as an “industrial” 12AT7. Sylvania made a 6201 in their Gold Brand line, and it came in the distinctive gold box, had gold lettering on the glass, and most (but not all!) had gold plated pins. Amperex made a very nice PQ version of the 6201 with gold pins. A few 6201 tubes, usually early ones, have an extra mica spacer on top, known as “triple mica”, and are noted for lower microphonics. Overall, this tube is a step up from the standard 12AT7, and most were made to meet military specs so that both military and industrial versions could be manufactured on the same assembly line.
- 6211: This unusual tube is listed as a frequency halfer, in addition to being a class-A amplifier dual triode with medium mu. Medium mu means it has lower gain than the typical 12AT7, in fact lower than even the 5965 listed below. It also has much larger, flat plates that have diamond shaped points at the corners bent inwards (in the versions I have seen) that seem to add rigidity to the plate structure. This tube also has slightly lower plate voltage and current ratings
- 5965: This industrial tube is close enough to the 12AT7 that audiophiles have started buying them up and commenting on the low noise of this tube. It most likely is because it has a lower gain, a mu of 47 compared to 60 of a 12AT7. It is kind of like using a 5751 in place of a 12AX7–the lower gain being the main difference. These tubes are still plentiful, and are really well made. GE made a fine version of this tube in their “5-Star” broadcast line. A few have an extra mica spacer on top, known as “triple mica” and noted for lower microphonics. There is also a rare Telefunken version of this tube with long plates.
- E180CC or 7062: Most often found in European types, typically Amperex or Philips, this tube is virtually the same as a 5965 in regard to electrical specifications. Audiophiles have been seeking it out since the price is generally lower than the premium ECC81 N.O.S. prices currently. Like the 5965, it has slightly lower gain than the 12AT7, and has the added advantage of a 10,000 hour heater life to make it another audiophile “secret” tube. A super bargain when you find them, since they are musical and smooth to listen to, and actually sound better than many standard 12AT7 tubes. This tube is about a half-inch taller than the standard 12AT7, so chassis space is an important consideration.
- 6414: An industrial, computer rated dual triode, virtually the same specs as the E180CC or 7062 listed above. Unlike the E180CC, this tube was made in the U.S.A. It is a more rugged build, with the Raytheon black-plate type sporting triple mica and military ratings. This tube is also like a 12AV7 and will work wonders in any 12AT7 spot, and may be just the ticket if you want to boost the gain of a 12AU7 circuit. The 6414 is getting very hard to find, but with a 10,000 hour heater rating you may never need a replacement. Discussion groups rate this type very highly for use in headphone amps. Stands about 1/2 inch taller than the standard 12AT7.
- 6829: Yet another industrial tube that is very similar to the 5965. This one has a mu factor of 47 so it is slightly lower than the typical 12AT7. Those I have heard are rich, detailed, warm, and thanks to the lower mu, very quiet and low in microphonics. GE made these in their 5-star and military line so they are a rugged and long lasting tube. Like the E180CC, this tube is about a half-inch taller than the 12AT7. An excellent audio bargain now, but prices may soon climb as audiophiles discover them!
- 6679: This is an industrial 12AT7 designed for mobile communications use. It maintains it’s output even if the filament voltage varies. Nice tubes, but they never have been too plentiful. They will work fine in hi-fi use, and are a step up in quality from the standard 12AT7.
- 7728: This was a high-end industrial tube made by CBS or Hytron (same factory made both, actually) with gold pins. Very scarce.
- ECC81: This is the European version of the 12AT7, and is identical to it. The Telefunken even has the wing shaped blackplates, just like the RCA. Great tubes, equalled only by the RCA blackplate or the military/industrial versions. Even those made for U.S.A. hi-fi products in the 1960s (Knight, Bogen, Daystrom, Dynascan, etc.) are excellent tubes, and have the fine sound quality of those with Mullard, Amperex or Telefunken labels. The rarest are the 1950s versions of Mullard and Amperex with a “D” shaped top getter.
- E81CC or ECC801S: This is a rare tube, usually found only in the Telefunken or Siemens brand. These tubes have been screened for audio use and low noise, and have a 10,000 hr. rated life on the heater. The ECC801S are by far the rarest, and the E81CC only a little more common. Both are excellent, low noise, finely crafted tubes, with the E81CC being the best value currently when compared to the recent spiraling rise in the ECC801S prices. They are both rare, but if you find some they may well be the last 12AT7 tube you will ever need to buy!
- CV4024 and M8162: Fine British military tubes from the 1960s and early 1970s. I have seen mostly Mullard examples of these, although Brimar did make some. Depending on the manufacturer, the tube may have both numbers on it. The rarest and best are the early Mullards labeled with M8162 and the shield logo. Most of the Mullard military tubes were made at the Mitcham, UK factory, which made most of the British military tubes.
- CV455: Another British military type of 12AT7. Usually Brimar was the maker of this fine tube, with the earlier versions from the 1950s being the most desirable. Watch for the versions made in the 1970s. These have wing shape black plates and a sound close to the earlier type, at often half the price!
- 6060: This rare tube is the Brimar answer to the Genalex A2900. It has a similar construction to it. These are sometimes called the “Yellow-T” tube due to the large capital letter “T” in a yellow triangle on the glass. The best of these were made by Brimar at their Footscray, UK factory in the 1950s. Virtually all of these are phono grade, they are very quiet. There are later versions with the “T” logo on a white triangle, but these are not as desirable, sonically, and when found should not be nearly as expensive.
- A2900: The even scarcer A2900 is in the “Gold Monarch” series by Genalex. Genalex was the premium line of tubes produced by the Marconi-Osram Company in England. It has wide, flat black plates and white lettering on the glass, the words “Gold Monarch” in white script, with the red Genalex decal logo around the top of the tube. There is also a British domestic version with the grey and blue GEC decal on the glass, that was not exported. These are true audiophile tubes, with tightly controlled noise specs, designed with audio perfection in mind. Any scarcer and these would be museum pieces!