12AX7 & 5751 Tube List

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The 12AX7 is one of the most widely used double triodes in audio circuits.  There are several variations of the 12AX7.  In most cases, they can all substitute for each other.  I will first attempt to describe each version and provide data for each.  Then, I will follow up with short descriptions of some that I currently have for sale.

Audio People Please READ!  There is no way I or anyone else can tell you how any given tube will sound in your equipment.  There are too many subjective variables for me or anyone to tell you unless they have your equipment and your ears.  You need to try different types to see what you like best.  I know that I would sell more tubes and make more money if I did, but I'm not in the business of pedaling  B.S.

Telefunken

Telefunken produced some of the best tubes that have ever been made anywhere at anytime.  They are remarkably devoid of any microphonic problems and tend to last a very long time.  It is fairly rare to see a weak or bad Telefunken tube.   The sound is characterized by many to be "mellow" or "dark" but really if you want to know how a tube sounds, you need to try it for yourself in your own equipment because it may sound totally different to your ears than someone else's. There are basically 2 different kinds of Telefunken 12AX7's.  The "smooth" plate version has as large totally smooth plate surface.  I believe that these are the earlier version.  The later version has ribbed plates that look much like RCA plates.   I really couldn't tell you the difference in the sound quality....like I said, buy one of each and try them both.  There are allot of "fake" Telefunken tubes out there, you need to be careful.  Any tube produced in the Telefunken factory should have a "diamond" molded into the glass in the base of the tube inside the circle of pins.  If you don't see this diamond,  then the tube wasn't produced by Telefunken. There are many Telefunken tubes out there that don't have the Telefunken name on them, but instead an OEM brand like Fisher, H.H. Scott,etc.  These are true Telefunken tubes as they have the characteristic diamond in the base.  This is true for other brand names of tubes like Amperex, etc.  It has also come to my attention that there are some perfect "fake" copies produced in China which are difficult to tell from the genuine ones.  The only way I could tell is by looking at the writing on the tube.  Original Telefunken printing is very poor and will wipe off easily with your fingers or just a little moisture.  For that reason, allot of Telefunken tubes are devoid of most of their writing.  These fakes have very nice writing and the printing will not rub off with your fingers or water.  So if the tubes you see have really indelible printing, suspect that they are fakes.  While these fakes look good, they apparently sound horrible (as I have been told).  Buyer Beware!  As with all tubes, we offer a 10 day evaluation period in which you may return an unsatisfactory tube.

Currently Available Telefunken 12AX7's

Amperex tubes were originally produced in Holland (The Netherlands) and are very high quality tubes.  Many prefer these to the Telefunken and are probably the second most sought after brand of 12AX7.  There are many versions of this tube.   The earliest versions from the 1950's have a "treble clef" logo or just the name Amperex (looks like the lettering above).. 1960's versions have the famous "bugleboy" logo.  1970's versions have the "world" logo in orange printing.  All sound very good and will last a long time.  Be careful of the orange printed ones.  Some of them say "Made in Holland" and in fact are Japanese.  The easisest way to tell the difference is to look at the side of the plate in the center.  There will be a half circular hole on one side of the plate and not on the other if the tube was made in Holland.  If there two holes forming a complete circle, then the tube was made by Matsushita Japan.  The Japanese made tubes will also have a dark colored getter.  Actually, the Japanese made tubes are nice tubes, too as the Matsushita plant was set up by Mullard..  They just aren't Amperex Holland.  Amperex tubes made in Holland have a curious characteristic.  When the filament is first energized, it flashes very bright for a second and then dulls down to the normal soft orange glow. Other tubes do this as well such as Mullard, Matsushita and Ei Yugosolvia. 

Currently Available Amperex 12AX7's

Mullard U.K.

Mullard tubes made in the U.K are also very well thought of tubes for good reasons.  They sound great and last long.  What more could you ask for?   The early versions had long narrow ribbed plates.  Later versions had wider shorter plates that looked mre like RCA plates.  These also flash brightly when first energized.  They normally have the 4 ridges or seams at the top of the tube and have 6 small black letters/symbols near the bottom of the tube.  IEC Servicemaster bought the rights to market Mullard tubes during the 1960's and 70's, so many Mullard tubes are dual marked "IEC Mullard".  These are still genuine Mullard tubes.   Mullard also marketed a special version called their 10M series.  The 10M series was rated for 10,000 hours and guaranteed for 2 years.  They were also were supposed to have been selected for low noise/microphonics.  These are usually IEC/Mullards. 

Currently Available Mullard 12AX7's

  Other European 12AX7's

RCA produced a very nice 12AX7 which is prefered by many people.  The oldest version (1950's) had black ribbed plates and did not have the "low noise" type filament which will be marked as 12AX7 not 12AX7A..  These were so well made, the low noise filament is not an issue with most of them.  1960's and 70's types were 12AX7A's or low noise 7025's which had ribbed gray plates.  7025's were specially designed to have low hum and noise characteristics and is the choice among many Hi-Fi types.  This is not to say the non 7025 tubes are noisy.  They are normally very quiet, too.  RCA 12AX7's seem to be very popular for electric guitar amplifiers.

Currently Available RCA 12AX7's

General Electric

GE tubes are good quality general purpose tubes that work well in most all applications.  Early GE made tubes are made especially well and have very unusual large plates that are ridged all the way around. Later ones have sideways plates like Sylvanias.  You can always tell a GE made tube by looking for the number on the tube.   It will be etched into the glass rather than printed.  These are just fine for replacements in guitar amps as well as HiFi.  They make good replacements for communications equipment as well.

Currently Available GE 12AX7's

Sylvania

Sylvania, like GE, produced good quality tubes.   The plates were always gray and "sideways".  Sylvania tubes are especially liked by ham operators due to their reliability and longetivity.

Currently Available Sylvania 12AX7's

Special 12AX7 Equivalents

Misc 12AX7

Type 5751

The type 5751 is a close relative to the 12AX7.  5751's have a lower gain factor than that of the 12AX7.  The gain factor of the 12AX7 is 100 and the gain factor for the 5751 is 70.  However, the plate resistance and transconductance are such that this tube makes a good substitute for type 12AX7.  According to the GE technical manual, the 5751 is a special quality tube for use in critical industrial and military applications in which operational dependability is of primary importance. Features of the tube include a high degree of mechanical strength (eliminates microphonics), and heater-cathode construction capable of withstanding many thousand cycles of intermittent operation.  Other technical manuals indicate that the 5751 also has matched triode units.  This tube makes a great hi-fi tube due to it's lower gain factor and a good guitar amp tube due to it's excellent microphonic performance.

Currently Available 5751's