Update / dialog addition to this entry:
This has always been an ongoing offer but I thought I would mention it here again: If there are any folks in the So Cal area that have any of the GT 12AX7M or GT 5751 tubes and would like to have them tested and traced I would be happy to do this at no charge. Drop me a note via email or send me a note on facebook if you are on facebook and we can set up an appointment. I do not mail things back and forth so you would have to be local enough to North Hollywood to bring the tubes into the shop. This would be done while you wait (and watch).
Neal Logdahl, a friend on facebook asked me: What's your take on the GT 5751M? Neil is the man behind HipKitty Products. He is the owner and chief engineer of http://www.hipkittyproducts.com/ in Nashville, Tennessee.
Here is a bit of info that may be of interest to a few folks.
The original 5751 was also intended for phase inverter use and also had a requirement that the two sides of the dual triode be within close match.
The GT (Groove Tubes) 5751M was developed during my tenure as tech support and tube development engineer during my six plus years in this position at Groove Tubes. Without going into too much detail and just hitting the bottom line, the GT-5751M bears no relationship to a 5751 from the past. The tube was put on the marked after the GT-12AX7M Mullard Reissue was released by Groove Tubes. The 12AX7M was the lowest gain tube of any 12AX7 tube on the market including any NOS offerings as well. The plate resistance was generally less than half of the 62.5K design spec at 250 volts with a 2 volt bias.
This made the tube crosstalk between the A and B side in many amplifier designs. The tube would act more like a capacitor than a tube. It would click and sputter during warmup and in many cases the noise would not go away. Tubes that were initially quiet would become noisy in a short time in many amps. The tube was a failure in every way except one. Their plate current was way too high generally but this factor is what I look for when I pick phase inverters for certain amps. Most 12AX7 production tubes made today are down 30% and more on plate current. The 12AX7M was too high. Current is what I look for in tubes that push the output set not gain so in this case the 12AX7M was a great tube.
I have two data sheets for real 5751 tubes. One is the RCA data sheet and one is the Tung Sol data sheet:
http://www.guitaramplifierblueprinting.com/Tube/5751.pdf - RCA
http://www.guitaramplifierblueprinting.com/Tube/5751WA.pdf - Tung Sol
The data sheet below from Groove Tubes:
What is that little * above where it says Specifications?
Originally, when I was at GT, part of my job was to test tubes and produce REAL data and not just reproduce spec sheets based on design spec from the past. I used to publish actual numbers. When I left this practice stopped. Perhaps GT no longer had the tools to do this type of testing as I took them with me as they were my tools. It seems as if these days GT is just reproducing design spec to produce their own data sheets.
Reality: The GT-5751M is a re-labeled GT-12AX7M tube with even lower gain than a 5751. It is sold on the basis of being a low gain 12AX7 but misses the mark when it comes being anywhere close (or usable) in regard to transconductance, plate resistance or plate current. It runs very hot, has curves that do not resemble anything close to a 12AX7, 5751, 12AY7, 12AT7 or 12AU7.
*SPECIFICATION MEASUREMENT PROCEDURES (below is from current GT data sheets)
The measurements on this sheet are actual test results of current production tubes made in a neutral and consistent manner for the purpose of fair comparisons. We decided not to use just reprinted “target specs” from an old RCA book like most, if not all, current production spec sheets released today. If these specs do not meet the old RCA or GE published spec sheets, do not be alarmed. In fact, NO tubes made today meet all these original specs, and only a few come even close in the critical areas of Transconductance and/or Gain.
We believe it is more important to provide our customers a meaningful standard by which ALL tubes made today can be compared. So beginning with our 2007 spec sheets, our first ever published, ALL the tubes we manufacture and/or made in other factories and/or other tubes that are in our catalog will reflect actual test results and averaged over at least 10 samples. As production quality rises and falls (frequently) we reserve the right to make these results subject to change as and performance rises, or falls. Current data sheets reflecting latest productions can be found on the GT website at www.groovetubes.com.
I was at GT from 2002-2008. The 2007 date above was describing one of my job functions at GT, a function which I developed. I was the one who came up with the concept of being the industry watchdog to assure all tube vendors had some degree of quality. I would publish my test results on various music forums and on my own GAB website on all vendors, not only GT. The current data sheets on the GT website are not my work and may be little more than editing of past data sheets. The tubes I test these days are just as awful as those in the past.
The 5751M is born: Aspen Pittman is a master of marketing. He was described by Richie Fliegler (please excuse spelling as it is probably wrong) as a mix of P.T. Barnum and Billy Graham the Evangelist. When the gain was found to be so low on the 12AX7M that it was something of a tragic selling point GT's founder and President decided to market the rejects as a lower gain tube, a new product.
Aspen was always coming out with new products. The SAG (special applications group) products of high gain kits, soft touch kits and matched phase inverters were all items I had been offering my clients at Guitar Amplifier Blueprinting for years. Aspen felt these products could be marketed by GT and sold by GT. That is what happened. People like Rick Benson (now at 65 Amps) can attest to how many thousands of dollars of these products were sold every week at GT. Did I receive anything for development of this product or from the sales of the product? No. But I digress. To use a crude old phrase, Aspen "polished a turd". He took the 12AX7M gain rejects and sold them as the 5751M. To repeat myself; the 12AX7M was also a low gain tube, generally somewhere between the gain of a 12AT7 and 12AY7 at best.
From the current GT website - 5751M description:
This is a lower gain 12AX7 tube type. Stevie Ray Vaughn used these to change his front end distortion to back end distortion ratio to have more of the touch sensitive output tube distortion.
These test as per 5751 spec with the proper transconductance and current output.
The slightly lower gain quiets many amps down and these are a very smooth front end tube but are also quite amazing in the phase inverter position.
In the Fender Hot Rod series these are really the rage these days with many of GT's folks that use these amps.
The 5751 is slight variation of the popular 12AX7. It is interchangeable in all circuits calling for a 12AX7. It can be effectively used to “customize” the amp's preamp stage to reduce gain when placed in the V1 position (first gain stage in the signal path). This will also allow for more clean headroom before distortion and soften the dynamics as well. Stevie Ray Vaughan used NOS 5751 preamp tubes in many of his Fender amps for this purpose.
Our exclusive GT5751M tube design, like its close sister the 12AX7M, is an exact replica of the original British built Mullard ECC82/12AX7 dual triode preamp tube, but with slightly lower 5751 Gain and TC specs. This famous Mullard dual triode design from the late ‘60s has a unique four (different) mica spacer design that reduces tube hum and microphonics by suspending the Cathode in a complex mica layered structure which acts like a shock mount. It is unlike any other preamp tube ever made (most all preamp tubes use just a single mica design!). This same Mullard design was licensed by other tube factories, such as Amperex, who used this design for its legendary Bugle Boy series of preamp tubes.
The GT 5751M has the same great rich classic sound of the GT12AX7M; a distinct tone along with high definition no other currently produced preamp tube can match. It is Aspen’s (our founder’s) personal favorite in ALL his modern high gain amps for adding that softer vintage touch and tone.
We highly recommended this tube for the Fender Hot Rod Deluxe amps to fatten up the tone and “tame” the higher gain preamp stage. You will get more natural power tube distortion that will be controlled more so by the player’s touch, than the Gain and Volume settings.
The above reminds me of the old story where P.T. Barnum purchased many boxcars of canned salmon without first seeing what he was buying. Upon inspection he found the salmon did not have the expected color of the canned salmon of the day, the nice rich pink color. It seemed that it was a washed out pale white color. He realized nobody would buy his product. So, he developed a marketing campaing "The highest grade salmon available - guaranteed not to turn pink".
Update 8/10/10 - Over on one of the music forums somebody asked what they should do when they needed to replace the Groove Tubes 5751M or 12AX7M when they failed or needed replacement in the future. This was my reply:
When you need to replace try a real NOS 5751. Less gain generally equates to less noise and this is one cool aspect of a 5751. You might find an NOS 5751 will give you the tone you like but they will last decades and be reliable with less worry. NOS 5751s are available and not all that pricy.
As a bit of a guideline, most of the GT 5751s actually spec out gain wise as a 12AY7 (gain about 40 rather than 60-70 of a 5751) The 12AY7 was the front end tube in the Fender tweed bassman, tweed deluxe, bandmaster and super. The average 12AX7M in the runs over the last two or so years at GT had gain of a 12AT7 at best on average. This is about 70. Many tubes are in the 60 range. When the tube fell below 60 they were screened as a 5751M but the plate current is off the map (generally three times or more than spec) and the plate resistance and other factors are nowhere close to spec.
It may have been better to market the GT 5751M as something like a 12UJ7. Some number that does not exist. Then it would not be compared. It could be said that it was a possible replacement for a 12AX7 type tube that just sounds different. That may have been the most honest approach.
Well Neil, here is the answer to your simple question. I am not a man of few words. Feel free to repost this wherever you wish including music forums.