I thought I'd post this here as there may be some folks that were interested....
12AX7 types at a glance:
Preamp tubes .... a few bits of info:
I received a question from one of the folks on a forum in late 2007 who wanted to retube his whole amp. For his objective I felt it was not necessary to replace every tube. He was making some classic mistakes and after giving him some guidelines and suggestions I felt that some of what I wrote might be beneficial for others.
On preamp tubes ... this is all personal preference but you DO NOT need to change them all. Experiment with V1 ... the tube most close to the input jack. This is 85% of the tone and gain in your amp and has the most effect.
Again ... this is personal taste but a little guideline here is:
12AX7R - lowest gain and darkest. Generally better in current driver spots such as effects loops or reverb circuits rather than in the front end. This tube is also known as the Sovtek 12AX7WA and Sovtek 12AX7WC. The gain of the WA is lower than any other 12AX7 type. The gain of the WC is about average compared with other 12AX7 types.
12AX7R2 - smooth but not bright, sort of mid range in response. Average gain. Great in phase inverter positions too. Also known
as the Sovtek 12AX7LPS. How a phase inverter breaks down and passes signal to the output section can affect the tone and feel of your amp. I know some say the phase inverter has no effect. I disagree and am happy to demo this to others and let them decide. In any case, my personal preference in many cases in a longer plate tube for the phase inverter in amps that use 12AX7 phase inverters. This is also a great front end tube for some tastes.
12AX7R3 - bright and articulate. Average gain. Can sound a bit thin to some ears in some amps. This tube is also known as the
12AX7EH (Electro Harmonix) and with a slightly different internal structure also known as the Tung Sol Reissue. These tubes are all over the map on specs so if you buy a EH or Tung Sol version get them from a trusted vendor that tests them well. If you get these in the GT Gold Series they are already tested for noise, output, etc. and will be within a good spec range.
12AX7C - Chinese 12AX7. The most smooth and linear of the 12AX7 family. A lot of Marshall folks swear by these and in Fender
type front ends are really nice. Make sure you use generation 9 only. Generation 7 and 8 are not as nice. Some early generation 4 tubes are prized by some high gain amp folks. There are a lot of "Chinese" 12AX7s on the market and lots are Gen 7-8. There are also a lot of Chinese tube companies and co-ops such as Sino. These are a great all around tube when selected by a good vendor. GT has nice ones but another great vendor of super 12AX7C's is http://www.arselectronics.com
as their process is top notch and their prices are great. ARS also performs reliability testing on every incoming batch of tubes they sell so they are one of the few folks aware when a new run is garbage. These are also the tube I select for tube preamps or amps with tube front ends and solid state power sections. This is also my pick for bass amps with tube front ends.
ECC83S - This is the JJ produced tube and has a short plate design that is very free of physical microphonics. These have a different mid range response than other 12AX7 types. They are the most gainy of any of the 12AX7 family and the tubes that have the highest percentage of ones that fall in the 85+ actual gain spec. The gain of a 12AX7 should be 100 at 250 plate volts with a 2 volt bias but most tubes made today are 75-80 or so. Many of the ECC83S tubes exceed a gain of 90. Just a five point drop in gain in the front end of many amps will turn the amp into an OK amp rather than a great amp as this is the main tone and gain stage in many amps. These have a classic British response; Vox, Marshall, Selmer etc. These are what I use to build the SAG-MHG kits after hand selecting for gain, current output, transconductance and plate resistance.
Ei 7025 long smooth plate - Not available at the moment from GT or perhaps anybody else as Ei is getting back on it's feet but ... if
you can find any of these out there they are the highest gain 12AX7 tube around but tend to be physically microphonic in many amps and if they are working nice today it is no guarantee that they will work that way tomorrow.
12AX7M - GT has re-tooled this tube almost a dozen times in the four or so years since it's release to make it more consistent,
more stable, and just better in every way. They were out of stock for a long time and will be back in January of 2008 with any luck.
In the most recent batch I tested (in production now), the gain is now on par with the ECC83S. They are smooth and in Fender tolex era amps are just terrific and with a Tele will tame the
brightness. There is a following of Marshall and 5150 folks that love these in their amps in the past and they look to be much more stable now. This is the tube that I use most often in the SAG-MHG kit as the phase inverter, the third tube in that three tube set. It is a long plate tube so I also like these in phase inverter positions along with the 12AX7R2 (Sovtek 12AX7LPS) as my other preferred tube for PI use. This is a GT exclusive tube.
5751M - Think of this as a lower gain (about 70) 12AX7. This is a tube to use in V1 when you want more clean headroom and a
smoother response. This was one of SRV's tricks in the first gain stage of some of his Fender amps. This is a GT exclusive tube.