Changing the tubes in your guitar amplifier can change the tone just as much as changing the speakers.  Many people claim the best 12ax7 preamp tubes and 6l6 power tubes are the RCA, GE and Tung Sol tubes that are no longer being made.  Those tubes do sound great but they are expensive and dwindling in supply.  Fortunately, there are also a number of high quality current production tubes that are available to upgrade the tone of your amplifier.

12AX7 Tubes

There are basically three types of 12ax7 tubes.  One way you can tell the difference is by looking at the plates inside the tube.  Shorter plate tubes (the types where you can't see the space in between the two plates) tend to be quieter and more reliable, but medium plate tubes tend to sound better.  There are also long plate tubes, and some claim they have the best tone of all.  But the long plate tubes tend not to be rugged enough for electric guitar amps.

Sovtek is the primary OEM supplier of preamp tubes. The Sovtek 12AX7 is what you would typically find in a Fender amplifier.  It is a quiet and reliable tube, but doesn't have the best tone. The 12AX7WA is most common; the 12AX7WB is a higher gain version of that tube; and I understand the 12AX7WC is the lower noise version. All three of those tubes have a short plate structure and sound fairly bright. 

If your amplifier is a high gain Marshall or Mesa Boogie, you may appreciate a more warm sounding short plate tube.  Examples of that would be the European style JJ ECC83 (ECC83 is the British name for a 12ax7) and the Chinese Shuguang 12AX7 (I think the Groove Tubes also sells this tube as the 12AX7C; and Tube Amp Doctor sells this tube as well).  The quality of the Chinese 12AX7's has gotten better in recent years, but I still like the tone of the JJ ECC83 tubes better, especially the gold pin version.

If you want an nicer clean tone or slightly distorted blues tone, the newer tubes Sovtek makes for Electro-Harmonix, Svetana and Tung Sol are better choices. Each of those tubes has a medium plate structure, like the original RCA 7025's.  To my ear, the Electro-Harmonix 12AX7 is the best sounding lower priced 12ax7 out there (the Svetlana 12AX7 looks like the same tube, but I've never tried one). If you don’t mind spending a little more, I think the Tung Sol 12AX7 "reissue" is even better (unlike the Electro-Harmonix version, the Tung Sol has a “halo” getter, just like the RCA). And if money is really no object, there is also the Groove Tubes 12AX7M, which is the most expensive 12AX7 in current production.  The 12AX7M is supposed to be a copy of the European Mullard 12AX7.  The 12AX7M is another great sounding tube, but for my money, the Tung Sol is a better value.

The third option for preamp tubes is the long plate tubes like the Sovtek 12AX7LP (the “lp” stands for “long plate”) and the tube Sovtek sells as sells as a Mullard 12ax7 "reissue" (the two tubes look identical, except for the name).  These are nicer, bright sounding tubes with great harmonic content, but the long plates make them prone to microphonics.  The old Ei 12AX7 and the new JJ ECC803 are similar long plate tubes.  The original grey, smooth plate version of the Ei 12AX7 was a great tube, but I didn't like the later silver plate version, and I understand they have now stopped making them.  I haven't tried the JJECC803, but I have heard good things about it.  If you play clean and you get your tubes from a supplier who tests them thoroughly, then a long plate tube may give you the best tone.  The problem is they tend not to do well in high gain amps and combo amps.  As a result, you will often seem them marketed as being more appropriate for high end audio.

Another thing to consider is the location in the amplifier of the preamp tube you are replacing.  The preamp tube that is located the furthest away from the power tubes will generally have the greatest effect on the tone of the amp; and the preamp tubes that are closer to the power tubes will have the least effect on the tone. 

6l6 Tubes

6l6 power tubes not only have different size plates, they also come in different size bottles.  But there are still basically three types to choose from.  The short plate, big bottle tubes are more reliable, but don't have the best tone.  The long plate tubes are the ones that sound better.  And those tubes come in two varieties: the big bottles, with the getters on top, and the medium size, clear top bottles, with the getters on the side.  

Sovtek is also the primary supplier of OEM 6l6 power tubes. The Sovtek 5881WXT is what you would typically find in a Fender amplifier. It is a short plate, big bottle tube.  Like the Sovtek 12ax7's, the 5881WXT is a quiet and reliable tube, but it isn’t the best performer out there.  A similarly constructed tube that has a nicer tone, and is especially good for distorted rock tones, is the JJ 6l6.  But the best of the short plate 6l6's is probably the Tung Sol 5881 "reissue."  The Tung Sol 5881 is made like the original; it is a short plate tube, and it also comes in a short bottle.  Like the original, it also puts out a little less power, so it will not be as loud as other 6l6's, and it has to be biased differently too. 

The Sovtek 5881WXT+ is a large bottle, full power tube that has a longer plate structure.  Electro-Harmonix sells the same tube as the 6l6EH.  The 5881WXT+ and 6l6EH have a nicer tone than the short plate Sovtek 5881WXT, and they still produce the same output.  But  the best long plate 6l6 is the SED "Winged C" 6l6.  SED (also known as "Winged C") is the company that was previously known as Svetlana.  Now the "Svetlana" brand name is being used by Sovtek.  In fact, Sovtek makes a copy of the SED "Winged C" 6l6 which is sold as the Svetlana 6l6.  I think the Sovtek "Svetlana 6l6" is a little better than the 5881WXT+ and the 6l6EH, but the SED C 6l6 is still the best choice, if you want to upgrade your power tubes, but don't want to lower the output.

If you are looking for a truly vintage sounding 6l6, you could also try the Groove Tubes 6l6GE, the Tung Sol 6l6GC STR or the Chinese made Tube Amp Doctor (TAD) 6l6WGC STR. These are also long plate, high output tubes, but they come in medium size bottles; and they have the clear top, side getter construction of the original GE and RCA 6l6’s.  The TAD and Tung Sol versions have the “black plates” the RCA tubes had; and the Groove Tubes version is mostly American made (it is also much more expensive than the other tubes).  But the tube with the best tone is definitely the TAD 6l6WGC STR. 

Keep in mind the market for vacum tubes is constantly changing. It used to be that the JJ ECC83's and Svetlana 6l6's were the best tubes available.  But then Sovtek was bought out by New Sensor and started working with Electro Harmonix to develop new tube designs.  Chinese tubes were notoriously cheap, but then Groove Tubes and Tube Amp Doctor started working with them to develop some top quality glass.  And if the Ei factory ever resumes production of their grey, smooth plate 12ax7's, that would make another great tube available again.

It also used to be that you needed to get tubes from a reseller who tested them thoroughly because they were notoriously unreliable from the factory.  But now Sovtek, for example, seems to be testing their own tubes more and selling the best ones as "Tung Sol" or "Mullard" reissues.  It could be my imagination but JJ's "gold pin" ECC83's also seem to be better quality versions of their regular ECC83's.  So I wouldn't hesitate to get any of those tubes directly from the manufacturer.  

The question of which 12ax7 and 6l6 tubes have the best tone is subjective.  I like tubes that have good midrange response (not so bright that they sound harsh or piercing, and not so warm that they sound dull or muffled).  But what sounds good to me may not sound good to you. 

The tone you hear depends on a number of things besides the tubes, including the type of amplifier, the speakers, the pickups and the strings, among other things.  I play Fender guitars and amplifiers; and I like vintage style, lower output speakers and pickups.  For my rig, I like the Tung Sol 12AX7's and the TAD 6L6WGC STR's.  If your setup is different, the tubes I like may not sound the same in your rig.

And it also depends on what type of tone you are looking for.  I play mostly clean with just a little overdrive.  If you play with a lot of distortion, you'll probably like different tubes than the one's I recommend (If I had a Marshall, I'd tube it with JJ ECC83's and SED EL34's).  Regardless of your situation, this information should make it easier to find the tubes that will work best for you.

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