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 American made RCA tubes that were produced in the 1950's and 60's, like the short, ribbed plate RCA 12ax7 and the black plate RCA 6l6GC.  Those tubes do sound great but they are expensive and dwindling in supply, since they are no longer being made.  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.  The most reliable tubes have a short, compact plate structure where the two plates are so close together you can't see the space in between them.  The better sounding tubes are constructed the same way the RCA 12ax7's were constructed in the 60's.  They have the two plates separated, so you can see the space between them, and the plates have a ribbed imprint on them that looks like a ladder.  There are also long plate tubes that look similar, except those tubes come in ribbed and smooth plate varieties.  Some claim the long plate tubes have the best tone of all, but most agree those tubes tend not to be very reliable in electric guitar amps.

Sovtek is the primary OEM supplier of preamp tubes today. 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, compact 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 (Groove Tubes also sells this tube as the 12AX7C; and Tube Amp Doctor sells this tube as a 7025).  To my ear, the JJ has more of a European tone, with a flatter eq, and a more prominent midrange.  I think it goes really well with EL34 or EL84 power tubes.  And the Chinese 12AX7 has a more American tone, except it's much warmer and rounder than the Sovteks.

If you want an nicer clean tone or slightly distorted blues tone, the newer tubes Sovtek makes for Electro-Harmonix, Svetlana and Tung Sol are better choices. Each of those tubes has the plates separated, so you can see the space betweent them, and the plates are ribbed, just like the original RCA 12ax7'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).

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).  Groove Tubes also sells a US-made 12AX7M, which is marketed as another copy of the European Mullard 12AX7.  Both these tubes have a fairly flat eq, which I associate with a more European sound, as opposed the the Tung Sols, which accentuate the highs and lows more and, to my ear, have more of an American tone.

Other long plate tubes include the old Ei 12AX7 and the new JJ ECC803.  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.  The problem with these long plates 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 RCA and GE 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 Jensen style speakers.  For my rig, I like the Tung Sol 12AX7's and the TAD short bottle 6L6WGC STR's. 

But I'd go for a different combination if I were looking for a more distorted rock sound.  So, as another example, if I were playing a Gibson through a Marshall with Celestion speakers, I'd tube the amp with JJ ECC83's and SED EL34's.

In my opinion, there is no one tube that sounds best, it all depends upon what you like and the other components you are using.  Regardless of your situation, this information should make it easier to find the tubes that will work best for you.

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