Changing the vacuum tubes in your guitar amplifier can have a substantial affect on the tone of your amplifier (the speakers and pickups have the biggest effect, but the affect of the tubes in the amp is noticeable too). 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, as well as the European made Mullard ECC83. 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.
There are five (5) types of 12ax7 tubes being made today, and the same tube is often sold under multiple different labels. If you want to know what you are buying, the best thing you can do is become familiar with the construction of the different types, so you can tell which is which regardless of the label each is being sold under.
Three (3) of the five tube types have a short, fat plate structure where the two plates are so close together you can't see the space in between them. The other two (2) tubes are made more like the way the RCA 12ax7's and Mullard ECC83's were constructed in the 60's. They have two thin plates that are separated, so you can see the space between them, and the plates have a ribbed imprint on them that looks like a ladder.
Russian Sovtek 12ax7
Sovtek is the primary OEM supplier of preamp tubes today. Sovtek tubes are made in Russia. As you can see above, they have a spacer below the short, fat plates, a gap between the plates and the second upper spacer, and a "dimple" getter over the top. If you see a tube that looks like this, it's a Sovtek, regardless of the label. Sometimes they are marked as Sovtek 12AX7's but you also see them marked as Groove Tubes 12AX7-R's and Fender 12AX7's. You will also see them marked 12AX7 WA, WB or WC. I'm told the WB version has a little more gain and the WC version is the most recent iteration. If you have the choice, go with the 12AX7WC, relabelled by Groove Tubes as the GT-12AX7-R.
Chinese Shuguang 12ax7
The second common OEM tube out there is the Chinese Shuguang 12AX7 (Groove Tubes also sells this tube as the 12AX7C; Tube Amp Doctor sells it as a TAD 7025 or 12AX7; and Mesa Boogie sells is as the SPAX7). The Chinese 12ax7's are constructed with short, fat plates, like the Sovteks, but they have an extra spacer above the tubes, a big nickel colored brace connecting the plates, and a "halo" getter on top. Again, if you see a tube made like this, it's a Chinese tube, regardless of whether it has Groove Tubes or Mesa Boogie or TAD written on the glass. These tubes have a warmer tone than the Sovteks and more gain (volume). I actually like the tone, especially for distortion, but I haven't had good luck with the reliability of Chinese tubes. If you want to try these, I'd get the Tube Amp Doctor TAD 7025 Highgrade.
European JJ ecc83 (12ax7)
The third OEM tube is the European JJ ECC83 (ECC83 is the British name for a 12AX7; JJ's are made in the Slovak Republic). They are constructed with short, fat plates and a halo getter, like the Shuguangs, but they have only two spacers and a metal disk above the plates that is connected to the getter. You may see these sold as Groove Tubes ECC83's, Ruby Tubes ECC83's or Mesa Boogie 12AX7's but the label doesn't matter; they are still JJ ECC83's. As for the sound, they have a more linear tone than the Sovteks or the Shuguangs, and a more prominent midrange. If that's the sound you are looking for, I'd recommend the Gold Pin JJ ECC83 (in my experience, the gold pins sound the same as the regular JJ ECC83's, but the quality control is better).
Russian Tung Sol Reissue 12ax7
Premium vintage American tone
If you have a vintage reissue amp or a boutique amp and 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 thinner plates that are separated, so you can see the space between them 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). If you want premium tube with a real nice American tone, I'd recommend the Tung Sol reissue 12AX7.
Russian Mullard Reissue 12ax7
Premium vintage European tone
The second option for a more vintage sounding preamp tubes is the long plate tubes Sovtek makes under its own name as the 12AX7LP (the “lp” stands for “long plate”), and the tube Sovtek sells as the Mullard 12AX7 "reissue." The two tubes look identical, except for the name, but the quality of the Mullards is much better, I think. Sovtek also makes a gold pin Gold Lion 12AX7 that appears to be the same as the Mullard and the LP, but will cost far more than any other 12AX7 out there. That tube may be worth the money to an audiophile but I think the Mullard reissue 12AX7 is a better choice for guitar players. Groove Tubes also sells a US-made 12AX7M, and it looks like TAD is apparently now selling this tube. And JJ has a new long plate tube called the JJ ECC803. I've tried the Groove Tube but not the JJ; and I didn't have much luck with the GT, so I'm sticking with Sovtek's Mullard reissue.
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. So you could put a premium Tung Sol V1, but then a less expensive Electro Harmonix might make more sense in V2 . Personally, I like JJECC83's in V1 and V2 for high gain amps; and Tung Sol 12ax7's in V1 and v2 for clean Fender amps. Many agree, a NOS 12at7 makes a good reverb driver; and a Chinese 12ax7 functions well for reverb recovery. Also, I prefer long plate 12ax7's or 12a7's for the phase inverter position.
6l6 power tubes not only have different size plates, they also come in different size bottles. There are basically three (3) types. The small bottle "stubbies" modeled after the original Tung Sol 5881, the tall bottles based on the original Sylvania 6l6GC STR, and the clear tops that have the side getter construction of the famous black plate RCA 6l6GC.
OEM 6l6 Tube
The most common OEM 6l6 is the Sovtek 5881WXT, which is sort of an anomaly. The Sovtek is a short plate tube in a tall bottle. It's called a 5881 but it's really a 6l6 . It can handle high voltages like the other 6l6's but it has the lower output of a 5881. And you will see it branded both ways. Sovtek calls it the 5881WXT but Groove Tubes sells it as a 6l6GC, Fender calls it a 6l6GC, and a Mesa Boogie sells it as a 6l6GC STR. Regardless of the name, the Sovtek is a very reliable, inexpensive tube, that has a relatively warm tone. A lot of people disparage this tube, but it has it's fans too. If you play loud enough to generate a lot of power tube distortion, and you want a tube that can take that punishment night after night, and still be reliable, the Sovtek 5881WXT is a good choice.
Tung Sol Reissue 5881
The only true small bottle 5881 currently in production is the Tung Sol 5881 reissue that is also made by Sovtek. Like the Sovtek 5881WXT, the Tung Sol has lower output than most 6l6's. But when it comes to tone, there is no comparison. The tone of the Tung Sol is much better than the 5881WXT. The problem is, unlike the Sovtek, the Tung Sol apparently cannot handle the high voltage of most modern tube amps (I've heard it should be o.k. in a reissue Bassman with the bias adjusted and using a tube rectifier, but that's about it). As a result, most people looking to upgrade the standard Sovtek 5881WXT will do better with a 6l6 tall bottle or one of the new clear tops.
Electro Hamonix 6l6GC
In addition to the Tung Sol, Sovtek also has a premium tall bottle 6l6 called the Sovtek 5881WXT+ (not the 5881WXT, the 588WXT"+", which is a very different animal) and the Electro-Harmonix 6l6EH (near as I can tell, the WXT+ and the 6l6EH are the same). Both the 5881WXT+ and the 6l6EH have longer plates, more volume and a nicer tone - with more harmonic complexity - than the Sovtek 5881WXT.
The highest rated 6l6 tall bottle is probably the SED 6l6. SED a/k/a "Winged C" is the company that was previously known as "Svetlana." The SED is a great tube with a brighter, more detailed tone, but it has a big downside, and that is that it typically costs about twice as much as all of these other tubes.
The "Svetlana" brand name is now owned by Sovtek. In fact, Sovtek makes a copy of the SED "Winged C" 6l6 which is sold as the Svetlana 6l6. The SED and the Sovtek Svetlana are not the same tube (the getters are different), and they don't have exactly the same tone (the Svetlana is not quite as bright). But the two tubes are very similar, and the Svetlana costs almost half as much.
In addition to the SED and the Sovteks, there are two other 6l6's that have a little different tone but are also an improvement over the standard 5881WXT. JJ makes a tall bottle JJ 6l6 that has a unique plate structure and a powerful clean sound. Like the SED, the JJ has been around a long time and has enjoyed considerable popularity. More recently, Tube Amp Doctor started selling a Chinese TAD 6l6GC STR that has become very popular. I think the TAD sounds more like the JJ than the SED or the Sovteks.
You really can't go wrong with any of these tall bottles. If you like big and beefy, try the JJ or the TAD. If you want a brighter tone with more detail, go for the SED or the Svetlana. If you would rather have something with a little flatter eq and smoother sound, I'd recommend the Electro-Harmonix 6l6EH or the Sovtek 5881WXT+. I especially like smooth overdrive tone of the 6l6EH and 5881WXT+ in a master volume amp.
But if what you want is the closest you can get to the original RCA 6l6 clear top, there are a number of other 6l6 tubes you may also want to give a listen to. Groove Tubes makes a clear top GT 6l6GE, Sovtek has a clear top Tung Sol 6l6GC STR and Tube Amp Doctor also sells a clear top Chinese made TAD 6l6WGC STR. These are 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. But the tube with the best tone is definitely the TAD 6l6WGC STR. The TAD is a nice, warm tube with a sweet, silky high end. To me, it is the perfect choice for a vintage series Fender amp, like the Super Reverb.
Keep in mind the market for vacuum tubes is constantly changing. It used to be that the JJ ECC83's and SED "Winged C" 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 Tube Amp Doctor and Groove Tubes started working with them to develop some top quality glass.
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 good ones under brand names like "Tung Sol" or "Mullard." The other thing the tube manufacturers are doing now is selling "gold pin" versions of their tubes (often for twice the price of the regular versions). I think it's worth it to buy the brand name versions, especially if you are buying direct from the manufacturer, but I don't think the gold pins are worth twice the price.
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 also 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 a nice clean tone, I like the Tung Sol 12AX7's and the TAD clear top 6L6WGC STR's. But for high gain distortion, I think I'd probably go JJ's all the way!!!.
One last thing, if you are going to experiment with the tubes in your amplifier, be really careful of two things: one, amps have very high voltages, like around 400 watts, so make sure the amp is unplugged and the voltages discharged before you do anything to the tubes. And two, be super careful if you don't have an amp that has the tubes mounted to the chassis (as they should be).
Many modern amps mount the tubes directly to the pcb board. This is a bad practice, in my opinion, because you risk cracking the board when you remove an old tube and insert a new one. Tube pins fit very snugly; it takes some force to get them out. If you use too much force, you could bend the pcb board and break one of the connections. If that happens (been there done that) you'll have a heck of a time getting the amp fixed (if the problem is intermittent you may also have a problem demonstrating there is something wrong). Nobody wants to work on pcb board amps. In my opinion, you are much better off buying an amp that has the tubes mounted to the metal chassis, not the pcb board, like a Fender Deluxe Reverb, Bassman, Super Reverb or a Twin Reverb. If you want a high gain amp, get a Fender Super Sonic, a Paul Reed Smith Archon or a Laney Lionheart. If you can live with fewer features, like no reverb, there are also the Chinese made hand wired amps made by VHT (special 6 and ultra), Vox (white tolex), and Blackstar (the Artisan amps). And, if you have the money, there are also a number of boutique American made hand wired amps, including Bad Cat, Top Hat, Carr, Dr. Z and Swart. All of those are well made amps. Be a smart consumer; buy a well made amp, so you can experiment with the tubes to find the tone your are looking for.
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.