I just ran across a very valuable piece of information on Vaccuum Power Tubes for amplifiers. I had seen it before, but lost track of it. I just thought I'd post it so I don't lose it again, and who knows, someone might benefit from it. I'll post a blog update soon, too.
Basically I can distinguish 8 types of end tubes that will fit in a Marshall plexi no problem, each with their own characteristics in sound, feel and response: EL34, 6CA7, 6L6, 7581A, 5881, KT66, KT88, 6550.
EL34: Current-production EL34's are designed to give you that midrange crunch rock music is known for. I never had the pleasure of hearing a new old stock or used old stock EL34 (Mullard) but from the descriptions George has given on this forum they sound warmer and more versatile than the newer EL34's. They are also cleaner with more headroom, which means that when they break up or are pushed by pedals the sound still doesn't get overly compressed or saturated. I find most current-production EL34's to be good only at producing that middy grind. They break up early and most of them tend to get shrill in the high end. The general consensus seems to be that the Svetlana EL34 and the JJ EL34L are the best new EL34's. I personally find the EL34's to be too much of a one-trick-pony for my taste but if you're looking for the classic 70's and 80's rock sound then EL34's are definitely the way to go.
6CA7: The term 6CA7 is often used synonymously with EL34. This is not correct as a true 6CA7 is an American big-bottle version of the British EL34 design. I don't know how it works exactly but from what I understand the 6CA7 is the basic EL34 design but with an added element of the American 6L6 design. The only decent new-production 6CA7 IMO is the GT-6CA7GE (remake of the General Electrics 6CA7 - yes, the one EVH used!). I own some original GE 6CA7's and I would describe them as a cleaner version of an EL34 which is slightly more American-sounding (i.e. more midscooped, less midrange grind, cleaner with more headroom, later breakup). The main feature of the big-bottle 6CA7 is its HUGE dark and tight low end response while still retaining its brightness. This tube is often used by bass players but you guitar players looking for a tube with lots of warm, clean output without sounding sterile might like it too. A very musical tube which is great for use in combination with pedals or heavy preamp gain because of their tight sound and clean headroom.
6L6: The tube you associate with Fender amps. The classic American tube sound: lots of lows and highs, midrange is not so well pronounced. The slightly midscooped quality of a 6L6 gives it its sweet and round 'glassy' character. The 6L6 is less 'forward-sounding' than a British tube like the EL34, yet in return you get a very sparkly tube with bell-like highs. The lows can get muddy when pushed so if you're gonna go for 6L6's in your Marshall I suggest you choose the 6L6GC which has a slightly higher output than a regular 6L6 and will sound tighter due to its increased headroom, especially in the low-end department. Best current-production 6L6's IMO are the JJ's and Svetlana's. I hear the GT-6L6GE (Groove Tubes replica of the GE 6L6) is very good too.
7581A: This is the military version of the 6L6GC, in other words a 'harder' tube (even cleaner and tighter). Expect HUGE low end and sweet, glassy, bell-like highs. This tube has all the qualities of a 6L6, only enhanced due to its bigger headroom. This tube is also favored by bass players but guitarists looking for the American midscooped 6L6 sound with later breakup will love it too. Great for use with pedals or heavy preamp gain to keep your sound nice and tight without it getting muddy when pushed. Unfortunately there's no current-production version of the 7581A on the market today so you'll have to look for NOS ones made by American companies like GE and RCA. They show up regularly on eBay and if you bid smart you can get them for decent $. The 7581A is often called the American KT66 because it's similar in design to that British tube. The 7581A does sound distinctly American (less pronounced mids), tho, whereas the KT66 sounds British (more pronounced mids).
5881: Also called the 6L6WGB and NOT the 6L6GC as they are often referred to. You guessed it, the 5881 is another version of a 6L6. This time, tho, it actually has a LOWER output with LESS headroom. Great for guitarists seeking the classic midscooped American sound with early breakup. Has a tendency to get muddy in the lows and shouldn't be pushed too hard. Not my favorite 6L6-type tube.
KT66: The British equivalent of the 6L6 design. The difference being they eliminated the mid 'kink', that slightly midscooped quality that gives the 6L6 its sweet and round character. KT stands for Kinkless Triode. What the KT66 gives you in return is a more pronounced and wider midrange and more clean headroom. The KT66 is capable of producing a midrange crunch which is similar to an EL34, yet it's considered a more versatile and musical tube. It's the perfect tube if you're looking for a hybrid British/American sound, i.e. the middy grind associated with British amps (Marshall, Vox, Hiwatt, Orange, etc.) coupled with the clean glassiness of American amps (Fender, early Mesa Boogies, etc.). The best KT66 ever is the GEC (Genelex). These go for hundreds of dollars on eBay. Best current-production, according to many, is the GT-KT66HP (Russian-made KT66 designed exclusively for Groove Tubes). Sounds great with pedals.
KT88: The British version of the American 6550 design. Again the mid 'kink' is eliminated, giving you a broader, more dynamic midrange. This tube has HUGE headroom and sounds darker than the KT66 because it enhances the lower frequencies especially. This is why the KT88 is liked by bassists. Guitarists looking for that extra bit of OOMPH! in their sound use either KT88's or 6550's depending on how much midrange they want their preferred sound to have. Again the GEC KT88 is considered the ultimate KT88 which explains their high $ value. Best current-production KT88 is without a doubt the JJ KT88.
6550: If you like the American 6L6 type of sound but you need even more clean headroom than a 7581A then a 6550 is what you want. These tubes sound very stiff and will take any kind of abuse gladly and not even blink an eye, which is great if you're looking for maximum clean headroom in a tube. On the other hand, the 6550 is less musical than its British counterpart the KT88 because of its less pronounced midrange. This causes the 6550 to always respond in a very stiff manner no matter how hard you dig in on your guitar. Bass players love this tube. Most guitarists will prefer the KT88 over the 6550. Yet it's a great tube for guitar if you're into HUGE clean output, especially in combination with heavy preamp gain (Zakk Wylde, Slash). Best current-production 6550 IMHO is the Svetlana 6550-C.