Some more good reading from the net:
EL34, 6550, KT88, KT66, 6L6GC and 6V6 are all the same pinout (called "octal base"). They're swappable to some extent, they have WILDLY differing performance characteristics and major internal adjustment (rebiasing) is needed. In some cases they're fully incompatible. All these tubes use the same socket.
Geek input:
"I just put 2 6CA7EH big bottles in my amp to replace Siemens EL34's and they sound great. They 6CA7EH is a direct replacement for EL34's (with a bias of course). The reason they are different than other EL34/6CA7 tubes is that they are a beam tetrode tube made with the same design as old Philips tubes. The beam tetrode 6CA7 is commonly called the American version of an EL34 due to having the same output, but the American made 6CA7 is tonally of the 6L6 family. It is basically a 6L6 with the high gain output of an EL34. Better Bottom end then regular EL34's, and more durable. I think they are pretty cool, and very cost friendly."
"If you want an alternate to EL34s, one user suggested 6550s. Zakk Wylde uses 6550s in his Marshalls, and with an input boost to push the amp to get a little extra gain, it should sound something like Zakk Wylde... pretty meaty! KT66s also maintain a full range of tones, but are quite expensive. I would go the KT66 route only if you plan on getting most of your gain from the power amp."
"kt-88's are replacements for el-34's, very similar to 6550's, if you have room for their huge bottle design. These suckers are expensive, but worth it if you can afford them. They have a huge bottom end that remains tight without losing any mids or highs. This is truly one the best all around tubes that can go to either end of the spectrum. Billy Corgan used these in his JCM800."
"Thanks for the info on the KT88's! Yeah, that's about what I've heard about them. I read somewhere that it has the low-mid warmth of an EL34 and the mid-high tone of a 6L6. Kinda the best of both worlds I guess."
WARNING: These tubes are not all created equal. Some combinations will explode or melt your amp. Do not swap powertube types without someone experienced in tube-based electronics involved!! This is not something for a novice or even an intermediate to be playing with. This is something for a serious electrical guy, not just a guitar player. Getting it wrong could melt your amp, or kill the guy touching the guitar, and that is absolutely no joke.
The EL84 is a physically smaller tube, the same socket size as 12xx but about twice as tall. They have much lower power (8.5w per tube vs. 30w per tube in an EL34). They are often described as "glassy", which I think of as a sort of sparkly quality to the clean which comes out as a brittle quality when driven. They are definitely not more midrangey than EL34s.
Groove Tubes sells a device called the 'Yellow Jacket' which lets you use EL84 tubes in an octal base amp. One geek had the following review of them: "They are nice. When I was contemplating downsizing my rig and looking at THD's Univalve and Flexi amps I had the chance to swap back and forth a slew of tubes in a Flexi-50, and used the Yellow Jackets for a set of EL84s. Other than the expected change in tone, there was no noticible difference/ noise/ buzz. I was very surprised. The idea that you can re-tube and have in essence a completely different amp is very, very cool."
<font size="1" color="#8e236b"><p align="center">[This message was edited by Ken Fox on 16 December 2005 at 06:59 AM.]</p></FONT>
<font size="1" color="#8e236b"><p align="center">[This message was edited by Curt Langston on 16 December 2005 at 05:41 AM.]</p></FONT>