6L6GC - KT-66 question

[Mooncusser]

Hell all,
In a quick nutshell, could someone quickly give me simple description of the tonal and performance differences I can expect if I switch to KT-66's from 6L6GC's?
Thanks a bunch,
Mass

[Fretts]

The nutshell version would be :"bigger". I think of a KT66 as being the halfway point between 6L6 and 6550. It all depends on how it's set up. You can drop 'em in, sure, but to really get the best out of 66's the environment around it has to be adjusted, voltage, input, screen, etc.

[Mooncusser]

So ,
If it isn't to much trouble, since a KT-66 is halway between a 6L6 and a 6550, could you let me know what the 6550 is charactoristic of? That way I could understand better about the KT-66 being a halfway point.
Thanks again,
Mass

[Fretts]

The 6550 is a a high voltage, high power tube that has a mixed reputation. For the longest time, they were only used in bass amps that were designed to play clean (six of them in a SVT) and then when Gulf & Western got the deal to import Marshalls to the USA they started drowning in bad tubes because the stock EL 34s didn't travel well. So they started replacing with big, American 6550's in there instead. The amp was not set up for 6550's and the sound was "off" to the ears of people who had gotten used to the English version. Because of bass amps and the Marshall USA tube swap, many people thought they were cold, hard tubes with no chance of sounding fat and juicy.
When they are in an amp intended for them, like VHT, you get a big, fat, round, hot powerful sound. The thing is, they have so much capability that you get tremendous headroom and clean-power capability. Great if you are a pedal-type guy and need a good base to run your pedal board from. But if you are looking for amp overdrive, it will have to get pretty damn loud before the drive sound we like starts to kick in. And the whole amp, from the power supply on up, has to be able to feed them what they need.
It's too late now for a nutshell, but it's like a big fat 6L6 WHEN it is operating in an environment that pushes it to the same degree that 6L6's get pushed in their amps.
These days, it seems like guys are going for multiples of smaller tubes instead of a few big ones like 6550s.