Differences in tubes.

Can anyone tell me the differences sonically between kt77s and 6550s? I know a lot of is dependent on the circuit the tubes are used in. I'm using a Jolida 302b with 6550s in it and was thinking of trying kt77s for something different.

Thanks in advance for any help!
The electrical characteristics of a KT77 are similar to an EL34/6CA7 and are somewhat different than a 6550 or KT88 even though the base pin configuration is the same. Most fixed bias amps cannot interchange the two tube types without circuitry changes, however many cathode biased amps can. And yes, they sound very different.
I'm not a fan of the 6550. At worst, it's a cold, harsh sounding, clinical tube. At best, it sounds relatively warm, yet flat and uninvolving.

I should say that my experience with the KT77 consists of the JJ and Shuguang varieties, and not the classic, Holy Grail vintage British variety. Along those lines, the KT77 is somewhere between an EL34 and a KT88. I would obviously recommend it over the 6550. The KT77 has a pretty fat mid/upper bass that rivals the KT88, and can be great in some amps/systems, though too boomy in others. Its midrange is more open and clear than a KT88, though it will not compete with the better EL34 variants.

Personally, I find the EL34 best for my tastes, but if I had to award a second prize, it would be the KT77. I see a lot of audiophiles on these threads actually prefer the KT77. To me, it comes down to midrange, and thankfully, I've found a way with my amplifiers to produce the bass of the KT77/KT88 without losing that magical midrange the EL34s I use produce.
I was also wondering how the KT-77 compares with the KT-66 tube? I'm also a fan of the EL-34 tube. Beautiful sounding mid range. I've been considering the Gold Lion tube. Running with Quicksilver amps...Thanks...
KT-66 is a 6L6 variant from an electrical standpoint and should only be used in amps that can accept the 6L6 types. Interstingly, the "KT" in these tubes originally stood for "kinkless tetrode" so they differ from the pentodes and beam power type tubes. If you like the EL-34s try the 6CA7 tube which is a direct sub, but is actually a beam power tube having the beam focusing element. They sound different and they sound better in some circuits than others. The GE 6CA7 is quite nice, but there are modern EH and Chinese versions as well. These are the "big bottle" EL34 variants.
If you're still using Triangle loudspeakers, you may want to call Kevin Deal at Upscale Audio in SoCal (see the A'Gon Guild). He's a Triangle dealer and (I think) a Jolida dealer who specializes in tubes/tube amps and may have some useful thoughts for you.