KT-77 vs. KT-66

All things being equal, KT66=6L6 w kinkless filament.  KT77=EL34 kinkless filament.  That being said, I prefer the Gold Lion KT77 over my Tesla, Svetlana, RFT, VEB and Psvane (the worst) EL34s.  KT77 has more of everything of the EL34 sonic signature.  
 

i have Sylvania 6L6 GBs and GCs, and Tung Sol 5881s.  Different sonic signature than EL34, less bass but more airy twinkling treble.  I don’t have KT66 (yet!) to compare, but going off KT77, if you like the 6L6 sound in your amp, you’ll love the KT66.  I’ve had excellent luck with Gold Lion new production from EH.  
 

My tube rolling amp is a Dennis Had Inspire KT88, best $2,000 SEP on the planet, IMO. Very revealing of tube changes.

Cheers

KT66 will definitely sound different than KT88.  KT88 is Europe’s answer to 6550.  My Inspire will run both.  I love the St Petersburg Svetlana 6550 in my Inspire, also have GL KT88 which is slightly different than the 6550, hard to put my finger (or ear) on difference.  Also have Shuggy Nature series black bottle which is a very nice sounding tube, closer to GL, further from 6550.  I really like all three.

What’s really fun is swapping the driver tube between 6SL7 (higher gain, more top end) and 6SN7 (lower gain, less top end detail, smoother and touch darker).  The Inspire runs both.  Great combo is the 6550 or Nature bb w a 1945 JAN RCA grey bottle 6SL7, get warm dark bass and mids, with the 6SL7 providing more detail and sparkle on top. 
 

obsessed and addicted 

Concur with wolf_garcia the GL KT77 is a fantastic output tube in a Had Inspire, detail, warmth, linear up and down the frequency range with excellent harmonics (my way of saying sounds great!).  
 

toddnig comments are helpful identifying KT88/6550 differences. More bass, lower mids, loose some sparkle and detail on the top.  Still a great sound depending on the music and mood.  But the GL KT77 is the overall winner, has it all top to bottom good tight  bass, rich warm mids, sparkly highs with detail 20Hz to 20kHz. Like I said earlier, everything the EL34s have but more of it and better.
 

Now I’m curious what a GL KT66 would sound like…oh jeez, here we go again!