> KT stands for "Kinkless Tetrode", an attempt to get the tube curves straightened out between the class A > class A/B transition zone.
No, the kink is at low plate voltage, and particularly at low plate current. This is not really an interesting area for Power amplifiers: they must swing to low plate volts at high plate current.
It has some slight effect with very oval loadlines, meaning speakers near bass resonance.
The main point of the KT line was so they did not have to pay RCA for Beam Tetrode patents.
[/i]if you look at an original 6550's internal structure and a us made 6CA7 (el34) they look pretty darn Identical.[/i]
Several different tubes were sold as 6CA7; some not much like Euro EL34s.
I think most of the key patents ran out by the 1950s. Anyway everybody was buying tubes from each other (an RCA or GE distribution catalog offered about 99% of all types ever made, but surely they never actually made that many types). Most types were minor variations of others. And TungSol was challenging the market with new types like 6550 (and 6336). So I think tube production "rationalized" in the 1950s.
In the same period, Ford Motor Company made well over a dozen "different" engines for Ford, Mercury, Edsel, and Lincoln. The "unique" engine was a feature of the Edsel. But there were just 3 basic blocks (2 blocks covered 90% of production) and about 5 different pistons. Mix-n-match, you get 239, 256, 272, 292, 312, 383, 410, 430, 462, 332, 352, 360, 361, 390, 406, 410(again), 427, and 428. Looks like a lot of engines, but they are just a few different parts stuck together in combinations. Contrast that with GM, where every division had its OWN truly-unique engines from the 1920s through much of the 1970s. GM later shifted everything to the Chevy and Olds engines; the tube companies did the same rationalization decades before. If a car idles and pulls well, if a tube idles warm and makes its power, nobody cares the exact shape of the torque or current curves or whether it derives from KT or 808/6L6 patents.