the kt66 is the british version of the 6l6GC,
kt or
kinkless
tetrode, so called was
to circumvent the american patent on beam tetrodes
that the 6L6gc and siblings belong to...
https://en.wikipedia.org/wiki/KT66so go ahead and use it....
thing about biasing is to get all three tubes at the same cathode currents, when paralleling tubes, it is important to get tubes that
are well matched at the operating point, and bias pots are provided for this purpose....
so how do you bias them?
the plate dissipation for the tube is your limit,
say your tube can do 30 watts, for reasonable tube life,
we do not want to run then at full 30 watts, 80% is a reasonable
target or 24 watts..
then knowing your plate voltage, you can adjust bias to get
your cathode current set...
say your plate voltage is 300 volts, so at 24watts, this means
80ma...
you may also follow recommended factory settings...
as for me, i will not bias the kt66 at more than 60mA cathode current, 40-50 is just fine...
That's why I'm trying to learn the bias adjustment without assistance from a tech.
I agree that not all boutique parts cost not always equate to level of improvement if there is.
Back to amp diagram, can I get your opinion and is it possible to use kt66 tube?