Patrick Turner implies that using a 35% screen tap lowers the distortion - at least from pentode mode.
"The power output is for 12 x 6550 with 20% cathode feedback connected,
and the tube operation is very similar an ultralinear amplifier with about 35%
taps for the screens. Power is marginally less than pure beam tetrode, but
the distortion is far lower."
http://www.turneraudio.com.au/300w-outp ... raphs.htmlHe recommends using a 30% screen tap for the 6550:
"The Rout from the output tubes should be equal to the primary input load, so use
6550 wired as Ultralinear with approximately 30% screen taps."
http://www.turneraudio.com.au/output-trans-theory.htmSome tubes may benefit from a 35% tap, but I know of
none off the top of my head. Dennis Grimwood delves into the theory and detail of screen grid ratios, but does not mention a 35% tap. I think some James transformers have or had a 35% tap?
http://www.oestex.com/tubes/ulo.html