RC coupling, although fast and compatible with overall loop feedback (highly desirable in pentode amplifiers), throws away about 1/3 of the potential swing (through voltage divider action) and also creates a steeper load-line, which increases distortion by 2 to 3 times (the load-line swings down into the low-current region).
But Job One in any feedback amplifier is bandwidth, otherwise the thing goes unstable. Can’t have that. So the rule of thumb is no more than one coupling cap in the entire forward path, plus the output transformer needs at least 60~80 kHz of bandwidth. The Williamson, which dominated from 1948 to 1956 in this country, was marginally stable unless you used the specified Partridge transformer, which had an astounding bandwidth extending to 120 kHz, and down to 4 Hz. So your options are limited if you want to wrap feedback around the amp ... only RC coupling, and only once per side, not twice, like the Williamson.
If there is no overall loop feedback, you can have any kooky topology you like, since phase margin no longer matters. But ... triode-connected pentodes have somewhat higher output impedances than DHTs (2 k instead of 800 ohms), and aren’t as linear. But this depends on the pentode. Don and I selected the triode-connected 6V6 because 1) 45 tubes are near-unobtainium these days 2) 6V6’s are reliable, been around forever, much loved by guitarists for their famous tone, and oh yes, sound good too. Last but not least, the 6V6 was purposely designed as the replacement for the 45 in radio use, so operating points are very similar.
Part of amp design is deciding what "tone" you want. Because it’s going to have a sound, no matter what technology is there ... Class D, MOSFETs, bipolar transistors, pentode, triode, or DHT. No matter what you design, it will have a sound, no matter what you do, or how clever you are. The perfect component does not exist. The designer needs to steer that little touch of coloration in a good direction. Fortunately, Don and I are on the same wavelength ... I’m looking forward to meeting him at the Seattle show, in person. Should be a lot of fun.
