"John Byrns" wrote in message
My question was why would a SET amplifier differ from an
equivalent PP amplifier in terms of frequency response
errors?
Mostly the lack of NFB, but probably something about the odd OPT
configurations that are used.
I ask because you and others here seem to single
out SET amplifiers for this particular criticism, some
even explicitly stating that PP amplifiers don't suffer
from these frequency response errors.
It would be appropriate to say PP amps with NFB.
I am not
attempting to contradict your characterization of SET
amplifiers, I am simply asking why you single out SET
amplifiers for this criticism when it applies equally to
equivalent PP amplifiers?
Since you seem to treat the "Radiotron Designer's
Handbook Ed. 4" as some sort of tube bible, it is
enlightening to read the text printed directly above Fig.
13.14 which reads as follows.
"A triode applies nearly constant voltage across the load
impedance. This is a standard condition of test for a
loudspeaker, and some models of loudspeakers are designed
to operate under these conditions (see Chapters 20 and
21). A triode is almost the ideal output stage for a
loudspeaker load when looked at from the load point of
view, with or without feedback."
This harkens back to the triode/pentode controversy that raged back in the
1950s, perhaps earlier. These were the days of the Brook p-p triode amp was
being compared to various amps with p-p pentodes etc. The early Brook amps
had loop feedback and both interstage and output transformers. Later on the
interstage transformer was replaced with inductive loading of the driver
stage.
The trade-off then became the comparative effectiveness of a triode amp with
inherently lower output impedance, but also lower voltage gain and therefore
less potential for loop feedback, all other things being equal.
http://www.ampslab.com/vintage1.htm