"Andy Evans" wrote in message
oups.com...
Note that most transistor amp designs tend to have a flat response, low

distortion, and a low output impedance. Whereas zero-feedback 'SET'
designs
tend to have a tailored response, distortion that rises rapidly with
output, and a relatively high output impedance. All of these factors
may
cause the SET design to change the signal patterns and cause audible
changes in some circumstances.

It may be useful to seperate out some of the commonly used expressions
regarding SETs and valve amps, since I know several have been confused.

a) One of the common characteristics of SETs (though NOT implicit in
the term "single ended triode") is the use of a DHT (directly heated
triode) in the output such as 2a3, 6B4G, 300B and 845 - this can be a
major positive factor in the overall sound, against which can be set
the drawbacks Jim has listed above. .
b) When we come to DHTs we are in another ballgame, in my opinion.
Those who habitually use DHTs, do extended listening tests with DHTs,
and are familiar with their characteristic sound will claim they offer
quite audible gains in clarity over indirectly heated valves. This is
only partly a question of output valves, but in practice only a small
handful of people will have used small signal DHTs for preamps, input
and driver stages of amplifiers. Reason being that while output DHTs
are available in current production, there are zero small signal DHTs
(except one or two rare boutique ones at huge prices). They are
available on ebay since they were used in countless old radios from the
20s and 30s, and there are good but dwindling stocks still to be had.
Some on the other hand have become quite rare. As you might guess, I'm
a huge fan and use them in preference to anything else.
c) DHTs and zero feedback can be used in push-pull amps with different
results from their use in SETs. Each of these factors contribute -
feedback, push-pull/single ended, indirectly heated or directly heated
and so forth (we could go on)

I don't personally use SETs and never have, and it is useful in terms
of general attribution theory to consider what we attribute to which
factors. In particular DHTs and SETs are absolutely not equivalents -
one refers to valve construction and the other to circuit topology. One
might consider that SETs with DHTs are a bit of a curates egg - both
good and bad factors. I'd personally opt for all-DHT push pull amps
given the choice. We haven't even come to another common SET feature -
interstage transformers - but that's enough to be getting on with. Andy

What is the theory behind an indirectly heated valve, having an inferior
performance to a directly heated one ? Not being contentious - just
interested ...

Arfa