(DHTs sounding better than directly heated valves) You may have to
explain why (Jim).

I may not - your need for theoretical knowledge is considerably greater
than the average, and many people build equipment empirically - they
put a circuit together, listen to it and measure it, then start
changing it until it sounds better to their ears. I'm sure there is an
explanation somewhere, but on a day of 35 degrees centigrade I'm damned
if I'm going to hunt around for anything.

Since I am not particularly expert on valves I am unsure why you
believe direct heating is a "major positive factor" in terms of the
results.

As I said, I and a few others (audio designers and EEs just for the
record) have been working with DHT small tubes for quite a while now,
building numerous circuits and subjecting them to listening tests. We
believe in DHTs on the basis of the results we have found, and our
responses to listening tests have been very close. The differences are
quite marked.

I had though that valve designers largely abandoned direct heating of
the cathode for various reasons.

They indeed did - the reason, as I said, being that current production
of them stopped in the 30s for the most part. Designers use current
production parts.

I suspect he (OP) would find it rather easier to get good results using
a design that does
not rely on such (DHTs).

Absolutely true. It's an esoteric pursuit.

The problem for the OP here is that you may simply be giving opinions
for which there is no reliable evidence. As, indeed, you warn. (ed.
???? do I) Hence there is a risk that what you say will simply impede
his progress.

I advised the OP to build a simple transistor amp as a first project.
But if the OP is interested in a) valves, which he noted and b) the
best possible sound, then it is the belief of myself and a number of
others (including a large number of respected designers and EEs) that
he should investigate DHTs. I go out of my way to point out that this
is an esoteric route to take and that the small signal tubes are not
current production. I should also add that they are complex to work
with and require sophisticated filament supplies. Having said all that,
in the interest of completeness it would be a dereliction of available
knowledge not to mention DHTs since if he wants the best in sound, NOT
knowing about them would impede his progress. It took me thirty years
to start building them, and this SUBSTANTIALLY impeded my progress.
Like the OP I was originally a simple DIY builder interested in valve
equipment. My "progress" was quite simple - I rejected what sounded
worse and used what sounded better. Along the way came a lot of theory
and listening tests, but the principle of using what sounds better
remains the same. Andy