Andre Jute wrote:
If you were promised that your dream tube would be made at a
reasonable price, if only you can describe it adequately, what would
you ask for?
Clearly, whatever you specify must not breach the laws of physics,
such as asking for a tube giving a hundred watts in Class A for a year
on a single AA battery.
I deliberately didn't specify my own idea to start with so that
everyone else could have all the court to shoot from. But it is
amazing how alike we all think, at least as long as we stay inside the
envelope; in fact it will look like I took my idea from John, from
Ronnie, from Chris and from Max!
My idea is a twin triode in an octal base glass tube the shape of and
no larger than a 300B, working off a real life (not max) B+ of 385V
and requiring no more than 60V of signal for full output, which should
be in the order of 10W per side (real output at everyday parameters,
not theoretical output at Pdmax; a WE300B is effectively a 6W SE tube
in real life, not the 10W American ponycar amp makers claim).
Indirectly heated filaments might be nice but I don't consider them as
essential as John does; DH fils have never bothered me much. I don't
care how hot the tube gets if the longevity is reasonable. It seems to
me that the 300B/845/211, that entire lot, are tubes whose performance
is depressed in favour of longevity; I have WE300B getting on for 20K
hours, at least, an entirely unreasonable life expectancy, in effect
indestructable in conservative designs.
Two tubes in the same envelope so it can be PSE or PP. Octal for cost.
10W per side so that 20W is easily achieved in PSE or more in PP up to
much more with variation among output classes. 385V is chosen for
component cost; it fits into the standard cap for 230V mains, so the
any iso-transformer will do, and the caps are cheap too. 60V signal is
easily achieved with a single or at most two stages of voltage
amplification.
I'd also specify a second tube as a driver: two 417A in the same
envelope, which can be bigger than the child's thumb of the present
417A, say up to 6SN7 size. (Actually, I'd ask for two 437A in the same
envelope, and settle for the two 417A if the man said 437A are
impossible. A 417A is good up to 24mA and has a mu of over 40; a 437A
is a 417A on a lifelong diet of steroids.) The reason for asking for
20mA and over is because that is what is necessary on a driver to
overcome Miller in PSE 300B or single 845, which have approximately
the same parameters I specify above. Or, if I can't have any of that,
how about 4x 6SN7 in one envelope, or 2x 6SL7 + 2x 6SN7? But these are
just convenience matters (except for the unobtainable 437A); as Chris
says, small signal and driver tubes are probably a done deal, at a
peak of perfection.
And, since I would have the guy pinned down, still trying to close his
mouth, I'd impress on him the necessity of a new rectifier, twin GZ37
in one tube, i.e. a complete one-tube high voltage high current bridge
rectifier). The reason I don't choose the venerated GZ34 is that the
-37 is a better rectifier in every respect and anyway its size and
shape match the 300B bulb I've already specified above. All that heat
may need the bigger tube too than the GZ34.
The reason for asking for a kilovolt tube bridge rectifier is many a
slip twixt the cup and the lip: the new tubes may turn out to be
rubbish and then we shall all want to go back to 845...
Andre Jute
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