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From: peterca@ento.csiro.au
Date: Mon, 11 Apr 94 16:04:14 EST
Subject: Indirectly heated better?

brad.sanders@circellar.com writes the following which prompted some musing:

>Even the _much_ better
>specs of an EL-34 in triode mode don't compare to those of the 300B or 845.
>
>The curves for a these high quality triodes are _very_ linear. This
>linearity is not seen in many _low_level_ triodes (12AX7, 12BZ7, 6922, etc),
>let alone power triodes. 
[Wow I can't wait for an all 300B phono stage and preamp!]
I was wondering whether there is any reason why a direct heated triode
should sound/work better than an indirectly heated triode?  I would have
thought that the indirectly heated cathode would be more rigid and so help
to maintain interelectrode distances more constant.  Also I would have
thought that the larger surface area might help.  Increased thermal mass of
the cathode might also be of benefit to smooth out any variation in
emission with any variations in the heating current (which could be
regulated in the case of indirect heating)  Wouldn't it be possible to make
a valve which measures and sounds just as good as a 300B but indirectly
heated?  Is it just that by the time indirect heating came along the demand
for new, high quality triode designs had just about gone?  

Peter Campbell (peterca@ento.csiro.au)
C/O Div. Entomology, C.S.I.R.O.
Box 1700 Canberra A.C.T. 2601    Ph.61-6-2464158 (w), 61-6-2516213 (h)
Australia                           61-6-2464173 (fax)