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From: glantz@enable.tay1.dec.com (Mike Glantz)
Subject: Re: Some questions on "pure triodes"
Date: 3 Feb 1994 21:46:08 GMT

In article <2ir8gr$krt@introl.introl.com>, hit@irene.mit.edu (KERMAN, A. K.) writes:
:I have a few questions concerning PA's built around classic direct-heated-
:cathode triodes, like the 300B:
:	1. What are the advantages of these tubes? It is rumored that musical
:passages with lots of harmonic content will benefit with these designs. I
:listen to a lot of Baroque music on 'original' instruments, and wonder if I
:might look into this class of PA's now that I looking to upgrade my system.

The direct-heated triodes have the most linear characteristics of any power
tubes, and the 300B is the best of these. You can design amps with them that
don't have any feedback, and they will still sound very good. Of course,
there's no reason why feedback is something to avoid, but a lot of people
seem to think so. It's quite a bit easier for a beginner to design an amp
without feedback, though, although direct-heated triodes are particularly
difficult to design with, so the benefit is probably cancelled.

In the absence of feedback, and in a single-tube amp (i.e., not push-pull),
the 300B exhibits a very regular, distinctive distortion spectrum. In my
opinion (having heard one), it sounds particularly good on music which is
rich in strings. I'm going to guess that this might be because the harmonic
structure of string instruments is similar to the distortion spectrum of the
tube, but haven't looked at a spectrum analysis of a violin in a very long
time.

Another psychoacoustic effect which I can report from personal experience
with no-feedback class-A triode amps is that the distortion spectrum of
such an amp gives an uncanny sense of increased dynamic range. That is,
one has the sense that a 10W amp delivers sound equivalent to that of a
much more powerful solid state amp. Others have reported similar findings,
but I would wait until some more objective tests have been done before
concluding that this is in fact the case.

:	2. What are typical lifetimes of tubes like the 300B? Are they sensible
:to replace (i.e. price and availability)?

The 300B has a fairly high plate dissipation (40W), so can be safely designed
into moderately powered amps without operating it at a point which would
cause it undue stress. I would expect that the useful life of a good
quality 300B (from Western Electric, for example) could be several thousand
hours under such conditions. I would hate to have to replace one, as the going
price for a Cetron 300B (the W.E.s aren't even available, as far as I know)
is around $200. There's now a Chinese tube available for less than 1/2 that
price, but I haven't heard from anyone how reliable they are, or how they
perform.

:	3. Any experience with PA's of this type from VAC? In particular,
:any comments on the VAC seventy/seventy?

No experience, sorry.

-- 
Mike Glantz			Digital Equipment Corporation
glantz@tay1.dec.com		Littleton, MA, USA