Re: 211 and 845 operating Ea/Ia bias points.

On Aug 9, 9:06 pm, "maxhifi" <n...@xxxxxxxx> wrote:
You don't need to build it (though I did) to know it will sound like
***. Just check the Ia-Ea-Eg curves, and ask yourself what sort of a
primary impedance you will have to put on the tube to get any power
whatsoever, and I mean less than 2W.

They say to use 3400 ohms, I did print the curve out and draw the load
line - and really, it looks decent - amazing in fact compared to say a
triode connected KT88. I believe their 15W at 5% THD spec, but I see what
you mean, it hardly uses the tube to its full potential. I was just
imagining new ways of using 'normal' parts I have laying around (i.e. output
transformers), rather than seriously contemplating building a new amp around
845's.
maxhifi <no@xxxxxxxx> wrote:

You don't need to build it (though I did) to know it will sound like
***. Just check the Ia-Ea-Eg curves, and ask yourself what sort of a
primary impedance you will have to put on the tube to get any power
whatsoever, and I mean less than 2W.

They say to use 3400 ohms, I did print the curve out and draw the load
line - and really, it looks decent - amazing in fact compared to say a
triode connected KT88. I believe their 15W at 5% THD spec, but I see what
you mean, it hardly uses the tube to its full potential. I was just
imagining new ways of using 'normal' parts I have laying around (i.e. output
transformers), rather than seriously contemplating building a new amp around
845's.

I've drawn the 500V 5K (warning magenta lines), 750V 3.4K (seasick
cyan lines) and 1000V 10K (reliable deep blue lines) operating points
on the curves to aid discussion:
http://members.lycos.co.uk/fiultra/Discussion%20Circuits.htm

The key thing isn't the voltage but the plate load necessitated at
that voltage by (often unrealistic) power expectations. I haven't
drawn the 500V 3K combination that was suggested at one stage but you
can easily see that it will be down in the curved grunge at the bottom
of the transfer function. Even with 5K this isn't such a hot amp: it
will operate at current cutoff rather often, right after it makes an
unplanned discursion into grid current, with the uncontrolled
consequences Patrick has already alluded to.

The 750V 3.4K suggestion is one made by engineers who think THD of 5
per cent acceptable... It might have been made acceptable by accepting
a lower power output consequent on a more conservative load choice. It
uses huge current swings for absolutely no good purpose. It isn't
brilliant even by engineering standards, never mind audio standards.

Notice also that my minimum acceptable combination of 1000V 10K has
*uses more current over half its output*, that is up to nearly 10W!
Notice that the output of the 1000V 10K combination is *visibly* less
distorted over its entire output: you don't have to measure, it hits
you in the face when you eyeball it. It is in fact virtually
undistorted to more power than the total output of the 750V
suggestion.

Notice "minimum acceptable combination": Ideally the loadline should
be tilted to cross the negative bias lines at right angles, but I have
never had big transformers of such elevated impedance, and Patrick,
who is presumably capable of winding something good enough for 100W at
12-14K, doesn't want to pay the price in lost power. I do however know
that a major improvement lies beyond 10K if you don't need all the
power. I discovered this when I seriesed two high quality 5K6 Lundahl
transformers for 11K2 altogether and loaded up an 845 with their
primaries.

Of course, the 845 is such a fabulously linear tube, what Patrick and
I and others are shouting about like high tragedy are in practice
minor pecuniations that in a less superior tube will not even be
noticed in its general raucousness. We can get altogether too bloody
refeened for our own good.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
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containing vital gems of wisdom"
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