> There has to be something said here for the fact that Carl
> (KM1H) and myself
> have very successfully converted SB-200's to 6 meters, and
> getting 700-800
> watts out of them depending on how "hot" the whole
> conversion works out. I
> don't think anything is specific, but to simply say a 572
> should not work on 6
> meters is hogwash.
Well Lou it all depends on the definition of "work". People
think if something isn't a good idea that means it doesn't
ever "work", but "work" is a very nebulous term.
The fact is tubes of the 811 family have a single long thing
grid lead that goes to a single pin in the socket and a very
long control grid inside the tube. This forms a series
inductance to the ground and the large grid forms a
capacitor. At some frequency this parallel tunes the grid
lead no matter what you do outside the tube.
The manufacturer understands normal stable repeatable
operation of the tube requires operation well below the
frequency where the grid is parallel resonant. This is one
of the primary limits, actually the most common one, used
when they give an operating frequency range. Now you and
Carl can convince yourself a 572B is a great tube to use on
six meters, but the fact is the tube is marginal already at
upper HF.
> They may rate it to 30mhz, but they do work at 50mhz.
A resistor loaded vertical will "work". So does using a nail
for a cotter key.
> If I was to have to answer the question "WHY", I guess
> something has to be
> said for the fact that we are using a SINGLE band tank
> circuit, which holds the
> stray "C" and "L" to a minimum ie..no long leads going to
> a tank circuit far
> away from the load and tune caps. This is where
> instability can arise. SO
> in a nutshell I'd say WE make them work because we make
> them a single band
> amp. The parameters of the tube are as good or better
> than what you see on 80
> meters.
Actually Lou it's all about if the anode circuit happens to
have a parallel resonance on or near the frequency of the
control grid, and what the grid to chassis impedance looks
like. If you happen to hit a lucky combination of lead
lengths and stray impedances it's certainly quite possible
to use a tube beyond the range where it is reliable or
predictable in other similar situations.
I've seen very sloppy homebrew amplifiers breaking all
common sense rules of circuit layouts using 4-1000A tubes
that were totally stable.They had long thin leads to the
grids and very long anode to tuning cap leads and almost no
input to output shielding. Yet these sloppy thrown together
amplifiers were totally stable. If you clean up all the
leads and shield things, and the PA takes off above ten
meters. It's a bear to stabilize.
Now I suppose the fellow throwing it together could argue he
did a better engineering job, but the fact is sometimes
there is just a lucky combination that has more to do with
finding a recipe where losses luckily exceed feedback on a
critical frequency area.
Sometimes using a tube beyond safe stable limits will work
just fine, but we shouldn't confuse that with the
manufacturer being wrong.
What people are generally saying is the 572B is such a poor
tube at high frequencies it requires neutralizing to be
stable on ten meters. This is true even in an exceptionally
clean layout. That's a fact not open for debate. Even the
tube manufacturer knows that.
That ISN'T saying the tube can't work in a particular layout
above that frequency, it's just saying you are going beyond
good design limits. That fact should be clear because by
your own admission you don't get consistent gain or output
between multiple unit of similar design. This doesn't mean
the amp is "bad" or you shouldn't sell them, it just means
you have found a lucky esoteric combination that happens to
work as a monoband amp.
The mistake comes in when people form an opinion based on an
esoteric "design" using Edisonian cut-and-try
experimentation. That isn't a good place from which we
should dictate to the rest of the world that everyone,
including the tube manufacturer, doesn't understand the
device.
There's nothing wrong with what you are doing except when
you extend the fact you got lucky to mean it's a great idea
and the rest of the world is substandard.
73 Tom
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