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Re: [Amps] 6 mtrs:

To: <amps@contesting.com>
Subject: Re: [Amps] 6 mtrs:
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Tue, 25 Sep 2007 07:47:33 -0400
List-post: <mailto:amps@contesting.com>
> 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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