Lou, you are wasting your time trying to get any meaningful answers from our
self appointed guru.
The 572B is rated to 60 MHz by at least one manufacturer who arguably
actually tested them there. The suggested Ep is 89% of HF ratings which puts
the SB-200 well within that limit. Just because Cetron had no intention of
initially selling the tube for RF use doesnt mean it wont work. It was only
after the audio market didnt materialize that they shopped it for RF. I
seriously doubt that they ever tested the tube above 30 MHz if indeed they
even tested it there and let the Heaths and others be the test beds. However
there were several other producers of the 572B that saw the potential and
quickly introduced their own version of the original Taylor T-160L.
Toms constant referring to lucky, chance or accident really makes me laugh.
My conversions are unconditionally stable at any settings and peak output
occurs exactly where it should. It was no accident. Several others have
tried to copy that design. Some have been successful, others I get in here
from customers who spent good money to have someone else do it for them and
it took off when they tried tuning into their antenna.
Of course output will vary. Using 30-40 year old well used tubes may only
produce 600W wheras new or well preserved oldies will do 750W or more. I
always start by testing an amp at 20M if it hasnt been butchered for CB;
what I get there I will get the same or a bit better on 6M. It is very
repeatable.
Toms repeated comments that he got a sweep tube to work on 6M as a dare is
another hilarious statement. Obviously he forgot about the Drake TR6 which
used 3 6JB6's very successfully. Of course in his eyes Drake didnt know what
they were doing.
Also note that Tom has a long track record of belittling other designs, be
they commercial or ham. Its just that "not invented here" scenario over and
over again until its finally time to just ignore them.
Finally, operating a tube above its frequency of maximum ratings is a well
established and accepted fact. A prime example is the Eimac 4-125A which has
a 120 MHz limit at maximum ratings. However it was used very successfully in
thousands of high band VHF amps including police and government. Eimac shows
typical circuits for 100 and 175 MHz along with suggested deratings. I guess
they didnt have an engineering staff either. I have one of those amps here
which does an excellent job on 2M FM driven by an old 10W Clegg xcvr. It is
used for chasing simplex contest contacts.
As far as Im concerned this thread is over, Ive better things to do than
waste my time in a never ending argument. Tom is especially proficient at
doing that.
Carl
KM1H
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <amps@contesting.com>
Sent: Tuesday, September 25, 2007 7:47 AM
Subject: Re: [Amps] 6 mtrs:
>> 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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