> > > You are making mountains out of what turn out to be mole-hills. It's
> > > pretty easy to find the extremes in phase shift and loss at several
> > > points, and determine if oscillation is possible.
> > If they were indeed molehills, the questions surrounding VHF oscillation
> > and suppressor effectiveness would have been resolved long ago.
> They are resolved in many people's minds. There seems to be only
> one person behind claims parasitics are the root of all evil.
There is no way for either of us to know how many people agree that
"parasitics are the root of all evil" -- whatever that means. But a quick
scan of this reflector's archives will remind you that the number of
commercial amplifier owners believing that they've experienced parasitic
oscillations is far greater than 1.
The true test of an argument is its ability convince detractors. While the
VHF oscillation questions may well be resolved in some people's minds, they
certainly have not been put to rest.
> Most everyone else with engineering experience on vacuum tube
> PA's and a science/physics/engineering background seem to
> agree.
Lets evaluate ideas solely on their merits, not the credentials of those who
endorse them.
> > > I think maybe the mental picture you have of the system is
> > > incorrect, perhaps you are assuming wild gyrations on multiple
> > > frequencies. Multiple complex resonances and feedback paths are
> > > typical for "unclean" layouts, like a breadboard wooden chassis
> > > PA with point to point wiring, but not in modern PA's with decent
> > > construction.
> > Words like "wild", "unclean" and "decent" are subjective, and thus not
> > conducive to proving anything.
> If I had twenty years, I could write a book on what makes an
> "unclean" layout using scientific terms. As it is, you'll just have to
> consider poor ground, long leads, and components that exhibit
> changing characteristics due to internal construction errors as part
> of "unclean" layouts.
> I gave the example of breadboard construction. Do you know what
> that is, and what it looks like?
Certainly. The term effectively conveys a style of construction, but is too
imprecise to be of use in a cogent argument. The point is that arguments
couched in natural language are doomed to everlasting debate. If you enjoy
the verbal tussle, by all means continue the practice. If you seek
resolution, consider a more precise formulation of your argument.
> Modeling eliminates the need for imprecise
> > formulations in natural language. One describes a system with equations,
> > establishes the domain of each parameter via measurement or
> specification,
> > and then runs those equations over those domains to produce results that
> > can be repeated and objectively assessed by everyone, not just
> one person
> > poking a scope probe around one instance of a circuit.
> You can not model a HF PA without know what the components
> behave like at radio frequencies. The only way to know that is to
> measure the components. By the time you measure the
> components, you might as well just have measured the
> components as a group in the actual circuit.
> If you measure the actual circuit, you can predict the chance of
> any problem.
Its difficult to respond to these points without repeating points previously
made and ignored. Modeling does not eliminate the need to measure, nor does
it take less time. It permits one to evaluate the range of circuits likely
to occur over the course of production, rather than a single "actual
circuit". Its "language" is mathematics -- precise, unambiguous, and
repeatable. Modeling can produce conclusive, incontrovertable results.
> Modeling does not save time or give good answers in this
> application because the model is generally too complex. All of the
> modeling programs I've seen can't even handle an inductor properly,
> let alone a vacuum tube.
As an earlier contributor pointed out, one can construct models of
user-defined-devices if the off-the-shelf models are questionable. And it
would be helpful to know which modeling programs you've evaluated, and
roughly when you evaluated them; software often improves from release to
release.
> > > My point is the model would take longer to construct than the
> > > measurements would take, and almost certainly not be able to
> > > include anywhere near everything.
> > The measurement-and-argue approach to resolving the issues at
> hand has yet
> > to succeed. Creating and validating a model is certainly more
> > time-consuming than taking measurements. But after years of
> > non-convergence, one should consider optimizing for results rather than
> > time.
> There is no measurement-and-argue approach, except for one
> person who has little test equipment except for a grid dip oscillator.
> Then when someone with test equipment makes a measurement,
> he argues his GDO is better.
I've seen nothing but "measure-and-argue" on the question of VHF
oscillation.
> I can't say there aren't any modelling programs that will model a
> solenoid inductor properly, all I can say is I haven't seen one. I've
> tried a dozen programs, but I'm sure there are a dozen more.
> If anyone else has seen a program that works, please let me know.
Good idea.
73,
Dave, AA6YQ
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|