RF design is not such a complicate matter, in theory, what's difficult
is to deal with and to realize objects that doesn't introduce too many
unknown parasitic parameters so modifying, in practice, the original
(or supposed) electrical configuration. The difference between skilled
and non skilled RF designer is mostly in the knowdledge of the above
reality and in the ability to choose a configuration, or a device,
that's not adding uncontrollable complications to the amplifier.
The problem with non professionals when building or dealing with RF
amplifiers (not necessarily power amplifiers) is not "to see" the
presence of inherent parasitic parameters in a circuit or, more
frequently, in the way those amplifiers are then effectively built or
modified by the amateur, a way that often introduces a number of
parasitic parameters and messes everything.
A good device, like a Gaasfet transistor with large gain and stability
at several GHz, has no problem at all to be controlled at much lower
frequencies and risks of self oscillations are virtually absent if also
the external circuit around the Gaasfet has been designed and realized
in a proper way, all the above story do not only apply to solid state
but is also valid for vacuum tube amplifiers.
On the other end, a badly designed tube (more frequently older tubes,
but not only) with a low gain already at 30 MHz can still have enough
gain (at some nearby VHF) to start parasitics when their internal
construction is not the good one to insure stability and the external
ciruit does nothing to fix such type of problems.
73,
Mauri I4JMY
> ---------- Initial message -----------
>
> From : owner-amps@contesting.com
> To : amps@contesting.com
> Cc :
> Date : Thu, 3 Aug 2000 22:12:00 -0400
> Subject : Re: [AMPS] SB1000 parasite question for Tom and Rich+
>
>
>
> > Rich it seems to me that one way to reduce vhf gain would be not to
use
> > tubes ( very expensive ones) that have max. ratings to 500 mhz on
> > frequencies of 30 mhz or below. if it will amplify at 200 mhz it
should
> > also oscillate.....yes?? HANK
>
> Hi Hank,
>
> How do all those big fancy amplifiers with lighthouse tubes or big
> tubes in cavities with extreme Q's not oscillate?
>
> Actually that is exactly backwards. The most stable HF amplifiers
> use tubes that are good well up into VHF or UHF.
>
> The LEAST stable amplifiers use tubes that marginally work at 30
> MHz, like 811A's or 572B's.
>
> It's always been that way and will always be that way.
>
> It isn't difficult at all to check an amp for stability, almost
anyone
> can do it.
>
> The real reason this nichrome thing has gone on so long is
> because people want a single simple answer to any problem they
> might have. It makes anyone an instant expert, and they don't have
> to understand a thing about tubes, oscillators, switches, radios,
> relays or anything else.
>
> All they have to do is click their heels together and say...."there's
> no problem but parasitics...there's no problem but parasitics..."
>
> Before you know it, your an expert on the sole cause of any failure
> or abnormal operation....and you know the sole cure....a sprinkle of
> something that doesn't hardly change the VHF Q.
>
>
>
>
>
>
> 73, Tom W8JI
> w8ji@contesting.com
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