> Parasitics are rare in vhf amplifiers. They are not rare
> in hf amplifiers.
By itself, that isn't an accurate statement. I would say it
differently.
Parasitics are more common in amplifiers that have long thin
leads, especially in the tubes to the grid connections. If
we examine systems that are more difficult to stabilize, we
find tubes especially and sometimes other components that
have poor VHF/UHF performance. The long skinny leads,
especially on grids, are often the root of the problem.
This is why 811's, 572's, and other tubes with a single long
skinny lead to a single grid pin are much more difficult to
tame than tubes using a grid ring.
> The difference is longer leads in hf amplifiers. In hf
> amplifiers, the
> bandswitch is not often connected directly to the plate
> tuning capacitor -
> it is connected with finite lead. If there is a vhf
> oscillation, the
> voltage occuring at the switch may well be different than
> the voltage at the
> capacitor. The sloppier the construction, the greater the
> lead length and
> the greater variation possible for a vhf oscillation.
You assume a VHF oscillation causes a switch failure. That
isn't a good assumption. I think you have been misled by a
claim the bandswitch always has higher voltage breakdown
than the air variable, or you are trying to answer an
intentionally loaded question that wasn't based on true
facts.
Voltage breakdowns in tuning caps and components like
switches vary considerably with the particular position and
even that particular component out of a batch from a
manufacturer. The distributed field gradient between plates
of a capacitor changes greatly with the mesh of plates, the
micro-sharpness of edges, humidity and air pressure, dust,
and so on. The bandswitch, because it has multiple taps that
have different voltages on and field gradients around each
terminal has a very wide range of possible voltage failures.
Even lead routing to the the switch and the sharpness of the
contact stampings and solder connections in any particular
switch greatly influence actualy flashover voltages.
It's a big soup of things going on, and NOTHING limits the
operating frequency voltage stored in the tank except
transfer of energy to the load.
Most people don't even realise that air variable plates are
often tumbled for hours in very soft abrasives like walnut
shells to polish off edges. One little nick in assembly, and
the voltage rating can seriously degrade. Rotate the plate a
few degrees, and the same sharp point might no longer be a
factor. It isn't just spacing.
Switches are even worse, since they are genrally raw
stampings. If you look at a RSC model 80 you will see very
sharp contact blades. That's why then huge RSC 80 has about
the same average voltage breakdown as the much closer spaced
Centralab (Electroswitch) JV9000 series.
As for voltage distribution, it is VERY easy to measure. No
need to make wild guesses based on grid dip meters or what
we might mentally picture. It's very easy to see the
voltage, even at VHF, is highest at the tube and steps down
as we progress through the tank. If the voltage didn't do
that, we would have a major problem with harmonic
suppression! Someplace common sense must have received an
eviction notice, and we just take the world of wild guesses
that make no sense.
You can follow the voltage distrubution when a leveled
signal is injected right at the tube anode through an SB221
at this link:
http://www.w8ji.com/SB221/sb-221.htm
The best thing is, anyone with test equipment can duplicate
this. You don't have to assume I am guessing correctly or
assume I know what I am saying because I know things or see
things no one else can actually see.
> Of course the voltage variation would not be great if the
> lead length was
> minimal and the frequency of oscillation was in the hf
> range. If the lead
> length is 3" and the frequency of oscillation is 144 MHz,
> then the voltage
> difference between the two points could be 15%.
It could be 100% also, but that's not the issue. The issue
is voltage distribution in the system.
I've measured dozens of HF amps, and the VHF voltage
decreases greatly as it works it way through the tank
system. I can 100% assure you if it doesn't, the amp will be
a TVI nightmare and never pass FCC technical requirements.
VHF energy must be attenuated as close to the tube as
possible, and that means low shunt and high series
impedances, or the amp will be a harmonic disaster.
I hope this makes things clearer and I am not just wasting
time.
73 Tom
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