View Single Post
  #2   Report Post  
Old March 4th 07, 08:48 PM posted to
[email protected] is offline
external usenet poster
First recorded activity by RadioBanter: Aug 2006
Posts: 1,027
Default Tube equipment question

On Mar 4, 9:48?am, Mike Coslo wrote:
Solid state transmitters are notoriously finicky about matching to the
antenna. Tube equipment is not, so I am told, and early experience seems
to bear that out.

I disagree some with that, having entered HF radio comms
QSYing 1- to 15-KW transmitters. Failure to properly
neutralize a final RF amp can result in heating sufficient
to melt the glass envelope enough to let air in and thus
destroy the tube (an 833 that was mounted on the control
console at Army station ADA for weeks as a reminder).

Certainly I can see one of my newer rigs start to fold back at 2:1.

That's a somewhat arbitrary level that is designed into
most solid-state PAs. One can run them without the
VSWR sensing but it isn't a good idea.

For one thing, RF power transistors are expensive and
replacement is not the relatively easy task of just
unplugging the old tube and plugging in the new one.
It's a mechanical task and one has to remember to
properly heat-couple the new PA transistor...the amount
of waste heat is concentrated in a much smaller space
than big tube envelopes-bases.

What are the practical limitations of the Tube finals apparent

That's in many decades of old literature and covered
extensively. Data from commercial service transmitters
is more comprehensive than amateur types as a
general rule. Some of that may be hard to get now.

Is it safe to compare the load, plate, and drive controls to some of the
functions of a tuner? (possible real dumb question)

Not dumb, it's more like comparing potatoes and peas.

The long-common tube tuning controls are directly
connected to variable, relatively narrow-band tuning
and impedance-matching passive circuits. The tube
plate source impedance is relatively high compared
to the antenna feedline, even if that feedline is 600
Ohms balanced. The common pi-network is both
a resonating network and an impedance-changing

An antenna tuner is very simlar to a tube pi-network
but operates either to change (narrowband) impedances
up or down relative to the feedline characteristic
impedance. Again, passive components do the work
of transformation.

By contrast, most of the solid-state power amplifiers
are broadband, much more so than common tube
circuits. Since their input-output impedances are
relatively low and known (and predictable) over a wide
frequency range, they can use broadband transformers
for matching. The end result in the design is one
without many of the tube controls' necessity.

Usually, but not always, either type of amplifier is
still suceptible to damage from mismatching load
impedances. The mismatching just takes on a
slightly different form between the two.


Remember: All electronics works by smoke. If the smoke
gets out, it won't work... :-)