On 11 mayo, 02:05, walt wrote:
On May 10, 7:28*pm, Richard Fry wrote:



On May 10, 5:48*pm, Richard Clark wrote:

The transmission line is bounded at either end by identical mismatches
in conjugation. *There is a "ringing" in the line, which at only one
end there is dissipation (neglecting loss and damage to the tube for
exceeding maximum tolerances, of course).

If, as you have posted, dissipation occurs only at one end of the
transmission line -- which presumably for the most useful benefit
would be at the end of the line opposite the source, e.g., the load --
and the incident power generated and delivered to that line by the
final r-f stage in the transmitter is a constant, then how could
"damage to the tube" occur?

Just to note that as a field engineer for RCA Broadcast for many years
I have had to troubleshoot and repair failures that occurred in
transmission lines, transmitters, and output networks that resulted
from reflections -- either from within the transmission line alone, or
together with the load connected at its far end.

RF

Yes Richard, I'm familiar with your work at RCA Broadcast Div.
However, in my posts I'm concerned only with tube rigs used in the
Amateur Service. In these rigs reflected power doesn't cause
overheating, or other damage to the tubes.

To Walt: (Steady state) Load conditions can be specified in many forms
(complex impedance, complex reflection coefficient, VSWR with phase,
etc), but depending on the actual integral of the Vp*Ip product, the
plate's dissipation may decrease or increase (due to a load change).

So changing the load (without retuning as you mentioned below) may put
your tubes at risk. Your qualification below seems to contradict your
statement above.

But I'll qualify that
statement--if the pi-network is originally resonated into, say, a
dummy load, and is then switched to a mismatched line without
retuning, the reactance appearing at the input of the mismatched line
detunes the network and the result is the same is if the network was
left off resonance initially. As you well know, a mis-tuned pi-network
results in excessive plate current, which, if high enough will damage
the tube. Consequently, it's the mis-tuning that causes the damage,
not the reflections per se.

Incidentally Richard, I was also an electrical engineer with RCA, from
1949 thru 1980, first at the RCA Labs in Princeton and beginning in
1958 with Astro.

Walt


Wim
PA3DJS
www.tetech.nl