[Hmmmmmm, an old topic comes to the surface, yet again. :) ]
Ten-Tec has for nearly 30 years included a very descriptive section in
their transceiver manuals regarding the effect of SWR on their rig's
transistor PA output states and how Ten-Tec chose to not use SWR
rollback circuits. Instead they used far healthier transistors (hence
the ability to run 100% duty cycle without a fan) and installed a
current limiter in either the rig itself, or more commonly, in the
matching power supplies. In the Ten-Tec description they refer to two
types of SWR, referring mainly to whether or not the ultimate (complex)
impedance at the transceiver output is higher, say 100 Zohm or lower 25
Zohm. In the first case there are no issues nor protection generally
necessary. In the second case, as the SWR increases, effectively taking
the output more closely to a condition similar to a dead short, the PA
will pull more and more current because of its design. Unless there is
some form of limit to the available current eventually the transistors
will be operating outside their design temperature range and of course
that is not good. This is not a shortcoming in design and actually a far
better choice than the SWR rollback and the less robust transistors
often used by competing brands. If the amateur had the appropriate
matching power supply the worst thing that might happen is the
occasional 'popping' of the power supply's power supply breaker.
It has been my experience, with over 30 years a Ten-Tec owner, that this
design works great. I have never damaged a PA in all these years, nor do
I personally know anyone who has. Of course there have been some Ten-Tec
designs that did not automatically include current limiting. Take note
of a circuit difference between the Pegasus and the Jupiter for
example... or study the schematics of the Century 21, and Century 22 and
you will see a few strays from this design philosophy. Nevertheless, the
Triton Series, all the Omni's, the Corsairs, and probably the Deltas (I
need to confirm this) when used with the appropriate Ten-Tec power
supply had the current limiting protection required.
Problem is that some put their Ten-Tec's on a power meter and don't see
100 watts output (who knows what meter they are using), now Ten-Tec
specifications do not specify 100 watts exactly. Some rigs may only put
out 90 watts on some bands, yadda, yadda. Durring alignment the output
stages are adjusted for a particular current draw at resonance on a
particular band. The output is then checked on all bands to see if it is
within published specifications. If not then there isn't sufficient gain
withing the transistors and they need replacing. Unfortunately, I know
hams who have changed the internal settings of the transceiver to allow
their rig to output more power (yes, it can be done... Don't ask) and
then defeat the current limiting to keep their power supply from
shutting down. Worse yet, they use a different power supply brand and
then they may have a combination for real problems. In particular I am
thinking about power supplies like the Astron 35 or 50 series that can
easily damage an output unless there is some way to limit the current
supplied.
It does amaze me how many buy a used Transceiver and either never read
the manual, or may not even bother to obtain a manual in the first place
and then 'plug-n-play'.... all caution to the wind. Oh well.
Once again, we are not talking about a design 'shorcoming' but rather a
design choice that has served thousands of Ten-Tec customers very well.
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