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[TenTec] Ten-Tec PA design

To: tentec@contesting.com
Subject: [TenTec] Ten-Tec PA design
From: Jerry Volpe <kg6tt@arrl.net>
Reply-to: kg6tt@arrl.net,Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Thu, 13 Jul 2006 10:01:08 -0700
List-post: <mailto:tentec@contesting.com>
[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 

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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