Ten-Tec PA Design Comments

From the Ten-Tec Reflector July 13, 2006

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. During 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 'shortcoming' but rather a design choice that has served thousands of Ten-Tec customers very well.

Jerry Volpe KG6TT