[TenTec] Tx IMD and Purity

[Jim Brown]

On 7/1/2013 10:50 AM, Bob McGraw - K4TAX wrote:
> There are limits as to what can be obtained with a 12V radio design.  
> Thus I do think, even for a 100 watt radio, we need to get away from 
> 12V radios.  Now that brings up another issue and that is the 
> "internal power supply" would be a must.  I certainly do not have a 
> problem with this and would think a 50 volt supply would improve IMD 
> values for a 100 watt radio by some 20 to 30 dB. 

YES!  I've had extensive discussions of these issues with K6XX, part of 
the Elecraft design team. We're in the process of preparing a joint 
presentation to the Northern California Contest Club on the topic of 
clean signals.  That's a big deal for us, because we're >600 members, 
mostly in the SF Bay Area, we often contest at high power, and most of 
us live quite close to other active contesters.

Several of Bob's observations bear repeating.

1) The K3 is much cleaner when run at 50W (typical to drive a power amp) 
than at 100W.  At full power, it is also much cleaner when run from the 
14VDC (the higher voltage of a fully charged battery) than 11VDC (a 
battery getting close to full discharge).

2) The power supply for Elecraft's KPA500 (rated 600w) is 75-80VDC (it 
is an unregulated linear supply).

3) The Hercules II (which both he and I own, and which runs on 12-4VDC) 
has much higher IMD than the KPA500.

4) Well designed solid state amps have roughly 10 dB greater IMD than 
well designed AND PROPERLY TUNED tube amps.

5)  Solid state amps should always be run into a well matched load. The 
IMD produced by a solid state amp increases significantly when the load 
is not well matched.

6) Bob and I live about 3 miles from each other, are good friends, and 
very active contesters.  We both own KPA500s, but we both use legal 
limit tube amps for contesting (I use Titan 425s)  and are quite careful 
to keep them tuned.  As a result, we can work within 500 Hz of each 
other on CW, and 3-5 kHz on SSB.

There's more to it than power supply voltage, tubes or solid state. 
W8JI, who has designed a lot of power amps, has written extensively 
about several mechanisms that increase distortion, clicks, and splatter. 
Some of the newer amps use circuitry that attempts keep the tube biased 
off until it sees RF, then turn bias on. He observes that this sort of 
circuitry can cause massive clicks and splatter.

73, Jim K9YC