[TenTec] Omni VI PA "cut back power?" Not really.

[Ken Brown]

The RF PA in Omni VI and Paragon rigs do not have "Power Cutback" 
circuitry. There is no ALC or reverse power protection circuitry in 
these PAs. They are designed to feed a 50 ohm resistive load. If you 
connect a 100 ohm resistive load the RF voltage produced by the PA will 
be about the same as with a 50 ohm load, and the current supplied will 
be about half, resulting about half the RF power delivered to the load. 
This is not a problem for the Omni VI, though it may not be enough drive 
for your amplifier. If you connect a 25 ohm load, the RF voltage 
produced will be about the same and the current will try to be about 
double. That would be about double the power produced, and that IS a 
problem for the Omni VI PA. Ten-Tec power supplies for these rigs have 
current limiting, which protects the PA from over current. If you are 
not using a current limiting power supply, you had best do something to 
protect your Omni VI or Paragon PA from over current due to low 
impedance loads. This is why there are frequent discussions about where 
to get fast acting (Airpax for example) circuit breakers on this reflector.

The power that can be delivered by an Omni VI RF PA is dependent on the 
load connected to it, not because of any power cutback circuitry in the 
PA though. It will only produce the rated power into a proper load.  
This is not the same as with PAs which do have reverse power protection 
or ALC circuitry.

DE N6KB

> My problem was that the GLA-1000 lacks a tuned input circuit. It was 
> designed for exciters with tube finals, so input from the exciter is 
> through a capacitor. This presented varying impedances (not 50 ohm) to 
> the output of the Omni VI causing it to cut back its power. I could only 
> get full power out of the amp by using a small tuner between it and the 
> Omni VI.