[Collins] parallel amplifiers

[Gerald]

On Mon, 2005-07-04 at 14:37 -0400, Drleborgne at aol.com wrote:
> Hi: I would like to run two 30L-1  Collins linear amps in parallel to  
> increase power. Anyone in this august group would have any suggestions on how  to?  
> Thanks in advance.
>  
>  Martin Leborgne

Why? It is certainly possible, but there are virtually no HF
communications circuits where a 3 dB increase in power will
significantly change the quality of a signal after the vagaries of
propagation which tend to change more than 3 dB per second. Rarely does
HF communication work at or below the noise threshold where a 3 dB power
change could make a larger difference in received signal to noise ratio.

Last weekend, my radio club ran two stations, one using 100 watts to
full sized dipoles on 40 and 80, the other running 25 watts output to an
8 foot whip fed through an L match on 20 and 15 meters. Working both CW
and phone and even though 20 was dead several hours in the night, the 20
meter station made more contacts than the 40 meter station. 80 was
better than 15 meters, just because of propagation. 6 dB power
difference cost -10 contacts out of about 300 total for each band 40 and
20. E.g. the 20 meter station made 10 more contacts than the 40 meter
station.

Besides its a lot of work to parallel amplifiers right and doing it
wrong can lead to much amplifier carnage.

What it takes is a pair of power splitters/combiners, and a couple dummy
loads. These splitters have to match but can be in phase, out of phase,
or 90 degree (quadrature). You have to match the gain and phase shift of
the amplifiers so that the sum in maximized in the output splitter
(which also may have a difference port with dummy load) and the
difference minimized. You can use a ratrace hybrid made of coax, but it
has the same problems of limited frequency range and the need for the
difference loads and their metering. The output difference load has to
survive half the power of the individual amplifiers when only one is
working. Its a pain, so its not a common technique with tuned amplifiers
(using tubes) but is used to combine solid state modules for higher
power but a couple KW gets into just a few makers.

If you just can't live with less power than two 30L-1, swap them for a
Henry 2K-3 or a 30S-1. Or a 208U-3 or 3A (www.fairradio.com) and a three
phase power line to the hamshack and be sure to be cited by the FCC for
running excess power.

Using good coax to a decent antenna can make up power faster than
running lots of RF out of the PA.

One of the phenomena I noticed early Sunday morning last week that a
number of home stations had strong signals, but couldn't copy any of the
100 watt FD stations in the pileups they caused. Because they were
alligators, and couldn't hear "normal" stations in their local noise. I
worked out to look for the weaker signals, which answered my calls on
the first try so I added many more stations to the log than by trying to
work the home stations with big amplifiers.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer