[CQ-Contest] Tuning Sensitivity Of Modern Amps

[David Pruett <k8cc@comcast.net>]

At 11:22 PM 2/4/2005, you wrote:
> I have two ex-PLA surplus 1kW+ solid-state amps still in storage
> waiting for a reason to do something with them... the timing of Dan's
> post is handy as I'm beginning to get annoyed at having to retune
> the GU74B when I switch between antennas - even if one is flat &
> the other has not much higher SWR than that.

Brett,

This is my big gripe about modern amps, or more precisely, the tubes used 
in modern amps.  These high gain designs are wonderful when you tune up on 
a frequency and the load never changes (like in a broadcast or commercial 
application) but for us hams (and particularly contesters) the load 
sensitivity is a pain.

Personal examples from K8CC:

Ten Tec Titan, three antennas on 28 MHz.  Go to a spot in the band where 
the Bird 43 says SWRs are equal and low.  Tune up for 1500W on one antenna, 
2nd is 1000W and almost no grid current, third is 500W and grid meter is on 
the peg.

Homebrew 8877, three antennas on 7 MHz. Go to a spot in the band where the 
Bird 43 says SWRs are equal and low.  There was no combination of tune and 
load settings where I could run the amp safely into all three antennas at 
anything approaching the legal limit.

I've gotten around this by using only old-style glass bottle tubes in my HF 
contesting station.  In the exact same examples given above, a pair of 
3-500Zs, a 4-1000A or a 3-1000Z shows essentially no change in operating 
conditions when the loads are switched.

With the modern amps, Tim/K3LR offered the tip of loading the amp slightly 
heavier than normal to accomodate load impedance changes.  With most of the 
modern tubes, grid current is the killer so loading the amp slightly on the 
heavy side (say giving up 50W of power output on top of 1500W) drops the 
grid current significantly, and gives you some "wiggle room".

73,

Dave/K8CC


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