[CQ-Contest] Tuning Sensitivity Of Modern Amps

[N4XM Paul D. Schrader]

Dave,

I am very aware of your amp loading problem.
But equal SWR does not mean equal load impedance.

I put an XMatch antenna tuner on each antenna and then the problem is
solved for a wide frequency range on each antenna.
The desired immediate switching capability becomes a reality.

73

Paul  N4XM

At 10:29 AM 2/5/2005 -0500, you wrote:
>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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