[CQ-Contest] Tuning Sensitivity Of Modern Amps

[David Pruett]

Paul,

I understand that equal SWR does not mean equal load impedance.  However, 
as the SWR approaches unity the number of different combinations of R & X 
diminishes.  In both examples I quoted, the "equal and low" meant under 1.2:1.

I've used the "tuner on each antenna" approach here at K8CC for many years 
on 80M dipoles using MN-2000s.  While this provides a 1:1 match on A 
PARTICULAR FREQUENCY, at least with the MN-2000s the SWR bandwidth through 
the tuner is much, much narrower than that of the dipole by itself.  In the 
past this has been tolerable with my Henry 2K-3 (2 x 3-500Z) but recent 
operating patterns seem to have spread people out (more SS activity in the 
extra bands, and people going higher, particularly on CW) so some
"tuner tweaking" is necessary.

I would think that different tuner circuit configurations might be better 
or worse for this.  However, I would think that the desire for high 
efficiency (and thus hi-Q) would work against the need for broad bandwidth.

Finally, putting a tuner on each antenna gets financially ridiculous if 
quality tuners are purchased.

73,

Dave/K8CC

  At 12:23 PM 2/5/2005, N4XM Paul D. Schrader wrote:
>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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> >
>
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