[AMPS] SB-220 conundrum

[Michael McCarthy, W1NR]

Pete,

It is generally accepted that if max. power output does not
co-incide with the plate current dip, you have a parasitic
oscillation.  Why such a subtle change causes it is anyone's
guess.

Mike, W1NR

Pete Smith wrote:
> 
> Here's a weird one for the group.
> 
> I replaced the T/R relay in my SB-220 with a pair of vacuum relays -- an
> HC-1 on the output side and a tiny Hi-G d'Italia on the input side.  It
> works great, but there is one anomaly.  The input SWR on 40 meters (only)
> is high (~2.2:1) and this seems to be paired with abnormally high plate
> current and output power.  All other bands are fine.  Just for fun, I tried
> bypassing the input relay entirely, but that had no effect on the input
> SWR, so I'm inclined to think the relays aren't the problem.
> 
> Running into a dummy load at ~ 1.1:1 SWR and using my HM-102 wattmeter (no
> Bird, but power levels presumably indicative, at least relative to each
> other), I find the following:
> 
> 1.  power output is ~1100w on 80, declining to ~950w on 10, all with the
> exciter showing 125 watts output and the SB-220 grid current ~200 ma.
> Plate currents are ~750 ma.
> 
> 2.  But on 40m, power output (PLATE and LOAD tuned for maximum) is ~1450w,
> grid current ~250 ma. and Plate current ~900 ma.  There is also a fairly
> striking difference between the PLATE setting in this condition and the
> setting for minimum plate current.  If I tune the PLATE for minimum plate
> current and LOAD for maximum output, the figures look much more like those
> on the other bands.
> 
> Any idea what's happening?  I find it hard to equate higher input SWR with
> apparently higher gain.
> 
> K9MA has suggested that in any case it would be a good idea to re-do the
> input tanks on all bands to raise the Q and improve the SWR, and has kindly
> given me values that he has tested in his amp, but before I embark on all
> that I'm curious what people think about this specific situation.
> 
> 73,  Pete N4ZR
> 
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> 
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-- 
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