[Heathkit] SB-200 10M Power Output

[Johnnie Walker]

Hi Steve,

I have an SB-200 that I've reciently given the
overhaul as well.  Let me 
throw out my 2 cents here.

With the old tubes, I had a similar performance drop
on the higher bands 
as you - but these tubes were with an unknown history,
and from an 
ancient vintage. 

With new tubes (generic China-made), the output jumps
up nicely on all 
bands, including 10M, but the output drops vs.
frequency - especially on 
the higher bands 15/10m.  The tuning on 20M and above
is razor sharp, so 
be careful there.  I'm retrofitting my amp with
verniers and (hopefully) 
that should make tuning in the future more easy.

In my testing, the effeciency of the amp at 10m is not
good.  About 50% 
vs. over 70% on 80M.  But that's a side efffect of the
tank losses, and 
the natural drop of the tubes (their 30 mhz top end,
or so)...  The 
limiting performance factor, by my calculations, is
plate and grid 
current on the lower bands.  Not plate dis.  But once
you hit 15/10m, 
the plate dis becomes the limiting factor.  so even if
you can push 500 
ma plate current, you will notice the anodes will be
very quick to show 
color vs. the same loading conditions on the lower
bands.

Regarding the input, I did need to tune my inputs as
well.  And it's a 
hassle with the GG configuration, because the input
SWR depends heavily 
on the tank output setting!  My FT2K folds back the
power output if the 
swr is more than 2:1. 

There is an official way to adjust the input that
involves removing the 
tubes, and some other things, then tuning that way
passivly. 

But for me, the easy and fast way was to do it "live."
 I tuned the amp 
for maximum smoke into the dummy load.  Then adjusted
the input slug for 
that band.  Of course, this will then change the drive
level (unless you 
are coming from a low SWR to begin with), which will
change the plate 
current/voltage meaning the optimal tune point has
moved as well.  So 
it's an itterative process. 

Keep going back & forth, allowing ample time for the
anodes to cool.  
And at some point, you will be able to push full drive
into the amp 
(stay under 100W - and be careful to not exceed the
max grid current), 
and when the amp is tuned for max power, the SWR at
that point will also 
be very low.  For most bands, you should see a drop
off in corresponding 
power output with an input power around 65w.  I think
the drive for max 
output roll-off is a bit higer on the higher bands.

Also, there is a huge amount of converversy as to the
proper max power 
output from this amp.  In extensive reviewing of all
available data on 
the net, I have not found a data-based opinion that
takes the ouput 
figures from a modern-era power supply and new tubes
configuration.  To 
figure this out, I will be putting an oscope and
lp-100 on this amp when 
I get some free time, to check the linearity and onset
compression point. 

I don't have a spectrum analyzer to do a proper IMD
test, but I do have 
a PSK IMD meter, and think I can indirectly determine
the onset of 
distortion by looking at the change in IMD indication
under PSK 
modulation at various power levels.  The LP100 also
has a peak vs. 
average power test as well, and in discussing the
possible used of this 
setting to check the compression level, that looks
like it may provide a 
reasonable (albeit informal) method of looking for a
top end.

I'm not an instrumentation guy, and not much of an amp
guy - so if 
anyone has any comments on that as well, I sure would
appreciate the 
advice...

73 and good luck.  This SB200, in my opinion, is
really the sleeper amp 
of all time.  Gets you to within 1 S-UNIT of the big
dog QRO, and costs 
less than $500, complete with every modernization
possible!!!

73/jeff/ac0c

sbjohnston at aol.com wrote:
>
> Not sure why Carl felt it necessary to attack the
poor defenseless MFJ 
> antenna analyzer (a most useful device in my
opinion)...
>
> If you can't reduce mismatch between the exciter and
the amp, you 
> might benefit from a change in the length of the
cable connecting 
> them, or try an antenna tuner between them as an
experiment.  You may 
> find that your rig has been reducing its output in
response to the 
> reflected power coming back from the amp.
>
> Is the grid current in the right ballpark on the amp
when driving it?
>
> Steve WD8DAS
>
> sbjohnston at aol.com
> http://www.wd8das.net/
> ---------------------------------------------------
> Radio is your best entertainment value.
> ---------------------------------------------------
>
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