On Wed, 10 Dec 2003 18:37:25 UTC, (LJ)
wrote:

I recently picked up a used Heathkit SB-200 Linear Amplifier. I tried
it out last night for the first time - on 20-Meters. Power out was a
little more than 700 watts, worked fine.

I then tried it on 10-Meters, and looking at the Drake W-4 Wattmeter
on the 2000W output scale, it was pegged! Looking at the reverse
scale, it was pegged on the 200W reflected scale. This tells me that
the amp was probably going into oscillation.

I wasn't too surprised to see this, as I have an SB-220 that just
spits and arcs continuously - it's unusable on 10-Meters (other bands
are okay). I haven't had a chance to install the Harbrach suppressor
kit yet, which I hope will fix that problem (it puts out over 1300W
output on 20 Meters, no problem, so the amp does work.)

I decided to then try 40-Meters on the SB-200, figuring that if
20-Meters worked OK, 40 should be okay, too. Wrong - the same thing
happened - nice 50 ohm load, keying the amp while driving with 60-100
watts, the wattmeter is pegged again - forward and reflected.

I can see why 10-Meters would have a problem, but with 20-Meters
working fine, I'm surprised that 40 would have a problem. I haven't
tried it on 80 and 15 Meters yet, I'll try that tonight.

Any suggestions?

Thanks & 73,

Larry K7LJ

Pretty weird. Unless the watt meter is VERY sensitive at the
frequency of the parasitic, there is something else wrong. No way a
couple 572B's are putting out 2,000 watts

OK, I thought about it and it makes sense. It's oscillating at
some frequency, maybe up about 50 mHz. At that frequency your
antenna exhibits very high SWR. Even if it's only putting out
400-500 watts, the high SWR is pegging the meter on forward and
reflected.

de ah6gi/4
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