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Author Topic: SB200 High SWR All Bands  (Read 5459 times)
N0GW
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Posts: 47




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« Reply #15 on: August 25, 2017, 07:35:54 PM »

I used an SB-200 for quite a few years. What I discovered early on was that the input SWR on the SB-200 varies with the plate tune and load controls as you are tuning up. The input tuned circuits help get the input impedance somewhat close to 50 ohms but I would be surprised to find it as low as 1.3 to 1. Some transceivers are too sensitive to SWR to work well with the SB-200. I had to use an automatic antenna tuner (AT-250) between my TS-140. In later years, I found that intermediate tuner was not necessary with my TS-850 and later yet with my IC-756 Pro III.

It was kind of interesting when using the Kenwood AT-250 automatic tuner between the TS-140 and the SB-200. As I adjusted the plate and load controls on the SB-200, the motors in the AT-250 would be move to maintain a low SWR for the TS-140. For reference here, the TS-140 was extremely sensitive to SWR. Its output would drop rapidly as SWR increased above 1 to 1. By 1.5 to 1, the its output was only 3 or 4 watts.

For those who might  be  concerned about having two antenna tuners in line, that was actually not a problem. The tuner between the TS-140 and the SB-200 was in line only when the SB-200 actually operating.

Gary - N0GW
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KM1H
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Posts: 5259




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« Reply #16 on: August 26, 2017, 08:16:00 AM »

Quote
I used an SB-200 for quite a few years. What I discovered early on was that the input SWR on the SB-200 varies with the plate tune and load controls as you are tuning up. The input tuned circuits help get the input impedance somewhat close to 50 ohms but I would be surprised to find it as low as 1.3 to 1

That effect is perfectly normal in a GG amp since the input and output is in phase and the reverse isolation can vary considerably depending upon the type of tube and how well the amp is neutralized.

The 811A and 572B amps in particular have low isolation due to the tubes internal construction plus the SB-200 is marginally neutralized which can cause problems on 15M and up in particular.

There were factory changes to the input circuits over the SB-200 product life and several attempts by individuals to improve them. 

Carl
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K6BRN
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Posts: 1293




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« Reply #17 on: August 26, 2017, 11:11:41 PM »

Gary:

After rebuilding an SB-200 with a blasted band switch, deformed (due to heat) tubes and a variety of other fun problems, I was able to tune the input networks  to hit from 1.0:1 to 1.6:1, ... at specific sweet spots I selected on each band.  Retuning 10M required a change to the mica cap.  CW power output with NOS Cetrons ranges from 505 watts on 10M (1.2:1 SWR) to 665 watts (1.0:1 SWR) on 20M.  80M had the worst minumum SWR, at 1.6:1.  All with the SB-200 running off of a 120V, undedicated 15A outlet.

Brian - K6BRN.
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K5WLR
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Posts: 311




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« Reply #18 on: August 28, 2017, 06:54:03 AM »

Say Will...K5WLR...W-il-L Rogers?

Clever vanity call sign or did it just turn out that way like mine.

I practiced my call sign when I first got it. So if someone screws it up, I can understand.

Kraus

Hi Kraus,

It's a vanity call... just my initials.

73

Will
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