Sb200 input question

Discussion in 'Amateur Radio Amplifiers' started by KB3SQW, Dec 5, 2018 at 6:54 PM.

ad: L-HROutlet
ad: l-rl
ad: Left-2
ad: l-assoc
ad: L-MFJ
ad: MessiPaoloni-1
ad: Subscribe
ad: Left-3
  1. KB3SQW

    KB3SQW Ham Member QRZ Page

    i seem to have issue and not sure how to solve it. With dummy load the output is 1.1 vswr. But my input from rig shows high . Should the input on amp be 1.1 . Match.
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Is this true on all bands or just some HF bands? Does it happen both in bypass mode to the dummy load (amp off or amp keying line removed) or only when the amp is active and amplifying the signal?

    There are a couple of things that can lead the SB-200 to present a bad match to the rig.

    - Bad relay contacts that are often visibly blackened. This typically impacts SWR looking into the amp both when the amp is active and when it's in bypass mode. If the contacts aren't too bad they can often be burnished clean with a dollar bill or something similar but don't use strong abrasives or you'll likely scrape off any remaining plating from the contacts.

    - Bad input matching through the band specific tuned circuits. If this is the case the match may be good on some bands and bad on others. A bit of tuning to the input networks can help the trouble bands.

    There could be other things like the tubes themselves or circuitry around the tubes going bad and changing the load the input matching circuit sees, but the two above are common and usually not too hard to resolve.
     
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    No, that's an unrealistic expectation.

    However, the amplifier does have bandswitched tuned input circuits and you can adjust those for a better match.

    Sometimes a capacitor (or more than one) in the input tuned circuit section fails and then you cannot adjust for a better match without replacing the capacitor first. The capacitors are C30 through C38 and they're behind the shield towards the rear of the amplifier so you have to do some dismantling to making tuning adjustments or capacitor replacements. (They didn't make it particularly easy.)

    See below, from the SB-200 manual...

    SB-200 tuned input circuits.PNG
     
  4. KB3SQW

    KB3SQW Ham Member QRZ Page

    Thanks got new relay and will check the caps too . Hope it is that easy . Hihi
     
  5. W1QJ

    W1QJ Ham Member QRZ Page

    The SB-200 amps are know for having lousy input swr on 15 and 10 meters and rarely does adjusting the coils help a whole lot. Usually a unit that has its mica caps in tolerance and the slugs can be adjusted, a very low input SWR can be seen on 80-20 meters, but 15 and 10 are usually no good and can't be made perfect unless they are redesigned. Often times however on 15 and 10 the amp may put out decent power but the "seen" swr is high which is sometimes an erroneous swr reading called phantom swr. A properly working input circuit will show a 1 to 1 swr and will remain constant with drive level. I find myself constantly having to tweak 15 and 10 meter coils and caps to obtain a good match with very low swr. It also seems that what works in one amp does not always work in another and have to be tailored to the specific amp. This is mainly due to the fact that the higher the frequency the inductance and capacitance becomes more critical. On lower frequencies slight differences are not readily noticible
     
    K1ZJH likes this.
  6. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    My Kenwood hybrid had no trouble driving the SB-200 but when I bought a new ICOM, it coughed on 15m and 10m. I could tweek the amp to run OK in part of those bands but getting below about 1.3:1 is iffy up there with a stock SB-200, especially on 10m.
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I've found that adding a 10 to 15 ohm 10 watt, non-inductive swamping resistor between the tuned input circuits and the filament driving capacitor(s) can help a lot in terms of improving match on all bands. It also lets you drive the amp with a bit more power which is convenient with today's common 100 watt rigs.
     
    KD2ACO likes this.
  8. KB3SQW

    KB3SQW Ham Member QRZ Page

    Where about would you place it can you show a pic. Thanks
     
  9. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    I used a nice little chassis mount Caddock that works well there. I’ll find a picture right after the steaks come off he grill... or maybe after dinner. :p

    I needed that resistor because the hybrid wants to run at about 90 watts out and the SB-200 only needs about 70 watts for full power.
     
    Last edited: Dec 5, 2018 at 11:04 PM
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Next time I pull the amp out of its case I'll snap some photos, but here's how PA0FRI placed a similar swamping resistor:
    Photo from: https://pa0fri.home.xs4all.nl/Lineairs/SB200/sb200eng.htm
    [​IMG]

    The swamping (aka feedback) resistor is the set of parallel resistors shown as '10 ohm' sitting between the input matching networks and the tubes.

    I used a single 10 watt non-inductive resistor instead of a set in parallel and the resistor is a bit larger but sits roughly in the same place.

    There's nothing magic about the resistor value, anything in the 10 ohm to 30 ohm range should do a good job assuming you'll drive this with a 100 watt rig. A bit higher resistor value as in 15 to 20 ohms further reduces the tube's influence on input matching and should make input matching easier on the difficult bands.
     
    Last edited: Dec 6, 2018 at 12:00 AM

Share This Page