Sb-200 weird thing happening

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, May 15, 2019.

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  1. N6UH

    N6UH Ham Member QRZ Page

    A tuned circuit which isn't on the original diagram and a missing plate choke bypass. My point was it might have been borrowed to make that extra tuned circuit?
     
  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    I made some new parasitic suppressors using the same size copper wire/number of turns and two 47 ohm 5 watt metal oxide resistors......didn’t have any carbon resistors on hand. I added another 3kv bypass cap on the bottom of the plate choke. I took it to me Elmer’s shop and we tested it on all bands......on a dummy load. Everything seems to work well now on a dummy load......no weird stuff happening. I was doing all of my testing through a tuner and dipole as I don’t have a dummy load (need to buy). I haven’t checked it out on my antenna system yet in my shack, but I will later on. Hopefully, all is well now. I’ll update later. Thanks to everyone for all the help, hints, suggestions!
     
  3. KN4CTD

    KN4CTD Ham Member QRZ Page

    Ok....someone correct me if I’m wrong, but this seems to be an issue with my antenna / shack setup. The sb-200 worked perfectly on all bands at my Elmer’s shop on his Palstar dummy load excited by a Kenwood TS-2000. The transceiver showed good/acceptable input SWR on all bands when driving the amplifier. Even on 10 meters, it showed 400-500 watts output with about 1.3 to 1 input SWR as viewed on the transceiver. I’ve hooked it back up in my shack and it works fine everywhere except for 10 meters. The cross needles on my Atr-30 tuner still go full tilt when keyed, EXCEPT when I use the tuner on the radio between it and the amp. The rig is an Icom 746 pro. The antenna is an inverted V center fed dipole that is fed with LMR400 equivant coax. It is currently resonant on 3.9mhz. I’ve had many amps hooked up to this setup using the same rig, tuner, and antenna combination and I’ve never seen this before. I don’t plan on keeping this amp so I don’t necessarily want to go changing a lot of things because of it, but I’d still like to figure out what is actually happening to cause this. The amp “seemed” to work 100% in his shack on a dummy load on all bands. Low input swr, good output, easy tuning.....even on 10 meters. We never tried it through his antenna and tuner combination though. The hv plate choke bypass cap was replaced also. Any other suggestions?
     
  4. KN4CTD

    KN4CTD Ham Member QRZ Page

     
  5. HAMHOCK75

    HAMHOCK75 QRZ Member

    This is just a thought but I once did an analysis of grounded grid amplifiers. The input impedance of the amplifier is affected by the load. Since a dummy load is resistive but an antenna is resistive only at resonance but reactive off resonance, I wonder if the reactance of the load is affecting the tuning of 10M input matching network. That matching network should be doing what you are doing with the tuner between rig and amplifier. If the load is affecting the input match then the amplifier will likely be affected by the length of coax between the 746 and amplifier.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I note in W8JI's writeups on common amplifier problems, Tom does state the SB-200 is among those considered "not very stable," even when everything is working okay.

    He has suggestions for improving stability, but as already mentioned, one is directly grounding the grids which means re-designing a biasing system and changing the grid metering circuit.

    I built an SB-200 from a kit around 1968 or something (they were pretty new then) and with zero modifications it worked fine when all components including the Cetron tubes were brand-new. I sold that only a few years later when I bought an SB-220 kit, and I'm not sure where that SB-200 is now. Maybe in a landfill.:p

    I didn't notice any instability back then, but used a vacuum tube transmitter with it (SB-400). These old amps were really intended for use with tube-type transmitters...in many ways.:)
     
  7. KN4CTD

    KN4CTD Ham Member QRZ Page

    I figure if it all works as it should into a 50 ohm dummy load with no issues, the amp is probably fine....no inexplicable stuff going on. There’s another ham who lives close to me. I may take it and hook it up at his station and see if it likes his shack any better!
     
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Not necessarily. One measure of an amp's stability is to continue to work well into moderately reactive loads. Working into a 50 ohm dummy load does not stress the amp very hard from a stability standpoint.
     
    WA7PRC likes this.
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    The missing choke bypass capacitor is a disc ceramic, not a big door knob cap like what's shown in the OP's photos for that added output network.
     
    WA7PRC likes this.
  10. KN4CTD

    KN4CTD Ham Member QRZ Page

    I’d still like to test it into someone else’s antenna setup at some point to see if it acts the same as it does on mine. Hopefully, I’ll get the chance to do that soon. I’ve traced all of the input and output circuits with an ohmmeter and I can’t find a problem there. I’ve also checked the grid resistors and caps and they all check out fine. This one definitely has me stumped. I’ve even tried the ol different length coax jumper thing and that had no affect either. I’ve searched forums online and have found at least two other owners of the sb-200 who described the same symptoms, but no fix. As many of these amps that are still around, I’m surprised I haven’t found a cure.
     

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