SB-200 oscillating with an antenna tuner

Discussion in 'Amateur Radio Amplifiers' started by KS9Q, Mar 29, 2017.

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  1. KS9Q

    KS9Q Ham Member QRZ Page

    What's bugging me about the terminal strip is that it is primarily used for the choke and decoupling components for the bias and the ALC circuits. The way I had everything laid out, the bypass caps were very close to the filament supply lines which carry drive power. Unwanted coupling could conceivably introduce RF where RF shouldn't be. I'm in the process of rearranging some furniture as it were to get a little more spacing and as little extra lead length as possible.

    I think reducing the resistor value on the suppressors would be prudent as well and I'll change out the plate choke bypass cap while I'm in there. You may not hear from me for a few days until parts show up and we are planning a few days of family activity as well, so I'll update everyone once I have some results. Thank you all for your input thus far. In my job as one of the only surviving full time component level avionics bench techs in my area, I'm usually on my own to figure things out and it helps a great deal to bounce ideas off of knowledgeable people who may very well catch something that I have consistently overlooked.


    Jim K. - KS9Q
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    One of the downfalls of that amplifier is how grid current is metered.
    The amplifier could be modified to measure grid current in the negative lead of the power supply and that would allow for directly grounding the grids. That can do nothing but enhance stability.
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    In fact, the W7RY kit for the SB200 provides for that. At the same time, it fixes a few other things.
    AF6LJ likes this.
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    That's the single best mod you could do to an SB-200 it actually adds value to the amplifier.
    WA7PRC likes this.
  5. KS9Q

    KS9Q Ham Member QRZ Page

    I've been looking at that mod - interesting idea. One guy claims he's getting 750-800W all bands and 900W on 10 m, which I think is horribly optimistic at best. I can't see an SB-200 putting out that kind of power, especially on 10. Maybe 700W with new tubes on a good day. Sure you take the SWR bridge circuit out (not the worst idea) but still - those tubes only put out so much juice. At any rate, I like the idea of triggering the relay before the tubes are brought out of cutoff, which seems to be the whole premise of the mod. And an actual standby switch to boot!


    Jim K. - KS9Q
  6. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I think yer right about the optimistic power claim!
    Mine started off as someone's mobile disaster so when I bought it, it was an easy decision to gut it and flip its topology the other way around to turn it into an SB-220 style... grounded grids and flipped the cutoff to run +100v to the cathodes key up. The amp is stable and runs well like this.

    Good skill with yours! (I never say, "Good luck.") :p
    ... and tell us how it comes out.
    Last edited: Mar 31, 2017
  7. W1QJ

    W1QJ Ham Member QRZ Page

    By all means grounding the grids directly is good for a considerable gain increase and I would not rule out an additional 100 watts more over the standard design. And not to say an SB-200 would never make 900 watts just isn't true with 572B tubes. Case in point....I converted an SB-200 to a mono-band 160 meter amp. I did not ground the grids, the original SB-200 design prevailed except that I put a .01 ceramic disc across the 200pf grid capacitors. Used a bigger UH filament choke and plate choke and more Pf on bypass. Then I designed a typical input input circuit just like the SB-200 originals but for 160m. I then did up a tank circuit using a toroid. Padded the load and tune caps appropriately. I obtained an easy 925 watts output from the amp. If I had gone through grounding the grids I am sure I would have been up to a KW. The grid current was a shade over the white section of the meter which is no big deal for SSB or light duty CW. I held down a dead carrier for 30 seconds and the tubes just started to make some color.
  8. KS9Q

    KS9Q Ham Member QRZ Page

    One of the (several) sites on the SB-200 suggests bumping those grid bypass caps to anywhere from 950-1500 pF. Not quite as substantial as direct grounding, to be sure, but a happy medium if you want a better path to ground without reworking the whole bias arrangement. As you increase that capacitance, at some point you have to start messing with the input network to get the matching right. I've already spent a crap-ton of time on this amp so I probably won't go quite that far with it. I can certainly see how reworking the circuit to a more efficient design can milk some more performance out of the tubes.

    Jim K. - KS9Q
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Problem with grid bypassing is the sockets don't permit any value of capacitor, no matter what you choose, to do a terrific job across 3 octaves of frequency coverage.

    Directly grounding the grids using wide copper straps (not wires!) via the shortest possible path to the chassis (which means drilling holes) does accomplish this.
    WA7PRC likes this.
  10. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Would someone explain how you can get more output by directly grounding the grids when it simply takes about 30% more power to drive the tubes to max out with the 200pf caps installed. If your driver will make full output with the caps, why would it make more with the grids grounded?

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