SB-200 oscillating with an antenna tuner

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

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It can, depends on the band. The "direct grounding" mostly increases gain, making the amp easier to drive, and helps increase output power on 10m/12m. It can also improve stability, since there isn't a capacitor anywhere that doesn't go through self-resonance at some frequency and above that become inductive.

    This is one reason why "broadband" coupling or decoupling often uses several parallel capacitors of different values. Hard to accomplish with one capacitor, especially when it has leads. Easier to accomplish with chip caps directly bonded to the required conductors, but the SB-200 doesn't provide well for that.
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Grounding the grids increases the gain of the tube compared to ungrounded. I don't mean the tube itself increases in gain but the design allows for more gain to be obtained.
  3. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Thanks for the response --- the ground or not to ground the grids has always confounded me so please humor me on this concept.

  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Every "serious" GG amp in the world has directly grounded grids, generally via a very short path using a very wide conductor, and they're all more stable than any kind of bypassing, unless the amp is single-frequency and the bypassing is selected to be perfect at that frequency.

    I don't know the guy(s) who designed the SB-200 or the SB-220 so I can't ask why he didn't use the conventional method, but I suspect it was just to make metering easier.

    Every SB-220 I've come across I've modified for direct grid grounding using very short 1/2" wide copper straps and they all became more stable, with increased output on the higher bands, after doing that.
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    I used soft copper strip, like this:
    You can't get any shorter than this.
    Hardware is #4-40.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's a good way to do it, and I've done the same thing (also using wide copper straps, so short they have almost no inductance at all).

    Some big power grid tube sockets ground the grids right inside the sockets. Some RF power tetrode sockets bypass the screens right inside the sockets, allowing the tubes to operate at several hundred MHz, which would be nearly impossible with any sort of "outside" bypass caps.

    The higher the m.u.f. of a tube is, the more stable it is likely to be at lower frequencies; one reason 8877s are so popular and very stable at HF.
    WA7PRC likes this.
  7. K9AXN

    K9AXN Premium Subscriber QRZ Page

    The 8877 is a coaxial tube, the 572B and 811A are not. There's a huge difference in P to K capacity .1 and .7 respectively.

    My comments were in regard to the 572B or 811A which have a 2" lead from the grid to the socket. That represents .040uh and there is no form of direct grounding that changes it. The capacitor reduces the inductance. I was not suggesting that every GG amp use the capacitor on the grids just those using the 811A and 572B.

    The genesis of the design was with the 30S1 Warren Bruene and was copied to the 30L1 by Gene Senti. That design was later rolled out to a good number of developers using the 811A or 572B and some using the 3-500 ----- Kenwood, Heathkit, Ameritron, Yaesu, Drake etc. The RCA handbook also used the capacitor with four 811A's. All of those engineers could not have been wrong.

    The original intent was to improve linearity and IMD through negative feed back, Increase the drive level requirement to a more natural point for common transceivers, reduce the need for neutralization by reducing the gain of the tube.

    This information is for those who might want to know the genesis of the design, the qualifications of those who created it, how successful it was, and where to read an authoritative source. "Single Sideband Principles And Circuits --- Pappenfus, Bruene, Schoenike" and "Electronic and Radio Engineering Fourth edition ---Frederick Terman".

    I don't have a horse in this race, folks can do as they please, just need to provide reasoning and understanding so people can decide for themselves what to road to drive.

    My original question: Why does increasing the drive to achieve max plate and grid current with the capacitors installed not result in the same output as the direct ground using less drive? Or is it assumed the exciter does not have the output necessary to do so on 10 and 12Metres with the capacitors to ground?

    Regards Jim
  8. WB1E

    WB1E Ham Member QRZ Page

    Try to see this oscillation without the microphone connected. Simply push the xmit button in SSB mode. This helped me when I was using a diy microphone that was had been picking up RF, but only on 20m on a 4x572b amp. I also found another harmonic, the better one in the tuner, with much less narrowness & allot less added inductance. Very close as the 80m setting actually. A good star design ground is most important as is the ground connection. Anyway, with the stock mic, the 20m oscillation went away. I don't use that diy microphone anymore needless to say. Some copper strap coming off the top of the plate coil to "C1" variable is also recommended. Half inch wide, also the plate cap, both sides.

    Best & 73
  9. KS9Q

    KS9Q Ham Member QRZ Page

    Definitely appreciate the responses and discussions. I'm going to be indisposed for about a week and a half, so will be back to give updates once I have finished the rework on the amp.


    Jim K - KS9Q
  10. KS9Q

    KS9Q Ham Member QRZ Page

    Sorry for the delay in getting back on this. I did finish the rework of the amp and am much happier with the layout at the base of the tubes. I also replaced the bypass cap on the plate choke and retuned the input circuits. To review - the oscillation was visible on my monitor scope and on my power/SWR meter. It occurred any time that I a) had the amp output going through my tuner circuit (not bypassed), b) keyed the amp and c) was set up for 20 m or higher. It would disappear with the loading cap fully meshed and get start (and get worse) as the load control was increased. Sometimes it would only occur when the radio was in the transmit mode (but the amp also had to be keyed - for testing purposes I keyed the amp separately from the transmitter). The problem also occurred with multiple exciters - an Icom 7410, a 7200 and a Kenwood TS-520S.

    What ultimately seems to have solved the problem, however, is increasing the value of the grid bypass caps to 470 pF from the 220's that were in there. I know a hard ground would be preferable but with the amount of work that already went into this amp, I didn't feel like reworking the bias circuit. Now I can have my tuner in circuit at 2o meters and haven't seen any hint of oscillation. The output of the amp has also increased to right around 700W on the low end. As expected, power falls off a bit toward 10 meters. At any rate, just wanted to pass that along. Will post some pics of the rework when I get a chance.


    Jim K. - KS9Q
    N0TZU likes this.

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