SB-220 Capacitor Bank

Discussion in 'Amateur Radio Amplifiers' started by VK4TUX, Dec 5, 2011.

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  1. VK4TUX

    VK4TUX Ham Member QRZ Page

    I am in the process of virtually rebuilding a sb-220 I got in here recently.

    I replaced the Plate transformer(secondary had shorted to core on previous),
    rebuilt the metering board with robust components
    (5408's, 1.5kv rated R's, 5w R3 etc)

    I am looking at the optimum value for the eight capacitors comprising the HV capacitor bank?

    i am thinking about 450v 470uF each.

    I will be using a new matched pair of Matchlett 3-500zg's in this amp.

    Adrian ... vk4tux
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

  3. W9GB

    W9GB Ham Member QRZ Page

    Adrian -

    Jim, N6OTQ has already provided you with feedback on the usage of the Machlett 3-500Z glass RF tubes.

    Tom, W8JI has documented his testing of various 3-500Z tubes .....
    Tom added Machlett tube measurements in October 2011.

    You get what you pay for with the Chinese tubes -- QUALITY is an elusive target for Chinese mfg. (statistically challenged).
    The Chinese need a modern day W. Edwards Deming, who transformed Japanese mfg. after World War 2.

    Deming was a student of Walter Andrew Shewhart,
    the Father of Statistical Quality Control (Shewhart started at Western Electric Hawthorne works, later moving to Bell Labs)
  4. VK4TUX

    VK4TUX Ham Member QRZ Page

    George, thanks, Harbach does ok there, I had a few issues with component size and value on their metering board.
    I use between 4 to 6 5408's (bsd) in the bias string, currently 5 in this amp, I use 4 in the other I use running virtually class A on two RFP 3-500zg's that colour up ok on the anodes at our lower allowed outputs.

    I note here that the local 235v-240v street 50hz street supply provides between 2.7kv to 3kv max on the plate.

    I get around ~180mA with 5 bsd, and ~220ma with 4 bsd.

    The 5408 meter protection diodes are a nice touch on the Harbach board, but they always short in a glitch situation, which is good for the meters, but I use 6A10's here at the meters themselves and dont see them short in glitch events whilst protecting the meters very well.

    I have the surge current covered and limited in this amp, so that is not an issue..

    There must be a saturation point where the stock sb-220 Plate transformer 0.9a ccs rating matches, and going beyond that point has no benefit unless the transformer is upgraded to a PT-220 or beyond.

    Also the tubes are Machlett (corr). The service and professional packing from K5SVC is 2nd to none. They test as good as the RF parts ver here, but less expensive.

    Adrian ... vk4tux
  5. VK4TUX

    VK4TUX Ham Member QRZ Page

    Gregory , What was you recommendation on the capacitor size? I missed it in there somewhere.

    Adrian ... vk4tux
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    IIRC, I used 330µF when I recapped my SB-220. That was BH (Before Harbach). I had to actually research what would fit, and design my own "soft key" and "soft start" circuits. Carl/KM1H likes 330µF too:
  7. W8JI

    W8JI Ham Member QRZ Page

    The SB220 has plenty of capacitance, even for 50 Hz. I would not upsize much, if any, because it won't do much to make things better for hum or regulation, and it can make things like surge current and transformer heating a little worse.

    Nearly all of the voltage sag relates to the transformer ESR, and the fact it is a doubler.

    The secondary to core, or secondary to primary failure, is common in the SB220. For a short period of time Ameritron switched to the same vendor Heath used, and had similar failures. It was inadequate insulation from that vendor. When we went back to Schumacker (sp?) Electric, all the problems went away.

    73 Tom
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Then, that's a good deal because they all come from the same place in China. I've had that confirmed about ten times, now. Machlett-RF Parts-et al., all the same factory. Machlett is just a name they place on the tube and carton, but they're all Chinese, so go for the best deal.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd shoot for lower as I doubt it has any real effect on linearity. Maybe 6 diodes would cause less heating, a lower electric bill, and no real change in the way the amp works.:eek:
  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    I use 220 mfd at 450 volts, 105 degree electrolytic capacitors in amplifiers like the SB-200, SB-220, etc. These are available from Mouser ( ) for very good prices. I also use them in things like the Heath HP-23- series, etc. For my single 4CX250B 432 MHz amplifier (salvaged from a Motorola "B" 100 watt FM base station), I have just built a new HV power supply and use them in this as well.

    Glen, K9STH
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