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SB 220 Glitch Resistor

Discussion in 'Amateur Radio Amplifiers' started by K5EQ, May 7, 2021.

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  1. K5EQ

    K5EQ Premium Subscriber QRZ Page

    I've been reading through the forums for a few weeks now as I work on my project SB-220 and have noticed a few different type of glitch resistors used on the HV side. 20 ohm 20 watt, 10 ohm 10 watt and a few other combinations. I would like to add the glitch resistor to my amp but can only source a 20 ohm 12 watt resistor at the moment unless I go with a cement version which I have seen quite a few recommendations from folks advising against using for safety reasons. Seems like there are a lot of back ordered parts right now at Mouser and Digikey and I am not able to source one from them at the moment. Would that be OK to use the 20 ohm 12 watt for now until I get my 20 ohm 20 watt that's on back order until August?

    On another note here is what I have done so far -

    1. Replaced caps
    2. Replaced HV board (with one that come with the amp but needed to be installed)
    3. Bias mod (moving wires on the relay)
    4. New tubes
    5. Replaced the muffin fan with the factory fan
    6. Cleaned up some of the wiring

    Here is what's left -
    1. Glitch resistor
    2. Return suppressors back to stock (have the resistors on order and they were not out of stock thankfully)
    3. Fix the ungrounded RF output cable by replacing it with a longer run)
    4. QSK mod (Waiting on backordered parts)
    5. Repaint the cabinet

    So far it's coming out pretty good.

    image0 (1).jpeg image1 (1).jpeg image2.jpeg
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

  3. K5EQ

    K5EQ Premium Subscriber QRZ Page

  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It's more a matter of resistor construction than anything else, but 20 Ohms at 12W is pushing the dissipation rating of the resistor for any kind of "CCS" application (long tune-ups, RTTY, whatever) since Ip can easily be 800mA and at 20 Ohms the resistor would be dissipating 12.8W. I'd try to keep the resistor rated at ~ 2x the actual required dissipation, so at 20 Ohms this would be a 25W resistor. The 15 Ohm, 20W resistor I referenced would only be dissipating 9.8W in a resistor rated 20W, so meets the 2:1 derating and shouldn't get terribly hot even key-down for a couple minutes.
  5. K5EQ

    K5EQ Premium Subscriber QRZ Page

    Thank you very much for the explanation. I will order it.
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Max current would be ~ 800 mA. Pwr = I*I*R or Pwr = 0.64*15 = 9.6 Watts so for reliability one would choose around a 15-20 Watt resistor.

    Each person has his/her own circuit preferences but I prefer to use a 10 ohm resistor, so Pwr = 0.64*10 = 6.4 Watts so a 10 Watt 10 ohm ceramic/cement power resistor can be used which is physically smaller and allows a 40% safety margin.

  7. K5EQ

    K5EQ Premium Subscriber QRZ Page

    That's one thing I have really enjoyed with this project is learning the theory behind what certain things have been done. It's not always easy to find out the why sometimes though. Now if I can just grasp the whole negative current idea. :)
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Currents are confusing!
  9. KD9IQO

    KD9IQO Ham Member QRZ Page

  10. K2XT

    K2XT Premium Subscriber QRZ Page

    Good, because those things are weird! Have you transmitted with them in there?

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