Heathkit SB-200 Red Plating Tubes

Discussion in 'Amateur Radio Amplifiers' started by K6LCM, May 23, 2020.

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  1. K6LCM

    K6LCM XML Subscriber QRZ Page

    Hmmm... LOAD at 1 (fully CCW) is happening on 80 and 40. I believe on 20 and 10 I can add some LOAD. Looking at the two variable capacitors that make up the LOAD control, I don't see anything obviously wrong. The condition of the plates looks good. All of the solder connections look good on the underside and there is continuity between the plates of the LOAD capacitor and the other capacitors. Perhaps this is a symptom of something wrong elsewhere in the circuit and not at the LOAD capacitor? I do see the padder and you are indeed correct, it is only engaged on 80m.
    Last edited: May 27, 2020
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Have you checked your Dummy Load hasn't gone low in resistance?

    That would cause the symptoms you describe, ie needing the Load Capacitor fully meshed to get full output.

    Roger G3YRO
    K6LCM likes this.
  3. K6LCM

    K6LCM XML Subscriber QRZ Page

    The LOADing (or lack thereof) is happening on the dummy load and a matched antenna.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Maybe I missed it in a previous post but how much power are you driving into the amp when you have to run the load capacitor fully meshed? Have you tried driving it with a full 100 watts?
  5. K6LCM

    K6LCM XML Subscriber QRZ Page

    Normally, I don't run the amp at full power (100 watts) on 80-40 because I can get 600 watts out with 60-65 watts from the exciter.

    I tried just now running it up to 100 watts in on both 80 and 40 but I was unable to increase LOAD without reducing output. However ...

    On 20m, however, I increase LOAD to 4 to achieve max power with 100 watts in.
    On 15m, max power is achieved just before LOAD reaches 5 with 100 watts in.
    On 10m, max power is achieved with LOAD set on 5 with 100 watts in.

    So it seems the issue with LOADing is only on the lower two bands: 80-40. Are there some capacitors that are part of the tuning circuits for 80 and 40 other than the big variable LOAD capacitor that could be out of spec and causing an issue on only these two bands?
  6. W1QJ

    W1QJ Ham Member QRZ Page

    The sb200 at full bore into a dummy load should have the load control opened somewhat. You need to check that 500pf padding cap and see if it’s ok and if it is getting switched in. Sounds like it’s not doing anything. But on 40 it’s no in play anyway so that is puzzling. Does anything look modified ?
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    There is a capacitor (C27) switched in to add loading capacitance on 80m but only on that band:

  8. AI3V

    AI3V Ham Member QRZ Page

    3 pages in....

    Glad you got a pair of tubes with the same no signal current, and turning red together.

    That is what a "matched pair" should do.

    You may want to ask the outfit that sold you the "matched pair" that was not matched just how they "match" them, and why you paid a premium for unmatched tubes.

    Having said that,

    When you apply nominal drive, and rotate the plate control, what plate current are you seeing at the resonance dip.

    If you are simply attempting to tune for maximum output power without regard to a specific current at the resonance dip, that's a problem.

  9. K6LCM

    K6LCM XML Subscriber QRZ Page

    The plate current is almost always below 500 mA when TUNEd for max power. Tuning into a dummy load, I can LOAD it and I will only load it such that plate current is below 500 mA. If in that safe range, I am still getting red plates after a sustained SSB transmission at 500 watts output power.
  10. AI3V

    AI3V Ham Member QRZ Page

    Start tuning for a dip in plate current instead of maximum output.

    In three pages you havn't figured out your DC power input, subtracted the RF output, and compared that number to the rated plate dissapation of your tubes.

    Also, how much "processing" are you using, that is to say, what is the average power?

    You are "red plating" OVER LOADING IN THE SENSE OF TOO MUCH HEAT your finals.

    The original problem of only ONE tube getting hotter than the other was caused by somebody selling you random tubes instead of the "matched pair" you paid for.


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