Heathkit SB-200 Red Plating Tubes

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

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  1. W1QJ

    W1QJ Ham Member QRZ Page

    I would try removing ( temporarily) C29 the 5pf cap at the grid of that one tube to see if that stops the problem
  2. AI3V

    AI3V Ham Member QRZ Page

    What is the no signal plate current of each tube, when you test the tubes one at a time?

    Run the pair that "matches" the closest.


    In other words, if the tubes run, for example,

    Tube#1 50ma
    Tube#2 75ma
    Tube#3 75 ma
    Tube#4 100ma

    Run tubes #2 and 3
  3. K6LCM

    K6LCM XML Subscriber QRZ Page

    W1QJ, I found and disconnected C29 and ran it up again with 20 watts from the exciter CW. I tuned for max output on 40 meters. I am still seeing some red plating after 10-15 seconds on one tube even with this capacitor disconnected.
  4. K6LCM

    K6LCM XML Subscriber QRZ Page

    I spent a few hours troubleshooting this evening. I was able to find a pair of tubes that were close matches. With those two tubes together both plates begin to turn red after about 15 seconds CW key down with 20 watts drive from the exciter.
  5. W1QJ

    W1QJ Ham Member QRZ Page

    Levi, 20 watts drive is actually low and the power output you are getting with only 20 watts isn’t in the “power band” of the design parameters of the tank circuit. You actually may have more plate dissipation at 20 watts of drive than you do at say 60 watts of drive. You might try running the drive up to 60 watts and make sure you are tuning for maximum output. Use the relative power meter option or look at a separate watt meter and tune for maximum output at 60 watts. You’ll be in the absolute area of the power band of the amp. Once doing so now check to see if the tubes get hot as quickly. With minimum plate dissipation the tubes should run cool in the short term. You must also be certain you are tuning correctly by tuning for maximum output but not with more drive as to send the grid current out of the White zone on the meter. Remember, running at reduced power doesn’t necessarily mean you are at a low dissipation point .
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    With only 20W drive, is the LOAD control still within its tuning range when peaked for max output power?

    Usually with such low drives, many amplifiers will have their LOAD control fully meshed and unable to actually tune through a peak. This is what Lou was describing as "more dissipation," which is less efficiency and can cause more heating than running at higher power where the LOAD control will be "unmeshed" and able to tune through a peak.
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Exactly Steve. Thanks for that clarification. That’s what I’m talking about.
  8. K6LCM

    K6LCM XML Subscriber QRZ Page

    I don't know if this may be part of the issue but on 80-40m at any power level, I don't really have to move the LOAD knob at all off of 1 (fully CCW). If I try to increase LOAD, I get a dip in power output. Is that normal? Should I be able to load it more and see a rise in output power once TUNE is peaked?

    Also, I measured bias voltage. It was -128 VDC unkeyed and -1.9 VDC keyed.
    Last edited: May 26, 2020
  9. G3YRO

    G3YRO Ham Member QRZ Page

    You transmit carrier at full power for 10 - 12 seconds ?!!

    My goodness, I would never key down for more than 2 seconds . . .

    If you want the 572Bs to last a long time, just tune in short 2 second bursts.

    (I have always done this with EVERY Valve PA or Linear Amplifier)

    Roger G3YRO
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The bias seems right.

    No, the LOAD control behavior is not normal except for if the amp is underdriven. When driven fully with ~100W, the LOAD control when using a decent load (dummy load or matched antenna) should not be fully CCW.

    On 80m, the SB-200 adds a "padder" (fixed value) capacitor in parallel with the variable LOAD cap and if that capacitor has failed (open circuit) then the LOADing won't work on 80m, but I don't think that's in the circuit on 40m where the parallel-ganged LOAD cap has enough capacitance on its own.

    If you can't tune LOADing for an output power peak so the only adjustment of that control reduces output power and cannot peak it, that's a problem.

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