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

    I bought a Heathkit SB-200 linear from a local University radio club a couple of years ago. It was my first amplifier and I finally got around to getting it on the air last month. It is in decent physical shape and tuned right up (tuned for peak power). It seems to put out the full 600 watts on all bands according to my external antenna tuner power meter.

    When setting the SB-200 up initially, I accidentally plugged in my Kenwood TS-480 exciter output into the ANT RELAY port of the SB-200 instead of the RF INPUT port and transmitted for a bit. I saw some smoke and checked the internals of the linear. I found that the R16 resistor was physically fried but still Ohm-ing out at proper value. I swapped it anyway and began using it. This time with the correct wiring :)

    There were a few non-standard mods done to the rig like a replacement antenna relay and a quasi-softkey that lowered current but not voltage of that circuit. I was still using an external relay box to interface with my Kenwood TS-480 just to be safe. I also added diodes across the internal meter to try to protect it (for whatever that is worth).

    After using it for a couple of months I have noticed that CW keying for more than 10-12 seconds (for tuning) and during prolonged SSB transmissions, I am getting red plating on one of the tubes in the unit.

    Here’s what I have tried to diagnose and solve the issue running into a dummy load and testing primarily on 40m using mostly lower power output from the exciter (20w):

    1. I have swapped the position of the 572B tubes but the particular tube that glows red will do so in the left or right socket.

    2. Thinking that maybe I have a bad tube, I have tried carefully running just the one "good" tube solo at about 20 watts exciter power but then that one started red plating. So it appears that when they are run together, the plate of the more powerful tube of the pair turns red. However, either tube will red plate when run solo.

    3. Thinking maybe these old tubes were just tired, I have tried swapping in new 572B Taylor tubes from RF Parts. Even with two brand new tubes, one tube glows as before.

    4. I thought perhaps some mods made by the radio club were causing a bias circuit issue so I removed the home-brew softkey and replacement relay mods restoring both circuits to original configuration per the manual. I found a used original Heathkit antenna relay and swapped it in since the coil resistance actually matters and is part of the bias circuit on this rig.

    5. I changed out the parasitic suppressors to new ones.

    6. I changed out the factory fan to a higher flow fan from Harbach Electronics which moves much more air over the tubes for cooling.

    Even after all of this, I am still getting a cherry red glow from the plate of one of the tubes. Looking for any help.
  2. G0JUR

    G0JUR Ham Member QRZ Page

    1 tube is working a little harder than the other (the red one) this happens a lot even with matched tubes. sometimes swapping positions cures it. usually the cause is slight differencesin the circuit construction, so leaving them unbalanced
    Good info here.
    Last edited: May 23, 2020
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    What is the plate voltage and current when keyed with no signal?
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd measure the grid resistors R21 and R22 to make certain they're equal in value. I'd also check for any shorts or opens in the associated grid bypass capacitors while I was at it.

    If one of those two grid resistors has gone open circuit or very high in value the bias on the two tubes will be different which could lead to what you're seeing. That doesn't explain why your problem seems to track the tube, not the socket even with new tubes but I'd at least verify that the bias circuitry is the same between the two sockets.
  5. K6LCM

    K6LCM XML Subscriber QRZ Page

    Plate current reads at about 60 mA and HV is 2100 volts.
  6. K6LCM

    K6LCM XML Subscriber QRZ Page

    I should have mentioned that I checked those two resistors also. Both tested within spec for 33 Ω.
    K7TRF likes this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'm not home to look this up easily, but 2100V sounds low to me. Either something's wrong (I think) or the metering resistor(s) have drifted high in value.

    It's been 50 years since I built an SB-200 but I recall the HV is substantially higher than that, up in the 2.5 kV area.

    Agreed one tube glowing red and the pair still providing 600W output sounds like the "red" tube is working much harder than the other one. In my old SB-200, under normal condx neither tube ever had a red anode.

    I'd keep looking for a problem.
  8. K9STH

    K9STH Ham Member QRZ Page

    First of all, don't run the SB-200 at 600-watts output on all bands. Full output on 80, 40, and 20-meters is fine. However, for maximum tube life, run closer to 500-watts output on 15-meters and 450-watts output on 10-meters. The station, on the other end, won't know the difference but the tubes will definitely know it!

    Try the original non-glowing tube with the non-glowing new tube and see if you still have a problem. If that works, then get a warranty replacement for the new 572B that does glow.

    Glen, K9STH
    K4AGO likes this.
  9. K6LCM

    K6LCM XML Subscriber QRZ Page

    Some more data:

    I measured the bias voltage this morning and it was -128 VDC unkeyed and -1.9 VDC keyed. According to the manual, -2 VDC is normal for keyed voltage. I can't find where it should be unkeyed but I think that it correct. I also checked R15 which was within spec for 1.5 Ω.
  10. K6LCM

    K6LCM XML Subscriber QRZ Page

    For reference, here is the schematic.

    Attached Files:

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