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 conducted a test today that I haven't done before. I keyed the amplifier with no input RF from the exciter for 45 seconds into a 50 Ohm dummy load. In that time, the plates of the new, matching 572B tubes began to glow orange-ish red. For the duration of this test, the meter showed flatline relative power, 80mA of plate current, 0 mA grid current and 2100 volts HV.

    This makes little sense to me. I would assume that if something were wrong with the bias circuit, the plate current would be higher. And if the red plating was due to a mismatched load this test would not have yielded red plates because there was no RF being input.
  2. W1QJ

    W1QJ Ham Member QRZ Page

    The test you did seems normal. 80-90ma is normal for idle current. So no problem there. in this test the tubes would be dissipated 168 watts. At some point they would show the heat they are dissipating. I believe you have a normal situation there.
    KD2ACO and W1TRY like this.
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I've read this whole thread a few days back but don't remember if you've cleaned up and oiled the fan and ensured that it's giving you proper air flow through the HV cage.

    I had an SB-200 that was a great amp. It ran at a high temp until I cut the fan hole an inch bigger and put in a larger fan. Running with the stock fan used to make the tubes go to just barely blushing after a minute of keydown even idling. I didn't think much of it at the time other than to put a larger fan inside.

    I used a large diameter Pabst fan so it could turn slowly and run quietly but still push a bunch of air through to keep the cage walls cooler.

    The cooling mechanism of these tubes is radiation. The heat inside the tubes has to have somewhere to go, and that's to the walls of the HV cage. Making sure that there is an aerodynamically clean path for cool air to get inside to remove heat from the metal around the tube is essential for their well being. This may include vacuuming the holes in the case and cleaning the fan blades.

    Even after I did this to mine, it seemed like the fan was just stirring hot air around instead of providing a flow through the chassis to shed heat.
    Last edited: Jun 1, 2020
  4. K6LCM

    K6LCM XML Subscriber QRZ Page

    Thanks for the detailed reply. Glad to know you had a similar experience. I did install the Harbach fan modification which definitely pushes more air around. Whether it cools, I am not sure :).
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    My amp came with Cetrons and I put Taylor 572s in them to try the new tubes while following a thread on the subject here several years ago.

    It was just serendipity that the room was dark and I saw a slight glow inside the cage while testing out the new tubes. With the lights on, I would not have noticed the slight blush.

    I suspect that many of these amps run like this and that usually, the situation is such that it goes unnoticed.

    I unkeyed the amp and let it cool down for a while, then put the Cetrons back in and those tubes behaved the same way.

    The thing I took away from that episode is that I let any amp cool down for longer than I did before then... say about 10 minutes in standby before shutting it off after a good run.
    Last edited: Jun 1, 2020
  6. K6LCM

    K6LCM XML Subscriber QRZ Page


    I chased down a major issue that seems to have addressed most of the issues with this amplifier. The band select switch terminal for 10 meters was bent and not making contact with the disc. Upon fixing this, the amplifier now tunes much better and appears to be more stable. The signal on the scope on 80-20m is very clean and I am getting 600+ watts and the plates are staying cooler, for sure. The only remaining issue is on 10 meters where I am unable to get the amplifier to put out more than 100 watts. This seems to be a separate issue altogether.

    The match on the exciter input appears to be nowhere near 50 Ohms on 10m and my Kenwood TS-480 internal tuner cannot tune it to allow the exciter to put out full power. I know that 10m is shaky on the SB-200. Any ideas for improving it?

    I'm starting a new thread since this is a different enough issue.

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