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Redplating on one tube

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  • #1

    Redplating on one tube

    More woes..!

    http://www.chambonino.com/carlsbro/carlcs100tc.html

    I heard a pop and then some hum, and the 3rd tube started redplating.

    If I turn the amp on, it doesn't redplate until I play a few chords.

    Could it be the 220k (carbon-comp) resistor that's causing it? I suspect it as the culprit.

    Other than that, what are the usual things that cause this?

    Any help is much appreciated as ever!
    Tags: None
  • #2
    Yes, check values with meter for those screens, Take the tubes out and turn it on, and check voltages at the pins, especially the bias voltage and plate voltages. compare them against the schematic, should be fairly close. a resistor or diode could have drifted or failed in the bias circuit. check around those 220's & 10k's, bias circuit cap as well, especially if voltages are off. Then on your final test after you've verified good voltages and nothing looks burned up put in the tubes and you may want to use fresh tubes in case that one is cooked already

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    • #3
      Originally posted by harry View Post
      Other than that, what are the usual things that cause this?
      A bad tube? Try switching the tubes around and see if the same tube still red-plates or if the same socket still red-plates.

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      • #4
        It's the same socket each time..

        Wouldn't the bias supply make all the tubes redplate, or a pair of them? I've replaced the 220k, still the same... 10k checks out alright on a meter.

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        • #5
          Also, seeing as it seems to wait until I've been playing for a couple of minutes to redplate, would a meter show up any incorrect resistances?

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          • #6
            Yes but weak tubes would redplate first (or more easily). With the amps that I've troubleshooted for redplating, I've encountered quite often the tubes biased too hot and there was one or two weak tube replating. If that wasn't the problem I just take voltages from the power tubes and work backwards, checking bias, and pinpointing the abnormality (unstable voltages usually for redplating). If I can't find it in the bias supply i will work from the tubes back to the PI checking voltages. This helps pinpoint the problem. Can't go around replacing every resistor!

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            • #7
              Don't forget that if the 0.1 coupling cap leaks any positive dc voltage it will kill the bias to that tube socket as well. Also check the socket itself.

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              • #8
                My mistake, it was a tube. I'd somehow managed to put it back in the same socket.

                Annoying though, I've only had the tubes about a month!

                Thanks for your help!

                Harry

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                • #9
                  Good idea, once you've had a tube redplate, is to check the screen resistor for that socket; also all pin voltages.
                  -Erik
                  Euthymia Electronics
                  Alameda, CA USA
                  Sanborn Farallon Amplifier

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                  • #10
                    If the screen resistors are what I think they are (the huge wirewound things), they look alright. Do they cause problems sometimes? These look indestructable!

                    Actually I'm still getting some crackling/rustling when I turn the amp on. I have no idea what that is.

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