Tube Amp repair help: Red plating after bias

Discussion in 'Amps and Cabs [BG]' started by pebo, Jan 23, 2019.

  1. Hi All,

    I've got a 1976 Eminar PA amplifier I use as a bass amp, teamed with a 15inch ashdown cab. It's sounds great... when its running. Power tubes are 2 x 6L6GC.

    The issue im having is the amp had a period of about 10 years where it was rapped in plastic and stored, before I got it. I've since replaced the electrolytic caps and slowly got it back up and running by limiting the current. I biased it using the internal pot to about 30mA.

    It ran like a dream, no buzz or hiss, for about 2 months then one tube started red plating. Checked all components for anything obvious, re-biased then it ran again for about a month with no issues. Started red plating again last night so I shut it down straight away.

    Has anyone experienced an amp with a fault like this, if so what should I be checking. Due to it being an Australian made amp there are no schematics I can find so im flying blind.

  2. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    What happens if you swap the two output tubes? Does the same tube red plate when it's moved to the other socket? Or does one socket cause the tube that's in it to red plate?
    Wasnex likes this.
  3. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    What is the grid voltage on each power tube?
  4. beans-on-toast


    Aug 7, 2008
    It could be electrical, or the problem could be mechanical.

    There is a chance that the bias pot is changing when you move (bounce around) the amp. Could also be that the pot is bad. Note if it is erratic or overly sensitive when setting the bias.

    Pots have a mechanical slop in them, some more than others. When adjusting the pot, turn it down counter clockwise past the desired bias voltage, then turn up to the bias point. If you pass that point, turn it down again and come up to the desired bias voltage. See if that helps.
  5. If it was the bias pot, both tubes should be glowing red, unless it's a balance pot.
    agedhorse likes this.
  6. Wasnex


    Dec 25, 2011
    The tubes or the circuit may not be balanced very well. It's pretty normal for the tubes on one side of the output transformer to run a bit hotter than tubes on the other side. Course I would be curious what level of current produces red plating...most likely a lot more than 30mA.

    If one of the tubes is gassy it can pull the bias down and cause both tubes to eventually go into thermal runaway.

    I have one amp that has a huge amount of resistance in the bias and grid circuit. I had to install a fan to actively cool the tubes. After about 20 minutes, the tubes would heat up enough that reverse current flowing from the control grids would start pulling the bias down. The amp would go into full thermal runaway within about 45 minutes.

    The offending amp uses only one transformer tap for all operating voltages. The plate supply is around 550V and the the amp uses a full wave rectifier instead of a bridge. I believe this puts the AC up around 775V. This same 775V is used to develop approximately -64V for the bias supply via a half wave rectifier....a lot of resistance is used to drop the voltage any reverse grid current passing through the bias network gets magnified by the huge resistance. The resultant voltage drop opposes the bias voltage setting up a feedback loop that continuously biases the amp hotter until the tubes runaway. This is obviously a design flaw, but the amp has worked fine since I installed the fans. Turn the fans off and the thermal runaway begins to ramp up alost immediately.

    Regarding the OP's amp. I would pull the output tubes and check all the operating voltages. If everything is good put the tubes back in, set the bias, and monitor what is happening. It's normal for the tube current to drift a bit as the tubes warm up. Check the current and reset to 30mA every 15 minutes for an hour. If the tube is gassy but has a good vacuum and good getter...maybe the getter can fix the problem. If the tube red plates, it could be informative to switch the output tubes to see if the problem follows the tube.

    If the current keeps increasing and eventually you run out of bias, it's possible the tubes or bad, there is too much resistance in the bias or grid network, or there is some other problem such as leaky coupling caps between the PI and the bias network. Could probably even be dirty tube sockets. A bit of carbon on the control grid contact could increase the resistance enough to cause a problem.
  7. Thanks for all the suggestions so far everyone. I'll measure the grid voltages again, swap the tubes around to see if it's the same socket offending.

    I think Beans-on-toast might be onto something with the pot not being stable. I remember it being very touchy.
    Wasnex likes this.
  8. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    I've had half an output section redplate from a driver/PI valve shorting one half.
    rufus.K likes this.
  9. BassmanPaul

    BassmanPaul Inactive

    It could also involve a leaky coupling cap.
    Bflat likes this.
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Hence measuring voltages.
    BassmanPaul likes this.
  11. Primary

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    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

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