Allen Encore power problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by moosie, Feb 28, 2015.

  1. moosie

    moosie Member

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    Cross-post from TDPRI.

    I built an Encore a year ago, and it's been working well, until a month ago when it developed an intermittent problem with the power supply... maybe?

    I normally run 6L6/5AR4, but I swapped those out thinking maybe they were damaged. Either causing the problem, or victims. Victims, turns out, because the problem persists with the 6V6/5U4GB configuration I'm running now.

    Yesterday, I had it on the bench, the tubes were nicely biased to 24mv, and the amp was behaving itself. I turned the amp completely off, and had lunch. Then, I came back, turned the amp on, waited, then switched it off standby.

    Bingo.

    I had the meter on the bias points. Both tubes read around 180-200mv. I had a guitar already plugged in, and confirmed I had sound -- one quiet strum, sounded okay. The tubes started to red plate, so I switched to standby. When I did that, I saw a big flash inside the rectifier.

    Upon inspection, the rectifier has tiny flakes of what look like glass loose inside.

    This same thing happened with the 6L6/5AR4 setup, except I wasn't watching it so closely when it happened.

    At this point I'm assuming the rectifier is damaged, and the tubes to some degree as well.

    Can someone point me in the right direction? If you're not familiar with the Encore, in many ways it's very similar to AB763. If you want to describe it in those terms, I can probably translate any differences.

    I'm still learning. If you say something like "bias supply", I'll need to ask where that is on the schematic.

    Thanks.
     
  2. Blue Strat

    Blue Strat Member

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    Bias supply! With only the rectifier installed (no power tubes) see what voltage readings you're getting on pin 5 of both power tube sockets. For 6L6s it should be around -48, 6V6...around -36. Turning the bias pot should make these readings change.

    While you're at it, replace your meter battery before you start.
     
    Last edited: Feb 28, 2015
  3. moosie

    moosie Member

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    Readings are as expected. But it's an intermittent problem. Maybe it's not acting up at the moment...? I could tell if power tubes redplated... but I wasn't using any...

    What next? And is there a bigger picture diagnostics flowchart so I can work on this myself?

    Thanks.
     
  4. tele_player

    tele_player Member

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    Bias supply is still your best suspect. It's just a few parts, double check each connection, reflow solder.
    Without power tubes, meter the bias voltage while turning bias pot, watch if bias voltage ever goes to 0.
    Maybe just replace bias pot...
     
  5. moosie

    moosie Member

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    That's helpful, thanks.

    David's kits are great. The instructions are so good, I was able to assemble it with no issues, and it ran cleanly for a year. The side effect of that was that I didn't have to actually understand what I was building. So now I'm learning. This stuff comes slow for me, so it may take some time, but I'll post my results once I figure it out.
     

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