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Sunn Model T reissue thudding

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  • #1

    Sunn Model T reissue thudding

    So I haven't been doing much amp work lately, 5 month old baby needs to nap, but I did take on a Sunn Model T reissue to change out power tubes.

    So it had one dead 6l6gc when I got it so I just put in a new set and biased them up, 30mA per tube, and played the amp loud for about 20 min.
    The next time I fired it up I get a constant thud, thud thud from the speaker and I figured I had some bad caps allowing the amp to motor boat.

    Well after removing the board, not fun, and changing all the main electrolytic caps I still have the thudding at the loud speaker. The odd thing I have noted is that in the "Club" mode i get a combined 44 mA through one pair of tubes and around 60 mA through the other pair. I have to check my boas voltage and see if I need to get some more negative voltage as I should have only 60 mA per pair in the "Arena" mode.

    Any advice or ideas of were to head with this problem would be appreciated.
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  • #2
    Ok so tubes out i confirmed my voltages are all ok. No oscillation with no tubes/ current draw.

    Hooked up my scope to the output and I have what looks like a low frequency oscillation. 1.6hz to be precise. this is very strange. I installed only 2 power tubes, and only 2 running into a corrected load (4 ohm switch position, 8 ohm load).

    Swapped to the other two tubes and the oscillation is gone. bad tube, go figure.

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    • #3
      Thanks for the follow up! It's nice to see this sort of thing played out. I just read this and was thinking "bad ground connection" when you re-posted about the bad tube. I probably wouldn't have expected a bad tube to cause a motor boat inducing phase shift, but now I know to consider that
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

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      • #4
        Agree, great courtesy

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        • #5
          I wish I had taken a screen shot of the occilation on my scope. it was a very weird charge and discharge, kinda reminded my of scoping secondary ignition on a car ...
          i was told the tube had an over current fault, though I'm not sure how, I never even got it up to proper bias current. But I got it replaced under warranty so all good, and a happy customer.

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