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Thread: Tube amp death (auto bias-trouble)

  1. #1
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    Tube amp death (auto bias-trouble)

    So I recently bought one of the many Chinese made tube amps available on eBay figuring it would be a good and inexpensive way to get started in the world of tube amplification. One it arrived it worked fine for about three months, sadly there was an accident involving someone throwing a ball and that ball hitting on of the tubes on the amp shattering it. Luckily I had been meaning to replace the stock tubes with some electro-harmonix EL84s. Now, at some point between ordering the tubes and them arriving one of my room-mates turned on the amp, which was now running one tube short. Apparently, from what he told me, the amp suddenly stopped working and the room smelled like "burnt maple syrup". Turns out one of the fuses blew, presumably due to the missing tube, still not sure what the "Maple Syrup" thing was.

    Now comes the part that has been driving me insane. My new tubes arrived the other day so I replaced the fuse and installed the tubes. After turning on the amp the plates from the tubes on the right channel started glowing hot red. After a little research on the problem I found out that the most likely cause is that the auto-bias is no longer doing what it should and they are running way to low (or is it high?).

    Either way I would love to use this amp again, so is there anything I can do to fix this problem? I opened up the bottom of the amp and could not see any parts that looked burnt or cracked in any way. What exactly should I look for/replace and can this even be fixed? I would immensely appreciate any help.

    P.S. The model is a Yaqin MC-84L, there used to be a website with the schematics however it seems to be down. I will do my best to look them up and post them if needed.
    Last edited by ThatNaysayer; 09-17-2009 at 12:09 AM.

  2. #2
    Senior Member Enzo's Avatar
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    I doubt it died because a tube was missing. However, when the old tube was broken, it is hard to say what additional damage to the circuit may have been caused.
    Education is what you're left with after you have forgotten what you have learned.

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    Is there anything I can do to find out what else may have been damaged? Also, is there any reputable place I could bring the amp to be repaired close to Chicago (the closer the better, right downtown would be perfect).

    Edit: On a side note, the left channel works fine. Is it safe to just run this as a mono amp (no tubes on the right) until I am able to fix the problem? Also, I forgot to mention that the amp was off at the time the tube was broken.
    Last edited by ThatNaysayer; 09-17-2009 at 01:23 AM.

  4. #4
    Senior Member Enzo's Avatar
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    I am sure there is somewhere in Chicago that could work on it. It is a home stereo unit, but a guitar amp guy with any competence would be at home in it.

    I love the specs - I have not changed a thing:
    Specification:



    Adopt outstanding improve ultra to recommend work set the circuit free while being getting linear.
    Grade has enlarged the circuit for SRPP ago, have made the one machine loud and wide , distorted and little frequently.
    Output transformer is it import audio frequency special-purpose silicon steel sheet (0.35mm ) and high strength high-quality enameled wire and special to is it make craft make to wind to adopt.
    Machine this adopt foreign famous-brand special-purpose to Hifi grade electrolytic having polarity electric capacity and high-quality Third Ring Road money to hinder and make stereo,
    the main sound channel selects for use and imports the special-purpose volume electric potential device.
    This machine use grade gallbladder *7 of 12As not famous with behind grade use by gallbladder the EL84s and top-grade English walnut side board not wooden not real not famous.
    Input Power : 240V AVAILABLE ("110v MAY NEED 1 - 2 WEEKS FOR SPECIAL ORDER")
    output power?12W2?4?-8??
    harmonic distortion? SNR?>/78dB(A weight)
    frequency response?6Hz--60KHz?Mark measurement technique according to country ?
    Signal inputs level? load impedance?8? or 4?
    The consumption of power source ?Valve: EL844 12AX72
    Weight: approx 14kg


    All data is based on Manufactory figure. Subject to change without notice
    One of your tubes is red plating. This is not a rare thning to see. Your tube is either defective or it has lost its bias. There could be a damaged circuit component, or it could simply be a spread pin in the tube socket, bent when the 0ld tube was smashed.

    Take it to a tube amp tech, and I am sure he will be able to figure out what it needs.
    Education is what you're left with after you have forgotten what you have learned.

  5. #5
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    I especially love this line

    This machine use grade gallbladder *7 of 12As not famous with behind grade use by gallbladder the EL84s and top-grade English walnut side board not wooden not real not famous.
    Thanks for the info, I will check local guitar amp repair places.

  6. #6
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    If the output stages was a push pull cathode (ie auto) bias type, sharing a common cathode resistor of around 120 ohms, then if it was powered up with only 1 el84 there would be inadequate current to develope sufficient bias voltage across the 120 ohm resistor. This would likely cause the remaining el84 to red-plate.
    But now you've fitted 2 new good (tested?) el84s, the problem is still there. I'm stuck to work out what would be likely to cause that. Need a schematic.
    Until one can be found, best you can do is to swap the 2xel84 from the bad channel into the good channel to confirm that they're ok.
    Have you got a good multimeter?

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    Ahh.. The delight of randomly coming across mangled syntax and bizarre synthesized imagery. Synectic poetry at its finest.
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