Yaqin MC-10L bias voltage too low

Discussion in 'Tube Audio' started by 20minc, Oct 11, 2017.

  1. 20minc

    20minc New Member

    Messages:
    10
    Hello.
    I bought used Yaqin MC-10L.
    When I check bias voltage it`s only 0.08v.
    It should be between 0.35v-0.4v.
    Amp sound is good I think.
    Should I adjust it?
    What kind tool should I use?
     
  2. datafone

    datafone Super Member

    Messages:
    2,229
    Location:
    Bris', Australia
    Yes adjust to the recommended, I suggest somewhere between between 0.25vdc and 0.40vdc depending on what you like better, lower will stress the tubes less and give a more relaxed presentation, while higher will give a tighter presentation.

    The driver used is Torx type of suitable size, insulate the shaft with shrink wrap.
    [​IMG]

    The bias pot in these can be very touchy, so just slight pressure on turning, sometimes just a fraction of 1mm will move the bias amount very noticeably.
    Edit: If you get a set of torx drivers one will fit snug, and don't let the tip wander around in there, look for the bias pot before inserting so you only contact it with the tip
     
    Last edited: Oct 12, 2017
  3. run_around

    run_around AK Subscriber Subscriber

    Messages:
    92
    Location:
    Upper Valley, NH
    I have the 13s which I think I have the newer version of the 10L and you will definitely want at least 0.35 on EL34's.

    I have reissue Tung Sol EL34's that seem to sound best at the higher end 0.38-0.40 on the bias.

    If you get a chance, you might try the JJ KT77's. I'm currently using a set adjusted to around 0.45 and think they are best sounding tubes I've used in this amp.
     
  4. 20minc

    20minc New Member

    Messages:
    10
    Thank you for reply.
    If I adjust bias to 0.35v-0.40v sound will improve?
     
  5. datafone

    datafone Super Member

    Messages:
    2,229
    Location:
    Bris', Australia
    You are the one to judge what sounds best to you.

    Just go for somewhere in the recommended range, that is why It's a range and not a single figure. Everything else is just what an individual prefers in his system with his preferences.
    Edit: and example is i know some that prefer a setting of 0.25vdc, some prefer a setting of around 0.30vdc and so on, you might prefer 0.335vdc, only you will know.
     
    Last edited: Oct 12, 2017
  6. datafone

    datafone Super Member

    Messages:
    2,229
    Location:
    Bris', Australia
    Hi ya'
    Was just thinking and thought to ask you something if I may?

    When you measured the bias, did you measure each of all 4 output tubes? Where they all reading 0.08vdc?

    The reason i ask is that it would be very strange if it had been set correctly and drifted down the same on all 4 tubes. So if they measured all the same for you I would think they set them at this level before selling, then I would wonder why so low as It's not a logical level to bias EL34's at. It makes me wonder if they set it at those levels for a reason and I can only think of bad reasons to do this, maybe it wouldn't set higher for some reason...worn tubes maybe or wouldn't run stable at higher settings :confused:

    BUT, before we look at that, you were measuring correctly weren't you, 20minc?

    The meter works correctly? and these were DC voltage measurements?

    Sorry for the questions but I think they need to be asked in order for you to proceed at this point.
     
  7. 20minc

    20minc New Member

    Messages:
    10
    Yes, I did measure each of all 4 output tubes and it`s all 0.08vdc.
    I thought too somebody set it to 0.08v because it`s all same reading.
    I did use 3 different DMM and read all same.
    Can amp works with that low bias voltage?
     
  8. datafone

    datafone Super Member

    Messages:
    2,229
    Location:
    Bris', Australia
    Not sure. To be honest I have never tried to run something like that at such low levels.

    Maybe someone else will chime in with an opinion on this.
     
  9. primosounds

    primosounds AK Subscriber Subscriber

    Messages:
    1,363
    I thought if the tubes are drawing that much current they would be in "cut off" or almost there . That is the bias voltage is too low (too much negative voltage) to allow the el34 to conduct, with the grid blocking the electrons from moving to the plate. The sound quality would be much improved if you put the right bias voltage on the tubes. It maybe that at the correct voltage the tubes were red plating hence the previous user turned down the bias to allow the tubes to operate safely. So watch your el34 tubes carefully when you adjust the bias to its proper level.
     
    datafone likes this.
  10. whoaru99

    whoaru99 Epic Member

    Messages:
    38,628
    Location:
    LoTL
    That far different first thing I think of is, measurement performed correctly?
     
  11. for_p1

    for_p1 Addicted Member

    Messages:
    5,354
    Location:
    NJ
    Amp can work even with ZERO bias. Sound quality will suck though.
     

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