Newbie Bias A Marshall Silver Jubilee Combo (2554)

Discussion in 'The Workbench' started by TubeAmp, Mar 31, 2018.

  1. TubeAmp

    TubeAmp New Member

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    Hello!

    So I measured the plate voltage to 450 V. According to my calculations, I should aim for a plate current of about 41 mA.

    So I mount my bias probe, mount a EHX EL34 tube on top, mount the other EHX EL34 and EHX 12AX7, set my multimeter on 200m DCA, connect speaker/dummy load, turn power and wait for 15 minutes.

    When I adjusting the bias, I end up in a range of about 5-14 mA, and I'm aiming for 41 mA. I try both high end low output. What am I doing wrong?

    Thank you in advance.
     
  2. pedecamp

    pedecamp Well-Known Member

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  3. TubeAmp

    TubeAmp New Member

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    Before I got started, I did research including the exact link you posted. I reread it and I believe it sates all the steps in my first post.

    It's a good answer if I asked how to bias a tube amp in general, but now I've already followed a generic guide and here I asked a specific question.

    Why do you think my plate current measurement range is so low (after completing the steps of the guide you've linked)?
     
  4. pedecamp

    pedecamp Well-Known Member

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    The way you described the steps in your OP I wasn't really following what you were saying so I posted those general links. It sounds like you have your multimeter set right, so all you gotta do is adjust the amp bias pot to get your desired bias. Are you maxing out at 14mA? You cant turn the bias pot further to 41mA? If this is the case send the tubes back, there might be something wrong with them, I've seen this happen to others before.
     
  5. TubeAmp

    TubeAmp New Member

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    Yes, it maxes out at 14 mA. Two things that I noticed:

    • It maxed out at about 14 mA both with the old tubes and the new tubes.
    • When playing, the tubes sounds great.
    These two reasons makes me think it's unlikely there's something wrong with the tubes. So it feels like I'm just missing something.
     
  6. pedecamp

    pedecamp Well-Known Member

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    If its a chronic problem no matter what tubes you put in it and you know youre doing it right then the amp needs a resistor change to fix it. A tech can fix this very quickly, easily and cheaply. Lets wait and get some more opinions from some of the techy guys heres, I've heard this problem here many times and this is the fix.
     
  7. charveldan

    charveldan Well-Known Member

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    Your bias should be .38 {not 41}.

    And that .38ma Negative grid voltage not plate voltage.
     
    Last edited: Apr 1, 2018
  8. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    What is your negative voltage reading at pins 5?
     
  9. TubeAmp

    TubeAmp New Member

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    Thanks for the input.

    Do you mean the 1 Ohm resistor which makes it possible to measure the plate current without a bias probe? Like in this video:



    Is my plate current supposed to be 0.38 mA? How did you calculate that? Why is your suggestion so different from post #5 in this thread?:

    http://www.marshallforum.com/threads/bias-a-jubilee.23113/

    I don't understand your last sentence.

    Could I ask what your followup suggestion would be? My amp is currently put together again and I'm waiting for new fuses to my multimeter (hehe).
     
  10. charveldan

    charveldan Well-Known Member

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    [​IMG]

    g1 [pin 5] is your negetive bias {control grid} measured in Milliamps. I don't recommend poking around Hot tube sockets if you're a Novice. Bias meters are a much better way to go.
    The Formula i & many use which is an Industry standard is :

    Wattage of tube in this case EL-34 {25 Watts} > Divided by > Plate Voltage X .7 {70% dissipation}.
    Or 25 divided by 450 = .055 X .7 = .038 or {38} ma.
     
    Last edited: Apr 2, 2018
  11. mickeydg5

    mickeydg5 Well-Known Member

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    No one asked what brand and model bias probe?

    TubeAmp, please provide the information so everyone knows for sure what is being used.

    With 450VDC on the plates the Jubilee will sound great in the 41mA/mV setting area. If you like to play it maxed a lot then opt for the 38mA/mV area. Do listen to its final sound though and chose what is best from that particular set of tubes and for you.
     
  12. pedecamp

    pedecamp Well-Known Member

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    Im no tech so I dont know, I've heard a hundred people post here with the same problem and that has always been the fix.
     
  13. mickeydg5

    mickeydg5 Well-Known Member

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    No resistor change should be necessary. The bias trimmer has adequate range. If everything checks out and those tubes do not bias up then they would be low output failed tubes.

    5 to 14mA is very low. Its just not right.
     
  14. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    That could tell ii something is wrong in the bias circuit or not. There are electrolytic caps in the bias circuit and that is a 30 years old amp.
     
    Jethro Rocker and TubeAmp like this.
  15. TubeAmp

    TubeAmp New Member

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    Thank you! Yes, I agree. It could kill me.

    Yes that makes sense, but you wrote 0,38 ma (not 38 ma) first and that's what confused me, hehe. :)

    But do you recommend 38 ma as PLATE current? Or NEGATIVE GRID current? I still don't understand your last sentence in your first post.

    I use: https://www.ebay.co.uk/itm/2-in-1-T...714612&hash=item41d09e96dc:g:AwkAAOSwBDtZzSEv

    To your last sentence: Yes, I agree with everything but it doesn't address the issue/question in my first post.

    But your recommendation is to do what's seen in the video? I don't plan on doing what's seen in the video because that would be dangerous, like charveldan said, but I want to understand what you're saying.

    Yeah something's off and I don't know what to do about it. But it sounds great with a randomly set bias now, hehe.

    Ok I should do this then.
     
  16. charveldan

    charveldan Well-Known Member

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    [​IMG]


    When you say "Plate current" i don't know what you mean.
    Bias current is a Negative current on the "Control Grid" it sets the idle of the tube. It meters the flow of Electrons from the Anode to the Cathode.
    Thats what you Bias.
    Plate Voltage {Anode} is measured in DC Volts, not current.
     
  17. mickeydg5

    mickeydg5 Well-Known Member

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    First, have you used this bias probe with success on other amplifiers before?

    Second, in your original post, you said the meter was set to 200m DCA. That is incorrect and should be set to 200m DCV or autorange for under 200mA having meter probes inserted in the proper meter jack.

    No dummy load; connect your speaker cable and cabinet with correct impedance match.

    If you still have issues then you will need to check the bias probe with your meter. I do not care for some of what I am reading in the bias probe instruction and you should actually figure out how this probe is connected and being used. The instructions also exclaim
    "EVEN MORE IMPORTANT!!!!:
    EVEN THOUGH YOU ARE DEALING WITH A FEW MILLIVOLTS' BETWEEN THE BANANA PLUGS ,IF THE CATHODE ON YOUR AMP IS CONNECTED TO CHASSIS ( PROBABLY IS ) YOU WILL HAVE THE FULL B+ POTENTIAL BETWEEN EACH OF THE BANANA PLUGS AND THE CHASSIS OF YOUR AMP. ALWAYS ALWAYS ALWAYS PLUG THE BANANA PLUGS IN THE MULTIMETER BEFORE SWITCHING ON THE AMP TO AVOID ELECTROCUTION."
    which makes me leery.
     
  18. Jethro Rocker

    Jethro Rocker Well-Known Member

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    You obviously have a probe that reads PV and current (current = MA.) No?? as the tube is mounted on the probe. The rest is semantics IMO. You are reading a PV of 450V and cannot get bias current any higher than 14. Sounds like Kinkless is on the right track if there are indeed electrolytes in that circuit, and he should know. It's that or 2 bad sets of tubes.
     
  19. mickeydg5

    mickeydg5 Well-Known Member

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    Charveldan, everything he has said so far is in line while trying to understand and communicate to the best of his ability information give by the bias probe seller (in Bulgaria).

    I believe your terms are more than just ambiguous and confusing the hell out of him.
     
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  20. mickeydg5

    mickeydg5 Well-Known Member

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    No, maybe, not exactly. That has to be determined.

    Tubeamp also noted he did not want to poke around inside the amplifier.

    Tubeamp, there is a way to check negative bias voltage on pins #5 of each power tube without opening the amplifier.
     

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