73 Marshall 100 Bass- tube bias/current under load question

Discussion in 'The Workbench' started by MarshallMV2203, Jan 27, 2021.

  1. MarshallMV2203

    MarshallMV2203 New Member

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    Hi, Question for the group, please-

    I recently got a four socket bias meter. I installed the sockets and old tubes to check to see how matched they still were after many months of hard use. Surprisingly closely matched. The bias is set at 31mA which is 60% (485Vdc plate, EL34, -41.8Vdc grid). Since this is the first time having a meter watching the output tube current instead of me measuring things the regular way (1 ohm or shunt method) my hands were free to grab the guitar. This is where it gets interesting. Under a hard strum the current would jump up to 90mA (one tube) when strumming the current sits around 55mA. Stop playing and it returns to 31mA. I guess it makes sense that the tube is passing more current since there is an input signal/voltage to start producing more power.... but since this is the first time I've actually seen it I'd like to please confirm this with the group.
    When someone is doing an amplifier output power test what is the current of the tube when fully cranked?? Thanks for any information.
     
  2. cruisemates

    cruisemates Active Member

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    You might get a better response in the "workbench" section. It's a good question, I'm subscribed.
     
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  3. South Park

    South Park Well-Known Member

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    What you are seeing is the voltage drop in the power tubes. The only reading that matters is the grid current that controls plate current . Thar is no power reading at the speaker . The best measure is with a signal gen and a scope voltage in voltage out that is the gain of the power section
     
  4. MarshallMV2203

    MarshallMV2203 New Member

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    Thank you both for the replies.
    I'll post in Work Bench, too. Thanks.
    South Park, I'll work on your suggestion. It did make me realize that the tube is following the load line, with signal/under load it's lower voltage = higher current through the tube. So when I was strumming the guitar and seeing 55mA on the meter the plate voltage could have been way lower. I will connect a meter to the plate and check it. Thanks for any other thoughts or suggestions.
     
  5. MarshallMV2203

    MarshallMV2203 New Member

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    Hi, Question for the group, please-

    I recently got a four socket bias meter. I installed the sockets and old tubes to check to see how matched they still were after many months of hard use. Surprisingly closely matched. The bias is set at 31mA which is 60% (485Vdc plate, EL34, -41.8Vdc grid). Since this is the first time having a meter watching the output tube current instead of me measuring things the regular way (1 ohm or shunt method) my hands were free to grab the guitar. This is where it gets interesting. Under a hard strum the current would jump up to 90mA (one tube) when strumming the current sits around 55mA. Stop playing and it returns to 31mA. I guess it makes sense that the tube is passing more current since there is an input signal/voltage to start producing more power.... but since this is the first time I've actually seen it I'd like to please confirm this with the group.
    When someone is doing an amplifier output power test what is the current of the tube when fully cranked?? Thanks for any information. P.S. Another member, South Park, helped me realize that the tube is following the load line (under load the plate voltage goes down and the current goes up). I've never had a meter on the plate while playing to see what it's actually doing. I will test this. Am I on the right track?? Thanks again for any thoughts.
     
  6. neikeel

    neikeel Well-Known Member

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    If you consider that you bias at 31mA = 60% of max dissipation at idle then 55mA is what you would expect at constant input load. Similarly with transient spikes as the signal is not heavily damped. If you were synchronously measuring the voltage on the output tubes as you draw more current (both anodes and screens) the voltage will drop.
     
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  7. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    merged threads...
     
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