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Biasing A 40 Watt Amp With El34 Tubes

Discussion in 'The Workbench' started by matttornado, Sep 18, 2018.

  1. Matopotato

    Matopotato Member

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    We swapped and lubricated and I feel it is a bit more stable now. Will try for some time before deciding if there is something wrong still.
    I bought it over the internet and there is probably enough warranty, but to send it off to UK from EU and back during the combo of pandemic, world wide shopping spree and postal companies + customs doing their best to fail makes it less attractive. Might have to pay shipping. Not sure. So if it boils down to new tubes, I would probably buy a new set and keep these in reserve.
     
  2. mickeydg5

    mickeydg5 Well-Known Member

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    This has nothing to do with biasing or measuring with a multimeter.

    We are talking the CLEAN channel issue and jumping some components with jumper/testing wires can provide insight.
     
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  3. mickeydg5

    mickeydg5 Well-Known Member

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    Not the amplifier, only the power tubes. Maybe.
     
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  4. Matopotato

    Matopotato Member

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    Ok, so I should pick up som alligator clips and cable and use to bypass some components under the chassi?
     
  5. Matopotato

    Matopotato Member

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    I know. So far any support requests to companies operating world/europe wide have resulted in "send the whole thing back, cables, boxes, everything and our technicians will investigate".
    So for me to only send the tubes in for them to test is unlikely imho.
    If it had been a shop, perhaps. But I doubt they trust my troubleshooting skills in relation how they operate.
     
  6. mickeydg5

    mickeydg5 Well-Known Member

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    Well then, time to invest in a matched power tube set in case needed. It is always good to have spares.
     
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  7. mickeydg5

    mickeydg5 Well-Known Member

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    Yes this would be inside the chassis on the board, so being careful is crucial.
    Something like this works.

    upload_2021-7-20_23-2-33.jpeg
     
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  8. Matopotato

    Matopotato Member

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  9. Matopotato

    Matopotato Member

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    I just realized that my music store downtown have a DSL40CR in the "playroom". So I went and tried it. Same thing.
    Using Clean/Crunch with low gain (~9 o'clock) and Volume and Master both at 12, it was clear that clean and crunch hade quite a big difference in output. Taking gain to 12 evens out their relative differences.
    So no fault. I just have to learn and adapt.
    I am so grateful for you suggestions and help @mickeydg5. And I hope you don't feel I have wasted your time on this.
    For the biasing part, it still fluctuates after the lubrication and swap, but not as much at all.
    I will study threads though to see if there are any tubes that would make the clean sound a bit "more".
    But a heartfelt huge thanks for your time and effort.
     
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  10. mickeydg5

    mickeydg5 Well-Known Member

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    The CLEAN-CRUNCH is actually a set circuits which pass a larger/more powerful signal than the CLASSIC-CLEAN circuit.
    It would have been nice if Marshall would have placed a GAIN control separately on both of those channels. Sharing a GAIN control makes for issues like this. It is kind of the way they do things. I mean Ultra has its own GAIN control.
     
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  11. spacerocker

    spacerocker Well-Known Member

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    So many people forget (or don't know) this simple fact. My JVMs usually measure about 4.5mA screen current, so I add this to the cathode bias current. (Actually this is what I used to do - these days I mostly measure anode (transformer) current instead - but this is potentially a much more hazardous method, and not for the inexperienced...)
     
  12. Matopotato

    Matopotato Member

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    Good point and agree.
    Even the OD1/OD2 ultra channel could have benefited since they show a similar difference in output, although not on the same level.
     
  13. mickeydg5

    mickeydg5 Well-Known Member

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    The main use of channels however within a song is to go from clean to dirty at the same or about the same volume but usually kick into a higher level dirty with more harmonics for lead or in your face chugging.

    Or maybe it is just what I like to do. :hmm:
     
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