El 34 voltage and bias

Discussion in 'Marshall Amps' started by swemarshall, Dec 24, 2019.

  1. swemarshall

    swemarshall New Member

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    Hey first post here!!!how come the jcm 2000 should be biased att ca 90mv and the jcm 600 att ca35-40mv and both have el34 tubes.
    I have measured platevoltage in my 600 and its 495v (if i remember correctly)but not yet done it in my jcm 2000 but i meen it must be similar voltages (am i right??) so why is it 90mv in the 2000??and can you bias it att 40mv and not kill it??
     
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  2. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    ~ 90 would be for two measured tubes. ~40 would be per tube.

    495 is rather high voltage. A DSL would be lower voltage of around 450-460. That makes a difference for the proper range.

    25 watts/ 495 volts = 50 ma x 70% = 35 ma

    25 watt / 455 = 55 ma x 70% =39 ma x 2 = 88 ma or ~90
     
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  3. South Park

    South Park Well-Known Member

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    When you bias the drivers you want the MA reading not the voltage
     
  4. EADGBE

    EADGBE Well-Known Member

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    Don't hesitate to bias the amp lower if it sounds better. My TSL60 sounds good biased at 67 mV. That's about 33.5 mV per EL34.
     
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  5. ken361

    ken361 Well-Known Member

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    dsl's sound best around there!
     
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  6. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    Correct, but in cases where there is 1 ohm resistor between cathode and ground mv = ma. This is done because many have meters that measure voltage but can not measure milliamps. And you don't need to disconnect anything or open up the amp in this particular case, as your simply measuring voltage across the 1 ohm resistor. It's easy and it's relatively safe.
     
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  7. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    In the case of the DSL40 it's still around 70% there, because the DSL40 runs fairly high plate voltage.
     
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  8. El Gringo

    El Gringo Well-Known Member

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    I love your photo in your avatar , jeez I thought I had it going on with 2 2555X half stacks ( with matching 2551AV cabs )
     
  9. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    LOL I wish too. That's just a photo I found in my documents files when I finally got around to posting an avatar. I don't recall where it came from.
     
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  10. MickeyJ

    MickeyJ Well-Known Member

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    Some el34 tubes brands sound better a bit colder than normal, some el34 tubes sound better a bit hotter. Make sure you turn the amp up to properly sound test.
    You can measure bias current via the 1 ohm resistor from pin 8 method, or use a bias probe, dial it up hot/cold in accordance with whatever the charts say, ( google 'weber bias calculator' ).
     
  11. wmachine

    wmachine Well-Known Member

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    Love those stacks!
    [​IMG]

    [​IMG]
     
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  12. Lukas

    Lukas Well-Known Member

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    I’ve biased a DSL40CR and a DSL100H in the last month both I biased at 35mv and 72mv on the 100H which is 36mv/tube and both sound amazing. That’s the sweet spot I’d say. 40 and higher is like holy shit your gonna wear those tubes out fast!!! I wouldn’t bias any EL34 at that so why would a JCM2000 be any different?
     
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  13. Lukas

    Lukas Well-Known Member

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    90 is for a pair than they would be 45 each and that’s pretty damn hot. They’ll work like that but have a spare quad ready to go soon.
     
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  14. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    I agree. But those are the Marshall recommended settings.

    If you read the Marshall Technical Service Bulletin issued to techs for the JCM2000 as explained by the Marshall USA tech at the time (posted online back then on another forum) it was because the amps sounded better biased hot when operated at low volumes (which most are), but if the amp was played cranked up to set the bias cooler.

    Also note that for the JCM2000 versions the plate voltage was less than for modern versions.

    The JCM2000 may be different if it runs a lower plate voltage. 45 per tube is hot if the plate voltage is relatively high. However if the plate voltage is lower it is not as hot.

    Take for example a JTM45 whose rectifier tube and power supply provides a plate voltage of about 405 volts: The KT66 or an EL34 has a max plate dissipation of 25 watts.

    25 / 405 = 62 ma

    70% of 62 = 44 ma.

    Running a DSL40 at 36 ma per tube is about the same as running the above JTM45 at 44 ma per tube. It's the % that matters more than ma number.

    In fine, I agree that running a JCM2000 dsl at 80% is still pretty hot and you should set it according to your needs rather than go by some number.
     
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  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    I think this is funny: most of the cabinets do not have any speakers inside.
    It's a stage prop.

    upload_2019-12-27_8-2-41.jpeg
     
  16. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    :welcome: Welcome to the forum
     

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