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Biasing A 40 Watt Amp With El34 Tubes

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

  1. matttornado

    matttornado Well-Known Member

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    I never read anything about this so I thought I would ask being that I am now an owner of a DSL 40 CR.

    So normally I would calculate bias current by dividing the tube power rating by plate voltage and then taking 60 -70% of that for adjusting the current. That works for 50 & 100 watt amps but what about 40 watt amps?

    Do we rate the power of the tube as 20 watts instead of 25 when doing the calculation?

    I realize I might get drilled for asking this and some may say I have no idea what I'm talking about but I'm asking anyway.;)

    Right out of the box, my 40 watt DSL was adjusted to 32mA with a plate voltage of 430 volts.
    That sounds about right, right?
     
  2. Chris-in-LA

    Chris-in-LA Well-Known Member

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    If you’re assuming 25 watts for the Tube, 34mv would be 60%.
     
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  3. mickeydg5

    mickeydg5 Well-Known Member

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    The bias and dissipation was discussed in another thread.

    Forty or fifty watt output has less to do with plate dissipation than if does with intended design and voltages. Tube watt rating and idle dissipation is only one aspect.

    From what I gather with your reported numbers Marshall biased it at about 50% like usual when coming out the factory. That is on the low side. Your 32-35mA is about 50% when calculating using the 1 ohm cathode resistor method.
     
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  4. matttornado

    matttornado Well-Known Member

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    That's what I calculated too.
    I couldn't go any higher than 34-35 mA on the bias trim pots like someone else has mentioned in a different thread.

    So to answer my question, a 40 watt amp should be biased by dividing 25 watts/ plate voltage * 60- 70% same as 50 or 100 watt amps?
     
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  5. J5684

    J5684 Active Member

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    40, 50 or 100 or 5 watt that is the basic formula (of course other tubes are different wattage).

    I thought about getting the new CR but this really makes me unhappy. If it's going to require a modification to be able to bias it in a decent range then I may just wait till I find a deal on one outta warranty. I wonder what Marshall is doing by by limiting the bias.
     
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  6. matttornado

    matttornado Well-Known Member

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    So with a plate voltage of 430 Volts & a bias at 60% dissipation, I should be at ~34.8mA.
     
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  7. mickeydg5

    mickeydg5 Well-Known Member

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    I have not seen schematics for the newer reissue amplifiers so no one out here can be certain. 40 watts is a target RMS value. The 40 watts comes from the amplifier design, not necessarily the power tubes. Yes an amplifier can make 40 watts RMS with either 30mA or 50mA idle bias. It all depends on the design.

    60% at 430VDC would be more around 40mA using the 1 ohm cathode method of monitoring.
     
    Last edited: Sep 21, 2018
  8. neikeel

    neikeel Well-Known Member

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    Bear in mind I that the 1R cathode method includes screen current too. (For EL34s with 1k screen resistors that is probably 3-4mA).
     
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  9. mickeydg5

    mickeydg5 Well-Known Member

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    25W / 430V = .058 (58mA)
    58mA x 60% = 34.9mA

    As Neikeel just pointed out the 1 ohm cathode method is measuring both the plate and screen current. So subtracting the 4mA from your maximum 35mA or so number gives about 31mA for the plate current alone, provided you are using the bias points and trimmers on the deck of the chassis.

    So your maxed out idle bias at this point is almost 54%.
     
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  10. Tom Drouhard

    Tom Drouhard New Member

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    Thanks for asking this question Matt tornado. I bought a dsl40cr last month and more info the better!
     
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  11. JP2036

    JP2036 Well-Known Member

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  12. mickeydg5

    mickeydg5 Well-Known Member

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    47 would be the highest I would go with the 1R cathode monitor resistor measurement. 40 is a nice average especially if you leave the volume control(s) cranked. :thumb:
     
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  13. 351

    351 Member

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    get it sounding about right, up to 47ma reading on that 1 ohm resistor, and anything less than 47.
     
  14. mickeydg5

    mickeydg5 Well-Known Member

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    So, are you backing me up? :)

    :yesway:
     
  15. 351

    351 Member

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    Yeah
     
  16. Matopotato

    Matopotato Member

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    I am trying to set bias again.
    1. Plate voltage was/is 427V but i understand many consider this plus the formula to be less relevant?
    2. The 1R cathode monitor resistor is new to me. I measure in the DSL40CR dedicated holes, thought that was correct?
    3. On stays solid 35mA Hot, Cold or over time. The other jump between hi 20s to low 40s and very hard to dial in as it jumps all the time, should I go for a new pair?
    4. This is possibly opening a can of tubes, but what is considered best tubes for DSL40CR's. Unless the originals are good. I am not looking to mod or push in any direction. Play mostly on 20W setting, bedroom no gigs to speak of.
    Thanks
     
  17. Matopotato

    Matopotato Member

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    #3: kept my phone on the dsl as i was tweaking. It overheard me. Bias voltage all over the place, then steady, the not. Shifting tubes too. I suspect I got a unit with some humor and high prankability level.
    Went between 35 an 34 and could not hear much difference so settled for 34. Really clear when cranked, but have to play from another room at 40 and all cranked. Still ringing.
     
  18. mickeydg5

    mickeydg5 Well-Known Member

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    @Matopotato
    I suggest use of a good electronics cleaner/lubricator on the tube sockets and tube pins.
    Swap the power tubes in the sockets to see if issues persist or change.
    Make sure socket prongs are /close together/tight for grabbing tube pins.

    AND if you are actually at plate voltage of 427 or so then bump up the bias to 40-45mA and see how things react.
     
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  19. myersbw

    myersbw Well-Known Member

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    @matttornado - I'm going to take a different direction on answering your original question. (With that amp, @mickeydg5 and everyone else confirmed what you needed to know for the DSL40 you have). But, here's a situation I ran into where it does make a difference to pay attention to data sheets with respect to what we desire when we divert to different tubes.

    I had a customer bring an amp in that he had EL34's in and wanted KT66's installed. If you used the "30 watt" rating and did your calculation for a quick bias at, say 60%...the current draw was such that the B+ dropped to where I was really uncomfortable. Treating it like a "25 W" tube for it's rating settled into an idle bias (and expected drop) that I was really comfortable with. The KT66's sounded amazing...it was a master volume amp so much breakup tone was coming from the preamp (not the output tubes).

    My point being, and like others have already said, the audio power out wasn't the factor for biasing to be considered. But, there is a consideration when you deviate from the original design to branch to another style output tube. With regards to 6L6 vs. EL34's...the one area we peek at is...will the filament current handle the extra load? So, in my case above...treating the KT66 like a "25 watt" tube at the 450V range was the correct thing to do in that amp.

    So long as you monitor current and voltages (I've got two meters going simultaneously watching the plate current and the B+), you'll see where you need to be. When B+ drops a bit too much out of spec, then you know you've either got a bad tube -or- you're at the edge of overtaxing the power transformer when it comes to gig time. So, if you only use that formula...your question is a valid one (just not relative to audio power out, although it could have a negative impact on the OT & PT if you bias some tubes to an upper operating level...depends on the amp).
     
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  20. Matopotato

    Matopotato Member

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    Thanks a lot for the suggestions.
    I just got it this February, so it shouldn't be too worn already. Will try the swap, and check the other.
    Today it felt as if the classic on clean was much quieter than the other three as far as Vol goes (Gain at 9), but I have been fooled before.
    EDIT: Or it is the Crunch and OD1/2 that just got louder...
     
    Last edited: Jul 10, 2021

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