Nad Crazy Bias?

Discussion in 'The Workbench' started by BrentD, Oct 17, 2017.

  1. BrentD

    BrentD Active Member

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    So I received a DSL50 in the mail today. The guy said he'd never had it retubed, but I don't know if he is the original owner. I fired it up and it sounded fine, but due to its age I wanted to check for bias drift. It also seemed really hot, with both tubes glowing more than I am used to. This is the first time I've worked with biasing an amp.

    I pulled off the back panel, turned the amp on and standby on (with a cab attached), and let it warm up for several minutes. I set my MM to 200mvdc. Using the common probe at the center pin and the test probe at either of the side pins, I was getting a measurement around 74 for each side (!). I gather Marshall recommends 45 and many people use 38 or so, and that the 74 would be fair if it were a DSL100, not a DSL50.

    Am I right about this? Was this set way too high? I turned the bias pots (oddly to me, right seemed to be down) until I got measurements as close to 38 as possible. This seemed to reduce the heat coming from the power section significantly. Am I nuts or a n00b, or did someone retube this and then follow the directions for a 100-watt head?
     
  2. Rokinroller

    Rokinroller Well-Known Member

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    You did all the right stuff . Seriously .... and yes you are pretty well correct with all you said. I would now get 2 new EL34's and bias them up as you did those that were suffering in there. 74ma / side was nuts for a DSL50 !
     
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  3. BrentD

    BrentD Active Member

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    The guy says he bought it new and never changed the tubes. Hard to believe the factory would have set it that way, but maybe some guy at the store messed with it.
     
  4. Rokinroller

    Rokinroller Well-Known Member

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    Its all believable and likely possible . By the way , on those bias test points , you dont have to go nuts trying to squeeze in 2 aligator clips ..... Its not absolutely necessary to clip your negative lead there on that center pin . Just clip your neg lead to any metal part of that chassis ... I always used one of the little screw heads near by . This way you dont have to worry about neg touching positive clip . Those 3 pins are so very close together. This way you can more easily move to left or right pin . Oh and yeah even after taking amp out of standby mode , let it warm up even half hour and more as you monitor the bias with your meter .... there will be some bias fluctuation all along ..... the bias should be more stable after being on longer and will give you a better reading / adjustment and hopefully a nice stable bias . It should all be good with a nice pair of well matched EL34's. Oh , always watch out for tubes not to "red-plate" ...... the orange heater filament glowing orange is fine and normal , but not those tube plates inside going red ... thats bad . If you see that .. reduce bias asap .
     
    Last edited: Oct 17, 2017
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  5. BrentD

    BrentD Active Member

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    Thanks again. I was actually using the regular pointed test leads instead of clips - I was trying to be so careful that I think I was shaking more, haha!

    The filaments are certainly glowing but no red-plating (even at the crazy bias). Stock SEDs that are 17 years old...I guess they must be pretty robust.
     
  6. Rokinroller

    Rokinroller Well-Known Member

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    If they are maintaining good stable bias and sound good , thats great . Rock-on . But always good to have spares . I really liked my DSL 50 and 100 . Sold them just to downsize to low wattage amps . But wouldnt mind picking up another DSL and or a JCM900 DR as I still have a V30 4x12 cab .
     

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