Would like to know about TSL 100 bias !!!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by okgo, Jul 18, 2006.


  1. okgo

    okgo Member

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    I have to change my new set of pre and power tubes for my TSL 100. Can I DIY-bias? I notice the 3-leads at the back of the amp. How can I DIY-bias this TSL and what tools that I have to use? thanks
     
  2. Wakarusa

    Wakarusa Member

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    1,477
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    Newnan, GA
    Next to the three pins are two small trim pots. The center pin is ground, the right and left pins tap into the two pair's cathode circuits in front of 1 ohm resistors.

    This would lead you to suspect that the voltage read between the center pin and the outside pins translates to amps (which, for the most part, it does). The Marshall manual will tell you to just set the trim pots so you read a certain number of millivolts across the pins.

    There are two problems:
    1) I've seen the Marshall recommended setting yield bias settings both way too hot and way too cold.

    2) You need to know the plate voltage for the tube. That reading isn't readily available.

    If you know what you're doing (e.g. understand all the ways you can die inside a live amplifier), the first time you do this you can just measure the plate voltage in the open chassis, note it, and use it as a ballpark figure for future biasing.

    'Zat help?

    Edit: if memory serves, the right trimpot adjusts the left pair of tubes and vice versa
     
  3. okgo

    okgo Member

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    Hi !! Thanks for your help!! :rotflmao

    Anyway, I just changed the tube on my TSL (EH 12ax7 and Svetlana EL34) and also biased my amp using the meter and adjust both trims to 90mV. And now my amp sounds AWESOME!! Very punchy and well-balance!!

    But i found that the powertubes run pretty hot (I'm not sure it's too hot or not) !! Is this gonna kill tubes earlier?
     
  4. Wakarusa

    Wakarusa Member

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    1,477
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    Newnan, GA
    You have, more than likely, biased the amplifier way too hot.

    I repeat: you need to know the plate voltage to correctly set the bias. The last two TSL100s that came through my shop had around 480VDC on the plates. This would lead to a maximum cathode current of somewhere around 65-70mA per side. You really need to find out what the plate voltage is in your amplifier. If you don't know what you're doing, or aren't comfortable working around high voltage (read: instant painful burning electrical death) then take the amp to a tech.
     
  5. okgo

    okgo Member

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    oh..

    So maybe I shouldn't do it myself!!!

    The reason i biased both trims to 90 because Marshall recommended it and I also tried to start the bias from around 75mV but It sounded dead!!! Not punchy and very muddy.

    But thanks for your suggestion. I think I'll better take my TSL to a tech.
     
  6. Wakarusa

    Wakarusa Member

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    1,477
    Joined:
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    Location:
    Newnan, GA
    It may well turn out that 90 is the right value, but, again, without all of the information it's tough to know. Depending on your tech he/she/it may well be willing to show you what you need to do (including the correct values to shoot for). Admittedly I have a biased opinion, but if you're not sure what you're doing it's worth paying for once to learn. Don't think of the charge as "cost to bias", try to get it to count as "tuition".
     

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