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1096 Views 5 Replies Latest reply: May 9, 2012 12:51 PM by daferalo RSS
Atminstone Just Startin'
Atminstone
10 posts since
Jan 2, 2010
Currently Being Moderated

May 8, 2012 12:05 PM

dt25 112 combo bias adjustment

Hi,

 

I have a DT25 112 combo. I measured the bias voltage V2 and V3 and got the values 32.8 and 35.4 mV, with the stock tubes. From what I understand, this is pretty high compared to the specifications (25 +/- 3mV)

The next step would be to adjust the bias voltage with the trim pot, but the only problem is that it's next to the speaker, and I can't see it.

 

Has anyone adjusted the bias voltage on their DT25 combo? Is it somehow possible without removing the speaker? If not, is it possible to remove the speaker without disassembling the whole amp?

 

If I leave the bias voltage at the current values, what does it do to my amp? Is it only wearing out the tubes faster, or is it going to hurt my amp? Are there any differences in tone for better or worse?

Tags: amp, 112, tube, combo, bias, dt, dt25
  • Rowbi Expert Line 6 User
    Rowbi
    7,745 posts since
    Nov 25, 2006
    Currently Being Moderated
    May 9, 2012 1:41 AM (in response to Atminstone)
    Re: dt25 112 combo bias adjustment

    make sure you're in NFL1, pentode and class AB.  make sure the amp is fully powered on with the master volume control at min and with no guitar plugged into the input.

     

    If the bias still reads so high then it needs adjusting.  you may want to have an amp tech or line 6 service centre do this as you may need to remove the chassis slightly to get to the bias pot, or use a special tool that only LIne 6 service centres have.  usually it's just a screwdriver though, so it's strange that you can't get to it.

     

    could you send a photo of where it is?

     

    could you make a small L shaped tool to get access to it?

     

    Cheers

     

    Rowbi

    • Atminstone Just Startin'
      Atminstone
      10 posts since
      Jan 2, 2010
      Currently Being Moderated
      May 9, 2012 6:51 AM (in response to Rowbi)
      Re: dt25 112 combo bias adjustment

      Hi Rowbi,

       

      I made sure I was in NFL1, pentode and class AB. Got pretty much the same results. I'll borrow a better multimeter tomorrow from work just to check (I got 14 mV from the "Ground" to the chassi?). Then I'll hunt for a L-shaped screwdriver, do you know/guess what kind of screwdriver it should be? (I can't really see the trim pot)

       

      Here's a few pictures:

      dt25 bias adj 1.jpg

      dt25 bias adj 3.jpg

      Not the easiest place to put a trim pot

       

      -Pekka

      • daferalo Iknowathingortwo
        daferalo
        270 posts since
        Apr 12, 2010
        Currently Being Moderated
        May 9, 2012 7:52 AM (in response to Atminstone)
        Re: dt25 112 combo bias adjustment

        Hi Atminstone,

         

        Maybe, you can find this thread interesting http://line6.com/support/message/333390#333390. Answer #8 from Andrewtorres. It seems that you might need to remove the amp out of the chasis to do so. However, I have never done this with my DT 25, so I cannot talk about the experience.

         

        Best Regards,

         

        Daf

        • Atminstone Just Startin'
          Atminstone
          10 posts since
          Jan 2, 2010
          Currently Being Moderated
          May 9, 2012 12:10 PM (in response to daferalo)
          Re: dt25 112 combo bias adjustment

          Thank you daferalo!

           

          This solved my problem. At first I hesitated to open the amp, but the post by andrewtorres in the other thread and the fact that there is only 4 screws holding the amp in the chassis (two of them are in the handle) made me brave enough. Just unscrew them and the amp comes sliding from the chassis. Then I placed the amp with knobs down on two large books and CONNECTED THE SPEAKER and power cables. It's very easy to measure and set the bias voltage when the amp is removed.

           

          Here's one more picture to show the amp removed from the chassis:

          dt25 open.jpg

  • daferalo Iknowathingortwo
    daferalo
    270 posts since
    Apr 12, 2010
    Currently Being Moderated
    May 9, 2012 12:51 PM (in response to Atminstone)
    Re: dt25 112 combo bias adjustment

    To my knowledge, I think that in this article the effects of bias voltage are fairly explained: http://www.aikenamps.com/WhatIsBiasing.htm

     

    Best Regards,

     

    Daf

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