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dt25 112 combo bias adjustment
by Atminstone on 2012-05-08 12:05:35

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?



Re: dt25 112 combo bias adjustment
by Rowbi on 2012-05-09 01:41:33

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



Re: dt25 112 combo bias adjustment
by Atminstone on 2012-05-09 06:51:05

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



Re: dt25 112 combo bias adjustment
by daferalo on 2012-05-09 07:52:21

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



Re: dt25 112 combo bias adjustment
by Atminstone on 2012-05-09 12:10:36

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



Re: dt25 112 combo bias adjustment
by daferalo on 2012-05-09 12:51:32

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




The information above may not be current, and you should direct questions to the current forum or review the manual.