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vile83r

DT25 bias is way off. How do I get it back in line?

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I switched out my tubes and was getting readings all over while doing the bias test. I got the v1 set to 25mV then while setting V3 the tube blew and so did the tube protection fuse. How can I get the BIAS pot back to a central spot so I can sort of start from scratch? If I BIAS with blown tubes does that give bad readings or will it still measure the voltage so I can at least get the BIAS pot back to something near factory?

 

Thanks for your help.

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I switched out my tubes and was getting readings all over while doing the bias test. I got the v1 set to 25mV then while setting V3 the tube blew and so did the tube protection fuse. How can I get the BIAS pot back to a central spot so I can sort of start from scratch? If I BIAS with blown tubes does that give bad readings or will it still measure the voltage so I can at least get the BIAS pot back to something near factory?

 

spaceatl just did a nice rundown on this. Let me try to find the link for ya.

 

Sure. Happy to share what I do, but keep in mind that I do NOT speak for Line 6 and if you want to learn about taking care of a tube amp, there is plenty of good information on the web...Lots of good how to's about setting the bias on a tube amp...I would never discourage someone from wanting to learn this stuff...Just keep in mind that the voltages are dangerous...THEY CAN KILL YOU...However, there are safe procedures anyone can follow and learn how to do it safely.

 

the bias adjustments only apply to the Class AB Mode...You would actually figure this out pretty quickly when you meter read zero on the test points in Class A.

 

Master Vol all the way down....After all, adjusting bias is about the "idle" state....If you put the amp on standby, you will have no readings...It becomes pretty clear once you are into doing it...You are just taking measurements...

 

One note is that you do not need to let the amp warm up to measure the plate voltage....You can simple take out the tubes, power up and measure it fro mthe tube socket...This can give you decent starting place for calculating safe bias level (in my DT25 case 414 volt plate; 70% of 12 watts = 20.4mv bias target)

 

My own procedure for adjustment with new tubes is to:

  • remove the old tubes with the power UNPLUGGED FROM THE WALL.
  • turn the bias adjustment(s) fully counter clockwise. (All the way down)
  • power on for 5 minutes on standby
  • Set as CLASS AB
  • power up off standby and take measurements immediately....bias should be very low...
  • Switch to CLASS A
  • let them cook for 5 or 10 minutes
  • Switch to CLASS AB
  • start adjust the pot(s) up slowly until I hit the target.
  • Let it cook for another 15 minutes
  • check again and adjust
  • play on it for a couple of days
  • check and adjust as needed

 

The voicings and pentode/triode do not change the plate voltage at all. I measured the plate voltage in each state in case an average would be needed....

 

I have my amp set Class AB/Pentode with Master down...NFL made no difference to the bias that I could measure...

Edited by Brazzy

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This looks excellent. Thank you so much!

The problem with that is that he doesn't tell you when to put the tubes back in.  This occurs sometime before "let 'em cook," obviously.  I think even in the original thread, which wouldn't be too hard to find as it was only a few days ago, that wasn't clear.

 

You didn't say, but what follows is assuming that you were in Class AB and will continue in class AB.  

 

Thing is, the bias is supposed to remain the same (in Class AB, natch) no matter which voice you are in (at least, that's what I read here in these pages).  When you went to V3 something happened.  Was it just a fluke, or was it something else?  For example, is it possible to have a bad board that causes your tubes to red plate in voice 3 or something along those lines?  

 

For you, the thing would be to power up with the bias pot turned all the way down and no tubes in.  Then at that point--assuming that you are a certified electronics technician and are trained so that you aren't putting your life in peril (or your amp), you would probably want to compare those voltages in voice I and in voice III.  If the bias voltage isn't constant, then based on what I've read, I'd say there must be something wrong with your electronics.

 

If you're at all unsure about what you're doing, you should take it in to a Line 6 service center.  If it's under warranty, then absolutely don't mess around or you could void your warranty.

 

Also, ONLY USE A MATCHED PAIR OF ELECTRO-HARMONIX TUBES IN THE POWER SLOTS!!!  It has been well-stated here that any other brand of tube may lead to problems.  I'd recommend searching out the recent threads about tubes and reading the warnings.

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Don't bias with a bad tube....Those guidelines are general and nothing but my own approach I take with "most" fixed-bias tube amplifiers...Certainly, one would take out the bad tubes and put in the new ones whilst powered down....These are NOT instructions, just a quick listing of the steps I take...IF you choose to follow them, I make no warranty...You do this at your own risk...and please don't kill yourself...it can certainly be learned...but don't do it using instructions from a forum...electrical safety is PARAMOUNT in this procedure...DT-25 is > 400 volt amp that can kill you dead...

 

Since there is a single bias adjustment in the DT25, a slightly different approach could be warranted...turning down fully clockwise might not be the best idea...better to measure and see what the effect is in very small increments on BOTH tubes...If you are not sure, get a tech to do it for you...

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Don't bias with a bad tube....Those guidelines are general and nothing but my own approach I take with "most" fixed-bias tube amplifiers...Certainly, one would take out the bad tubes and put in the new ones whilst powered down....These are NOT instructions, just a quick listing of the steps I take...IF you choose to follow them, I make no warranty...You do this at your own risk...and please don't kill yourself...it can certainly be learned...but don't do it using instructions from a forum...electrical safety is PARAMOUNT in this procedure...DT-25 is > 400 volt amp that can kill you dead...

 

Since there is a single bias adjustment in the DT25, a slightly different approach could be warranted...turning down fully clockwise might not be the best idea...better to measure and see what the effect is in very small increments on BOTH tubes...If you are not sure, get a tech to do it for you...

 

I really hope I didn't pass along faulty info!!!

 

I inferred that it's possible check the bias voltage with no power tubes in the sockets.  I wouldn't think that would be completely accurate, but I thought it might tell him if something is wrong with his electronics.  What say you?  (something happened when he changed from voice I to voice III)

 

As for turning the bias down all the way, that doesn't hurt anything does it?  (other than provide for a cold, sterile sound).

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No worries, my posts are nothing more than my opinion...take them with a grain of salt...Others might have different approaches like lighting incense and scented candles and adjusting the bias in the nude exactly 15 feet from the power source using a 7.25987 Ohm dummy load...

 

You can check plate voltage with the tubes removed...it is a little safer to probe the tube pin from the top of the socket IMHO...

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Others might have different approaches like lighting incense and scented candles and adjusting the bias in the nude exactly 15 feet from the power source using a 7.25987 Ohm dummy load...

 

People and rituals......never understood the point of that stuff, lol. Good One "S"!

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Here’s a video detailing replacing and biasing tubes in the DT-25 combo amp. 



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