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358 Views 12 Replies Latest reply: Feb 21, 2013 10:37 AM by bmoyer RSS
Pezza Just Startin'
Pezza
75 posts since
Jun 18, 2007
Currently Being Moderated

Feb 12, 2013 5:42 AM

Curing the DT25 noise problem - 12AX7 or EL84s?

I have had the DT25 head for just over a year now and have started to get the infamous 'wind blowing across a mic' noise. I have a few questions:

1. Have people found that it was the 12AX7 phase inverter causing the problem or was it the EL84 power tubes?

2. If it is the 12AX7, can I just swap it without any adjustment/biasing etc?

3. Which 12AX7s do you recommend? I gather that it won't have a huge impact on tone as it's not a preamp/poweramp tube.

 

I'm not familiar with replacing tubes so I'm hoping that a simple swap of the 12AX7 will cure the problem!

 

Thanks

Tags: head, tube, noise, dt-25
  • tormod1 Just Startin'
    tormod1
    12 posts since
    Mar 4, 2007
    Currently Being Moderated
    Feb 12, 2013 9:22 PM (in response to Pezza)
    Re: Curing the DT25 noise problem - 12AX7 or EL84s?

    I have the exact same problem, so I really hope someone has got the answer to this..

    • MGblade Just Startin'
      MGblade
      110 posts since
      Dec 5, 2007
      Currently Being Moderated
      Feb 13, 2013 5:40 AM (in response to tormod1)
      Re: Curing the DT25 noise problem - 12AX7 or EL84s?

      `12ax7 can be replaced without biassing,

      i would replace it with the same model/brand

       

      you can check the power tubes with the pencil method, tap the tubes with a graphite pencil,

      if they make noise while doing this through the speakers it,s a sign that the tube is worn, and need to be replaced.

       

      the power tubes need to be biased and hat is fairly easy on the dt25 head.

       

      grts Marcel

  • Krontab Just Startin'
    Krontab
    201 posts since
    Mar 15, 2006
    Currently Being Moderated
    Feb 13, 2013 4:59 PM (in response to Pezza)
    Re: Curing the DT25 noise problem - 12AX7 or EL84s?

    Had the windy mic issue as well. Not only did I have to replace the tubes I had to install a power conditioner. Line 6 support helped me identify the power supply issue.

  • geppert Just Startin'
    geppert
    188 posts since
    Nov 8, 2010
    Currently Being Moderated
    Feb 15, 2013 9:10 AM (in response to Pezza)
    Re: Curing the DT25 noise problem - 12AX7 or EL84s?

    I am willing to bet it is the power tubes and I also 2nd the motion for a power conditioner. Ever since Spaceati mentioned it, I bought the "Triplite LC1200" (same as he uses). It runs all my computer stuff (and gtr amps) at home and I take it out for practice and gigs. Nada had a problem with the amp (DT25 Head/Cab). Have changed all the tubes though. Did have some noise at one point. Definitely was due to the original E.H. EL84's that were the culprit. Also had "noticeable hum" in Class A & Triode mode, again positively attributed it to the EL84's. Replaced with Mesa. Very happy and quiet now. Don't forget to adjust bias if you replace tubes. Very easy to do with a multimeter but requires removal of chassis to get at test points. Caution is always advised however. If nervous about doing it yourself it is a 30 minute job for a tech. Maybe less if he s really fast.

    • spaceatl Expert Line 6 User
      spaceatl
      4,456 posts since
      Jan 24, 2007
      Currently Being Moderated
      Feb 15, 2013 12:09 PM (in response to geppert)
      Re: Curing the DT25 noise problem - 12AX7 or EL84s?

      +1...I like the SPAX7 or GT-SAGAX7 for inverters...

       

      I cannot recomend the Tripp Lite AVR enough...price is really reasonable I think...

       

      One thing a lot of folks do not realize is that using an AVR (transformer type) eliminates ground loops...why? It's isolated power...the transformer removes the potential of any voltage on the ground from the house side...Basically, my rigs have never had hum of any kind in any venue I have been in...I think you have to be gentle with these hybid amps...They are basically embedded computers with high voltage tube power amps...the less voltage swings and no voltage potential on ground is a good thing I think...

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