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366 Views 10 Replies Latest reply: Jun 22, 2012 10:17 AM by Martijndrift RSS
Martijndrift Just Startin'
Martijndrift
3 posts since
Nov 30, 2009
Currently Being Moderated

Jun 19, 2012 2:52 AM

No tube breakup from the power amp?

Hello everyone,

 

I bought a dt25 to go with my pod hd a few months ago, I thought it sounded pretty good.
Lately though I start to dislike the tones more and more. For some reason it seams like all the power amp is doing is making things louder and harsher. I especially notice this in topology II. When you turn the master volume up on other marshall style amps they get this really smooth distortion and sag. My dt 25 though just sounds more in your face and harsh when I turn it up (comparable to a solid state amp), I also don't notice any more distortion than at lower volumes. I was also getting popping noises every now and then, and I was receiving russian radio in topology 4 (I live in holland)

Hoping this was a tube problem, I replaced the tubes with a matched set of sovtek el84 (the same as standard). I also changed the phase inverter tube just to be sure (EH 12ax7).

 

Although the popping went away, and it started to pickup less noise, it still sounds incredibly harsh and boxy at higher volumes, there just doesn't seem to be any warm power tube distortion.

When I take topology I for example and I turn both the volume and master up to 10, shouldn't there be a bit of distortion or should it still be as clean as running the master volume on 2?

 

When I run it in class A it does seem to get a slight bit of power tube distortion at higher volume, although it still sounds harsh and undefined, it can be nowhere as warm and still sparkly as for example a real vox ac30. This kind of rules out the tube bias being off, (although I swear the tubes in other tube amps send out alot more light)

 

All in all I just have the feeling like the power tubes just aren't doing anything except making things louder and brighter, even at lower volumes my ears start to fatigue in 20mins. Does anyone have an Idea what this problem could be, or is it just me not liking the sound of the amp anymore?

 

Thanks for your help!

 

Martijn

Tags: amp, 6, line, power, up, tubes, break, harsh, bias, warm, dt, 25
  • Crusty_Old_Rocker Expert Line 6 User
    Crusty_Old_Rocker
    3,096 posts since
    Jan 24, 2007
    Currently Being Moderated
    Jun 19, 2012 5:32 AM (in response to Martijndrift)
    Re: No tube breakup from the power amp?

    Okay, you need to check the bias on the power amp valves.  The amp when run in class A mode is cathode biased, so you won't experience the bad or brittle tone that can come from a bias that's too hot or too cold.  You can't rule out the bias without checking it.  The light coming from the valves comes from the heaters, the heaters aren't affected by the bias, they run on 6.3 volts AC straight from the power transformer.

     

    If you want the power amp valves to break up more, I would suggest you get in touch with Bob from Eurotubes.  When ordering EL84s you can specify an early break up on the valves and Bob will deliver.  I still have my stock EL84s in my DT25s but in my home brew class A amps, I am running JJs that I got from Eurotubes that are graded for early break up and I love them.  My 30 Watt class A gets some real power amp goodness happening at lower volumes, which is great.  Without those valves, the amp would be deafening before the EL84s would break up.

     

    So, get yourself some early breakup graded JJs from Eurotubes and don't just put them into the amp, you need to have the bias adjusted otherwise when in class A/B the amp isn't going to sound good at all.

     

    Cheers,


    Crusty

  • Martijndrift Just Startin'
    Martijndrift
    3 posts since
    Nov 30, 2009
    Currently Being Moderated
    Jun 22, 2012 10:17 AM (in response to Martijndrift)
    Re: No tube breakup from the power amp?

    Ok guys, I just checked the bias, it was at 22 volts which is a bit on the low side, so I changed it to 26, this already gave quite abit more warmth.
    However I also ordered some jj el84, put those in, checked bias (which happened to be correct) and gave those a go.

    I must say it's a pretty big difference especially when you crank them. The highs sound softer, the bass is less flubby and the overal tone is just warmer!

    Thanks for your advice!

     

    Martijn

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