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Cliff - how can I ever have doubted you!

Discussion in 'Axe-Fx II Reviews' started by zack, Jan 16, 2013.

  1. zack

    zack
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    Just to satisfy my curiosity about the actual behaviour of a JCM800 following this post here:
    http://forum.fractalaudio.com/axe-fx-ii-discussion/62898-does-sound-right-you.html

    I pulled my trusty original 80's 2204 JCM800 50 watt head out of the loft today and took it to the rehearsal room...

    It's fair to say it's seen better days - it's been 7 years since I last tested it and more like 22 years since it was last used in anger...and what with the strong smell of burning, the faint blue glow in one of the EL34's and the fact that none of the controls work except the preamp gain ( master vol is stuck at 10 !!) it was quite exciting getting it up and running again!

    Anyway, bottom line is that the amp model behaves pretty exactly like the real thing - got the same white noise 'interference' sound during the attack phase of the note that was concerning me- identical really.

    I tell you what though, I totally lost any nostalgia I might have had for my old tube amp today, the whole experience for me was akin to playing an old 78 grammophone - a novelty at first then thinking thank goodness things have moved on somewhat - thanks Cliff!

    Cheers

    Zack

    PS: dodgy iphone video of the test:
     
    immortal_soloist and Spartacus like this.
  2. teddis

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    Sounds like she's gonna blow !!!
     
  3. FractalAudio

    FractalAudio
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    That "interference" is due to all the treble peaking used in the design. JCM800s have quite a bit of treble boost. When you first attack the string the note is unpitched. It's basically a brief explosion of noise. The treble boosting amplifies the upper frequencies of this noise burst which is what you hear.

    However, when you playing in a loud mix that treble boosting helps the sound cut.

    Modern designs use more carefully crafted treble boosting to retain the cut but tame some of the harshness of the attack. This is accomplished typically by putting a capacitor in parallel with the plate resistor(s) which rolls off around a few kHz. This preserves the treble boost in the upper midrange but softens the attack. I believe the SLO100 was one of the first amps to do this. You can see this in the Axe-Fx II as your Triode Freq parameters. You can soften the JCM800 attack by lowering the Triode Freq values. Most modern hi-gain designs use some form of HF rolloff like this.

    Another reason is that JCM800s use relatively low amounts of negative feedback (which is why they're so loud). This causes a treble boost in the power amp. Increase the Damping parameter to increase the negative feedback.
     
  4. zack

    zack
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    Thanks for the explanation - I guess this that is why the bright switch accentuates the noise?

    Appreciate the tips too - time for me to get tweaking!

    Cheers

    Zack
     
  5. FractalAudio

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    Yes, because that adds even more treble boost.
     
  6. connor66

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    I'll second the thanks from Zack, that's a much appreciated response and very helpful to boot
    Gary
     
  7. iggypop

    iggypop
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    Great explanation, Cliff, thank you.
     

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