Anyway to reduce amp compression?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by devbro, Jun 1, 2005.


  1. devbro

    devbro Member

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    I have a Triaxis / 2:90 that is too compressed. I guess I never noticed it until I bought a Shiva.

    What causes this?
    Is there a remedy?
     
  2. justonwo

    justonwo Member

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    I think amps generally become more compressed as they are biased hotter. I think one solution is to bias the amp a little colder.
     
  3. Blue Strat

    Blue Strat Member

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    True. Also, if the amp has a tube rectifier, using a higher rated tube recto or solid state rectifier will reduce compression.

    http://kcanostubes.com/content/newsletter_details.asp?ArticleID=3
     
  4. Swarty

    Swarty Member

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    It depends on where the compression is occuring. If it is in the preamp, some lower gain tubes may help. If it is the output tubes, higher power tubes will help. If it is the output transformer a better OT will help.
     
  5. John Phillips

    John Phillips Member

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    I'm reasonably sure with that rig it will be the Triaxis; unless you're running the 2:90 in 'Class A', anyway. If you're running it at full power it's a huge-sounding and very uncompressed power amp.

    The Triaxis I tried did seem very compressed and 'insensitive' - as I said in another thread, I think it may be deliberately designed like that to make it more consistent in complex setups... IMO it imposes its own signature tone on anything that goes into it, despite the large number of apparently different tones it can produce. I didn't like it actually - I much prefered the Formula, or even the older Studio Preamp (which is a 2U rack, so you may not have room for it - and if you have, you could try the new Rectifier Pre too).

    I do feel that all pre/power amp combinations I've heard are like this to some extent though - a single amp always seems more dynamic, responsive and just plain 'right' sounding. I don't really know why, but it does seem to be... even if the amp has an FX loop so it's really the same configuration as the pre/power setup.
     
  6. Ayan

    Ayan Member

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    That's my vote as well. Like the earlier Boogies that it was in part based on, the TriAxis is a very compressed sounding preamp.

    I don't think anyone will be successful trying to dial that out by pairing the TriAxis with different power amps.

    Gil
     
  7. devbro

    devbro Member

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    Thanks for all the feedback. My 2:90 is "fixed bias" and I don't think I can adjust the bias to run any cooler. I run the 2:90 wide open and the triaxis at about 5 with a volume pedal in between to act as a master volume.

    It sounds like the issue is with the Triaxis. I was hoping that someone had experience with different tubes or possibly a mod that could reduce this compresssion.

    :(
     
  8. mesafied

    mesafied Guest

    Maybe mod the triaxis...
     
  9. Reeek

    Reeek Member

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    Fixed biased is adjustable. It's just not cathode biased.
     
  10. markdurham

    markdurham Member

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    One small note here is that most (all?) Mesa power amps that are fixed bias don't have a bias pot to change the bias voltage. So the bias voltage is "fixed" in that case. They require the use of power tubes that have been matched to run cooler or hotter at that fixed bias voltage. The bias resistor can be replaced with a pot for true adjustable fixed bias.
    markd
     
  11. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    It's not a difficult mod to add bias pots.
     
  12. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    [I had to transform from long-time lurker to a participating member because this post asks a question I've focused on for years.]

    I've been a Triaxis user for over 12 years, and ran it into a 2:90 (with the adjustable bias mod, which I recommend) for 'bout half that time (now using a Mesa 20/20). And I've been an intensive tube tweaker, mostly using the Triaxis, for something like four years.

    In my opinion, the stock Triaxis is very compressed, and doesn't lend itself to touch-sensitive subtleties as well as most heads/combos. In devbro's case, I recommend looking at the Triaxis as the place to focus efforts. I set out to change my Triaxis compressed, buzzy, and overly-gainy tendencies, using tube substitutions, and found that the compression could be vastly improved with one inexpensive tube change: put a 12BZ7 in the V5 (output) position.

    The 12BZ7 helps provide a more open, dynamic and louder signal while reducing the noise floor. I also use one as the main input valve for my 20/20 amp. It works better than any of the over 30 12AX7 variants I've tried in V5. Note that the tube draws slightly more current than a 12AX7, but it works fine in the Triaxis (and 20/20). The larger size glass (nearly 1/2" longer) is easily accommodated in the Triaxis as well. Using a 12BZ7 will void your Mesa warranty, but it will put a smile on the faces of those who've come to terms with that meager threat. Also note that if you're recording direct or using higher volume effects in the loop, you may want to be careful with the 12BZ7 - you may cause the opamp to clip.

    A couple of other recommended tubes, for more dynamic overdrive (clean to crunch in Triaxis lead modes): Raytheon black-plate/halo-getter 12AX7A and RCA black-plate/square-getter 12AX7. The Raytheon is brighter and twangier, the RCA is thicker, but both transition wonderfully from clean to expressive breakup.

    Here's how I have my Triaxis tubed:
    V1 (Lead 1 input) Raytheon black-plate/halo-getter 12AX7A
    V2 (Rhythm and Lead 2 input; Lead 1 gain) Mullard CV4024/12AT7WA ('80s production)
    V3 (Lead and Rhythm mixer) RCA black-plate/square-getter/3-mica 5751
    V4 (Lead 2 gain) Mullard long-plate/halo-getter (f91 or f92 type) 12AX7/ECC83
    V5 (Master and record output; Effects return) RCA black-plate 12BZ7

    My taste has evolved over the years - I'm much more touch-oriented in my playing. I would have moved on from the Triaxis if not for these tube changes. Hope it helps others explore some fruitful avenues off the beaten path.

    - T
     
  13. devbro

    devbro Member

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    Timbre Wolf,
    Wow, where were you 3 months ago:(
    Ended up dumping the Triaxis and 2:90. Can't say I regret it though, the Shiva is turning out to be everything I ever wanted :D
     
  14. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Glad the Shiva is giving you satisfaction. Hold on to that state of mind - it's precious. I'm an obsessive perfectionist, so I always seek ways of improving what I've got. But every now and then, I like to bask in the magnificence of stunning tone, and put my can't-get-no-satisfaction persona on the slow boat to China.

    Rock on :dude

    - T
     

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