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400+ running on less tubes?

Discussion in 'Amps and Cabs [BG]' started by K-Funk, Mar 17, 2008.


  1. K-Funk

    K-Funk

    Sep 24, 2007
    Auburn Hills, MI
    Just curious, but if a tube goes out at a gig and I don't have a spare, can I just pull it and its match out and keep going, or will this hurt the amp? is there any long-term detriment for running it for extended periods with missing tubes? thanks.
     
  2. K-Funk

    K-Funk

    Sep 24, 2007
    Auburn Hills, MI
    bump?
     
  3. fenderhutz

    fenderhutz Supporting Member

    Jan 28, 2007
    Harpers Ferry WV
    Wow with the Mesa auto biasing I have no clue.
     
  4. For the sake of avoiding a confusing misinterpretation (at the least ;)), let's not use the term "auto biasing" WRT Mesa amps.
    They don't bias automatically; but, rather, are a non-adjustable fixed bias scheme.

    I'd just hate to see someone new to the instrument figure "cool, it biases automatically" and suffer consequences from that.

    Then again, perhaps I just jumped and you already knew that. ;)
     
  5. fenderhutz

    fenderhutz Supporting Member

    Jan 28, 2007
    Harpers Ferry WV
    Well yeah, contact Mesa and buy the #3 tube.

    Sine they don't sell the 400+ tube kits anymore I guess the "Auto" biasing is gone.
     
  6. K-Funk

    K-Funk

    Sep 24, 2007
    Auburn Hills, MI
    see.. I know with the SVT and it's "compete tube protection" thing, a resistor is supposed to blow when a tube shorts out, which lets the amp continue running at 5/6 power without a problem (or so says The Tube Amp Book)

    What about the 400+? does something protect it if a tube dies? and if something does happen, can I take the tube (and it's match) out and keep playing (and if so, for how long before it causes trouble?)
     
  7. chaosMK

    chaosMK

    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp
    I've been running my 400 with 4 (out of 6) KT-88's for at least 6 months, maybe longer.
     
  8. anderbass

    anderbass

    Dec 20, 2005
    Phoenix. Az.
    I've read a few posts by 400+ owners successfully pulling a non-functioning pair for fairly brief periods of time.
    I dont know if anyone is gonna know exactly how long you can get away with it. That's kinda like asking "how long can I get away with driving 60 miles an hour through school zones. (maybe quite a while... maybe not)

    I believe there's some references to the effects of power tube pulling in this thread. (Psycho Bass Guy and Jerold's posts)
    http://www.talkbass.com/forum/showthread.php?t=238662
     
  9. If you lose a single power tube during a gig, chances are good you won't even notice it. Besides, how do you intend to determine which tube to pull if you think something may be wrong?

    Dave M already covered it, but since alot of people on TB only skim the threads, it bears repeating that Mesa amps do NOT "auto-bias;" they simply have preset bias voltage (that is often wrong, BTW)

    The screen grid resistors act as fuses in the event of a power tube short. Instead of a shorted tube drawing more and more current until the mains fuse blows, the single resistor will burn up and turn the tube "off." It isn't healthy for the amp in the long term to run with an unbalanced power section, but it's worse for your speakers.

    Tubes just don't generally up and die. As I said before, if you lost a single 6L6 in a Bass 400+, you probably wouldn't even know. Why are you worried about it?
     
  10. K-Funk

    K-Funk

    Sep 24, 2007
    Auburn Hills, MI
    not necessarily worried, I just want to know as much about my amp as possible. I have almost no knowledge on the subject of amp function and am really interesting in preventing issues in the future, caused either by the amp or my sheer lack of experience.
     
  11. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    guitarists do it all the time with 100 watt heads to tame the volume and/or overdrive the power section. but the trick is that you have to half the impedance setting on the amp to keep things properly matched - so if you were playing with the amp set to 4 ohms, you'd need to set it to 2. BUT, this is assuming you have 4 power tubes and are pulling 2 of them. if you have a sextet, not sure what you would do, probably not a good idea. with 12 tubes, i would imagine you need to pull six of them to get the impedance correct. at that point, you'd be better off just having a backup amp.


    JR
     
  12. K-Funk

    K-Funk

    Sep 24, 2007
    Auburn Hills, MI
    does the 400+ have screen grid resistors, or does the main fuse just blow?
     
  13. Most tube amps have screen resistors, but the SVT is the only one I can think of that designed them to work that way.
     

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