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Could an active bass blow a tube in my Avalon?

Discussion in 'Bass' started by thatjeffguy, Mar 10, 2010.

  1. thatjeffguy

    thatjeffguy Active Member

    Had a session the other night. Plugged the bass into my Avalon 737 with the Avalon set to "Instrument".
    Was getting signal thru the Avalon into my board, where I set the trim to appropriate level. Recording went fine over the two hour session.
    I only found out later that the bass was an "active" bass with it's own preamp circuitry. Don't know what brand.
    The next day I went to do a vocal through the Avalon and it wasn't working. It would pass only a very low-level signal through and no amount of knob twisting affected it.
    Put in a new set of tubes and all is well.
    So, is it coincidence that the tube blew or did the active bass cause it? I would have expected the active bass to perhaps distort the input but not blow a tube.
    What do you think?
     
  2. dvdhawk

    dvdhawk Well-Known Member

    The Avalon specs a max. instrument input level +30dB - that's a ripping pile of headroom. And as you've alluded to, you would expect distortion if it were somehow out of range. I can't imagine it's possible the input level from any active bass would be outside that range.

    That being said... If it was a really percussive style of bass playing I believe it could shorten the tube life. Guitar players like Stevie Ray Vaughn had to re-tube their amps constantly due to that cleanish percussive high-volume style.
     
  3. thatjeffguy

    thatjeffguy Active Member

    Thanks for the reply, dvdhawk!

    The perplexing part is that the resultant bass tracks were consistent in level throughout the session, so whatever happened must have happened right when we plugged the bass in.

    Anyway, all is well now and I have learned that it is probably a good idea to inquire as to whether an instrument is active or not before plugging in!

    Jeff
     
  4. Boswell

    Boswell Moderator Distinguished Member

    Whether it's possible to damage the tubes in the pre-amp by excessive signal level is largely a matter of how the Avalon has been designed. I don't know the circuit details of the 737, but it could be that to get the distinctive acoustic colour, the tubes are operated in a mode that results in grid current flowing on the positive peaks of large signals. If this is the case, the tubes will poison and require replacement from time to time. As Hawk said, some top players get through tubes quickly, and probably changed them as often as they changed their guitar strings. You may have to accept it as an operating expense, although it's hard to see how your particular pre-amp deteriorated suddenly overnight.
     
  5. Davedog

    Davedog Distinguished Member


    That and the fact that they were always screwing with the voltage to the amps.....Try a Blackface Super with only 85 to 90 volts to it. Brown sound fer sure. And no tube life at all.

    I believe this is total co-incidence. Theres too much pad available at the input of a 737 and as Hawk said, you'd know the instant it was plugged in if there was too much input.
     

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