12AT7 in V4B?

Discussion in 'Amps and Cabs [BG]' started by Soulfinger, Apr 22, 2017.

  1. Soulfinger


    Sep 20, 2004
    Houston, TX
    The problem with my '73 V4B is that the volume knob is so touchy. The total range of the volume knob that I use only goes to about the 9-o'clock position, particularly when using an active bass (an EBMM Sterling, which isn't even that high of an output as active basses go). Could I use a 12AT7 as a preamp tube to lower the overall gain and get more range out of the volume knob?
  2. Probably not.
    You say the problem happens especially with an active bass.
    That should be your clue that it may not a tube problem.

    Have you tried any attenuation with the active to see if that helps?
    Do you have an active passive switch, or seperate active passive inputs on your amp?
    Or have you tried the active bass volume at a different place? Lower?
    This might help to get your amp volume into a better range.

    Also it may be a failing volume pot if it's "touchy."
    Does the volume cut out or jump when operating the pot through it's range?
    Or does the pot have any scratchiness to to?

    Also, what do you mean by "tonal range" of the volume knob.
  3. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    The V series use a linear volume pot, the usable range is going to be shorter and the taper different than a log pot which you might be more used to. Try backing off the volume on your bass instead of on the amp and see if you can get a little more control.
  4. Biggbass


    Dec 14, 2011
    Planet Earth
    Doubt a tube swap will accomplish your goal. Might just want to replace the volume pot.
  5. jnewmark

    jnewmark Just wanna play the groove. Supporting Member

    Aug 31, 2006
    Stax 1966
    Third St. Cigar Records staff musician.
    I don't know if it's the solution to your problem or not, but, a 12AT7 is sometimes used to replace 12AX7s where lower gain and more note definition/clarity is desired.
  6. jmattbassplaya


    Jan 13, 2008
    Yep. I did this in my Fender Blues Deluxe amp. It made the volume jump from 1-4 much more useable.
  7. ThisBass


    Aug 29, 2012
    The problem is caused by the lin pot only. Same issue is for svt amps of the same time line.
    There is no need to swap the tube cause schematic wise there is no issue with the tubes.
    If a 12AT7 makes a lot of difference in level then a 12AT7 will very likely also makes lots of difference for the bias point. May be the schematic has to be modified to get correct bias again with a 12AT7.

    The Treble Boost is directly linked with the volume pot. So a 12AT7 will cause a very different sound characteristic if the Treble Boost is engaged.

    If your car front wheels do lack of some air you don't want fill air in the rear wheels.
    Last edited: Apr 23, 2017
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