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    Driver Tube Options ST-120

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    WntrMute2

    Posts : 96
    Join date : 2010-11-21

    Driver Tube Options ST-120

    Post by WntrMute2 on Sun Jul 31, 2011 10:23 am

    So the tube guru in our group gave me a few driver tubes to roll into the center position of my ST-120 and I wanted opinions and advice. He thinks there may be sonic benefits. Here is a list:

    1) E180CC (IBM 5965)
    2) 6829
    3) 12AU7
    4) 12AV7

    Any opinions as to pro vs con? Also, can I exchange the center tube for any of these without damaging the amp? Lastly, do I need to turn the amp off before swapping out the center tube or can I just pull it and replace it "on the fly".

    Thanks in advance.
    Dave

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Driver Tube Options ST-120

    Post by Bob Latino on Sun Jul 31, 2011 11:55 am

    Hi Dave,

    Always shut the amp off and wait 30 seconds to a minute before you replace any tube in any tube amp. During the wait period the high voltage will bleed off and you won't get a transient spike when you pull the tube. Of course use a cloth or something to pull the tube so you don't burn your fingers.

    The 6829, 5695 and 12AV7 are plug compatible with the 12AT7 and will work in the amp in the center position. They are also somewhat lower in gain than a 12AT7. How they will sound in comparison to a 12AT7 is anybody's guess? I have never used any of these three tubes in my own ST-120. The VTA board on the ST-120 has been optimized for 12AT7 tubes.

    My personal favorite tube on the ST-120's center position is the Mullard CV4024 which is a 12AT7 equivalent tube.

    As an aside > Substituting LOWER gain tubes into a circuit usually does no harm BUT how good it will work is questionable. From personal experience I would never substitute a HIGHER gain tube in a circuit. IOW never substitute a 12AX7 tube (mu = 100) for a 12AT7 (mu = 60) Higher gain tubes can sometimes cause an amp to oscillate, squeal, motorboat or become unstable especially at higher volume levels.

    Bob


    WntrMute2

    Posts : 96
    Join date : 2010-11-21

    Re: Driver Tube Options ST-120

    Post by WntrMute2 on Mon Aug 01, 2011 7:07 pm

    Right now I've rolled in a 12AU7 Eico labeled(Mullard) tube in the center spot and am digging the sound! I am not an especially discriminating listener but the combo seems great to me. Bass still the same but midrange and treble extension are really nice. Try it iF you have some 12AU7s lying around. Did not notice a big difference in volume but it is a lower gain tube than the 12AT7. Just another option for Bob's great amp!

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