preamp tube swap?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mojoworker, Feb 10, 2012.

  1. mojoworker

    mojoworker Member

    Jan 22, 2012
    i own a small amp with 2 x el84 and 2 x 12ax7 plus the rectifier. if i wanted to achieve a change in headroom by changing out one of the preamp tubes for a lower gain tube is it the phase inverter that should be exchanged or another position? for the novice is the proximity to the power tubes enough to identify the phase inverter?

    thanks -

  2. Prattacaster

    Prattacaster Member

    Jul 12, 2010
    Nashville, TN
    Usually the case.
    Well it helps to have a good idea where the overdrive is coming from. Another thing is are you trying to achieve more headroom for a live band situation or just a home recording. For a live band situation you would be better off buying a bigger cab or a more efficient speaker(s).

    As far as the tubes go, a 12ay7 in first position would clean things up because a 12ay7 is an inherently higher headroom tube so the second gain stage wont overdrive as much. You should be able to get a cleaner sound at the same volume level but you'l have to turn the volume knob up a little higher to make up for the gain difference. If it seems like a lot of your overdrive is coming from the power amp you can try the PI(phase Inverter) slot as well. Maybe a 5751 would do the trick. The only thing is to get the same volume you will have to turn it up which could overdrive succeeding stages and lower your headroom. It all depends on your specific circuit. Just put them in and see for yourself.
  3. Blue Strat

    Blue Strat Member

    Aug 2, 2002
    Sterling, VA (not far from Washington DC)
    What Prattacaster said. Once you get about half way up on your volume control, you run out of power amp headroom in most amps. In this case, nothing you do in the preamp will help but will only create the illusion of more headroom because you can turn the volume CONTROL up to a higher number. The actual volume will be the same when you run out of headroom.

    Generally, if your pickups are single coils you won't run into preamp distortion earlier than power amp distortion. With humbuckers you're more likely to.

    Here's a quick test: Play your amp the with your typical settings (where the amp starts breaking up) on the amp with the guitar volume all the way up. Now, turn the guitar volume back to about 8. The amp should be cleaner but the volume lower. Now, turn the amp volume up to match your previous loudness. If the amp is now cleaner, the headroom issue is in the preamp and lower gain preamp tubes will help. If not, nothing is going to help, you need a more powerful amp.

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