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Swap 12ax7 for 12au7?

Discussion in 'Amps and Cabs [BG]' started by fuzzy beard, Jul 23, 2012.


  1. As most of you must be tired of hearing me talk about my Fender Bassman 20 i have a new question!

    I was told some people like the 12au7 for clean bass tone over the 12ax7. Now my amp is very clean anyway but would it make a differance?

    Are there any pro's or con's?
     
  2. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    The 12au7 has less gain so it would stay cleaner higher on the dial. But personally I think you're just moving the distortion point from a lower spot on the dial to a higher spot on the dial so I never saw the point of it.
     
  3. Well it doesnt matter to much. At 10 on the volume i have to dime the bass and dig in to get some breakup. And i think in this cab i get more speaker fart than breakup:)

    Just didnt know if there was any advantages.
     
  4. I've had good results doing that particular tube swap on a guitar amp, but I like clean guitar with almost no breakup at all.
     
  5. How do i find out if they are bias'ed the same? The amp is fine as it is. But if i could just pop in a set and hear what they do with out having to get the head serviced i would give it a go.

    Head was serviced a few months back with all new JJ tubes.
     
  6. LowEZ

    LowEZ Supporting Member

    Mar 29, 2011
    Central NJ
    Yep, the line out is tapped from the NFB loop and the volume control very much affects this level.

    Into the input of another amp or power amp. You just have to watch the level as it's pretty hot. I used to do this with a Champ II which is essentially the guitar version of the Bassman 20.
     
  7. bigmuff113

    bigmuff113

    Jul 4, 2012
    San Antonio
    You only need to bias power tubes. Preamp tubes not at all
     
  8. So i could put new pre tubes in it at any time with no i'll effect?
     
  9. So what are the down sides of a hot signal?

    Also it is a plus to have my line out after the power tubes wouldnt you say?
     
  10. bigmuff113

    bigmuff113

    Jul 4, 2012
    San Antonio
    Yup
     
  11. LowEZ

    LowEZ Supporting Member

    Mar 29, 2011
    Central NJ
    Just make sure you're only swapping V1.
     
  12. Thanks!
     
  13. Why? Dont want to be a pain just wondering.
     
  14. LowEZ

    LowEZ Supporting Member

    Mar 29, 2011
    Central NJ
    Just that you can overdrive or overload the input of whatever you're connecting it to.

    I would.
     
  15. LowEZ

    LowEZ Supporting Member

    Mar 29, 2011
    Central NJ
    V2 is the phase inverter tube, not a pre-amp tube. You do not want to change the value of the phase inverter.
     
  16. Cool thanks for the info! I learn alittle more about these tube amps all the time!
     
  17. Ok new problem. (That didnt take long)

    Went back and looked at the schematic to see wich one was v1 and wich was v2. It shows 2 v1 tubes and 2 v2 tubes. I ony have 2 pre amp tubes! I dont have 4 so do i have a bad schematic or am i reading this wrong?
     
  18. You can swap any 12A_7 tube without ill effect. They are all dual triodes with the same pinout. It is a pretty common thing to do on guitar amps. A google search on "12AX7 swap" will probably cough up a chart showing the different gains of each. 12AX7s have the highest gain of this family of tubes. My preferred replacement is the 5751. It is the most similar to a 12AX7 but with only 70 % of the gain.

    But if your amp is already totally clean, any swap will just make it less loud.
     
  19. LowEZ

    LowEZ Supporting Member

    Mar 29, 2011
    Central NJ
    Two triodes in each tube. So, V1A and V1B are both in the same physical tube, V2A and V2B are both in the same physical tube. Four triodes, two tubes.
     

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