Which tube is the 1st gain stage? How do I know so I can try a different tube?

Discussion in 'Amps and Cabs [BG]' started by mr80htz, Apr 27, 2019.

  1. mr80htz


    May 15, 2018
    I have a Traynor YBA 1 Bass Master and want to try something different. I want to replace the 12ax7 in stage one with 12 ay7 but don't know if the tube closest to the power tubes is the right one or the pre amp tube on the opposite end?
  2. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Generally closest one to inputs.
  3. Wasnex


    Dec 25, 2011
    There is quiet a bit of variation in gain and tone between various 12AX7 types. ECC83, ECC803, 7025 are all basically names for common 12AX7 types. If you want slightly lower gain you can also try 5751.
  4. ThisBass


    Aug 29, 2012
    Can you tell us the reason why you want to change (on your own) the initial stage of the amplifier?

    edit, guitar preamps initial stage are sometimes rather less cathode biased which may make tube rolling noticeable (at some content).
    Bass guitar (and all purpose) amplifiers are most often "standard" biased at the initial stage which may make tube rolling like an esoteric science.
    Last edited: Apr 27, 2019
  5. lowendrv


    Dec 12, 2007
    Wasnex likes this.
  6. mr80htz


    May 15, 2018
    Thanks for this...this is for the anniversary tribute head though . I have an old one where the 3 pre amp tubes are in a straight line. I'm guessing that the one furthest from the power tubes is V1. Thank!
  7. mr80htz


    May 15, 2018
    The amp starts smacking it's headroom ceiling at around 3 /10. I want to have a wider volume sweep that's usable before breakup.
  8. ThisBass


    Aug 29, 2012
    The cathode resistors for V1 and V2 equals 820 Ohm which predicts a rather highish (total) gain.
    Furthermore the vol.-pot is directly (AC coupled) wired to the anode resistor of the initial stage which then (also) improves the total gain structure.
    Obviously the preamp schematic was designed with respect to generate some distortion.

    I don't think that tube rolling alone will bring you to the desired gain characteristica.
    It might be beneficial to mod the schematic a little bit just to meet your personal taste and needs.
    This mod could be easily done by an experienced service tech.
    Modifications should be done with care and respect to provide the possibility to also bring the amplifier back to original specs
    Last edited: Apr 28, 2019