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preamp tube question

Discussion in 'Amps and Cabs [BG]' started by phoenix742, Sep 28, 2009.


  1. phoenix742

    phoenix742

    Apr 19, 2008
    Portland, OR
    So I have two Marshall Valvestate amps... one is a valvestate 8080 112 combo, and one is a valvestate 8100 head. They both started having problems at roughly the same time, the combo would occasionally howl and feedback, and the head was having volume drops and swells, and I figured replacing the preamp tubes would probably fix the problem.

    When I opened them up, I realized they are pretty much the same amp internally, the only real difference is one is 80 watts and the other 100. They had different kinds of tubes, but when I bought new tubes I just put the same kind in both amps.

    Now we get to the problem. Both amps now have VERY low volume on the gain channel. The clean channel works normally, but the gain channel is just not there. Anybody have any ideas? This is the first time I have ever replaced tubes before, but it kinda seemed hard to mess up...

    Thanks in advance.
     
  2. You did use a 12AX7/ECC83, didn't you?

    If you used any of the other small twin triodes (12AT7, 12AU7, 7025 etc), they'll work as they have the same pinout, but the lower gain (much lower in the case of the 12AU7) will lead to the result you describe.
     
  3. jastacey

    jastacey Supporting Member

    Feb 8, 2009
    Houston,Tx
    Did you check your cords?
     
  4. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses

    a 7025 has the same output gain as a 12AX7 only it is 'supposed' to be lower noise .
     
  5. 7025 IS a replacement for 12AX7(a,wa ect.). Are you sure the pins are seated? You can bend pins down if the tube is forced in when not properly aligned.
     
  6. jetofuj

    jetofuj

    Jun 16, 2008
    PL
    Only gain channel uses preamp tube, clean channel has solid state preamp.
     
  7. RickenBoogie

    RickenBoogie

    Jul 22, 2007
    Dallas, TX
    Aren't those both guitar combo's?
     
  8. phoenix742

    phoenix742

    Apr 19, 2008
    Portland, OR
    yeah, they are both for my piccolo bass.....

    yes, i checked all cords...

    and I swapped the old Groove Tube into the head, and the problem went away... the new tubes I bought are Electro Harmonix, I don't know if that really makes any difference. I don't think any of the connecting pins bent, everything looked fine when I pulled them back out.

    At this point, I'm just gonna take them back and maybe get some Mesa tubes or something. The combo had a Mesa tube to begin with, so it should work for sure...
     
  9. hrgiger

    hrgiger

    Jan 11, 2009
    What are the white numbers that are printed on your new tubes???
     
  10. phoenix742

    phoenix742

    Apr 19, 2008
    Portland, OR
    the new tubes i put in are 12AX7 EHG. I'm gonna try some mesa tubes and see if that helps...
     

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