Preamp tube works in one amp - not in another?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Randy, Dec 5, 2005.

  1. Randy

    Randy Member

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    I just tried a GE 12AX7WA in V1 of my Elmwood Modena. I got no signal and saw that only half of the tube was lit up. I figured the tube just shorted out but for kicks I tried another one - same thing. I then tried a Chinese 12AX7 in V1 and it worked fine. Then I tried the GE's in a Marshall and they work fine in that amp.

    Why would these tubes not work in the Elmwood? I've never had a problem with any other tubes - could the pin arrangement be different?
     
  2. VacuumVoodoo

    VacuumVoodoo Member

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    I would suspect V1 socket in the Elmwood. IFINM the sockets are mounted directly on the pcb, check pins 4, 5 and 9 for possible intermittent failure.
     
  3. VaughnC

    VaughnC Supporting Member

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    The pinout for all 12AX7's should be the same. It sounds like certain tubes aren't making good contact with the tube socket in the one amp. Sometimes tube socket contacts can get bent outward and if a tube pin happens to go right down the center of the bent contact, there won't be any continuity on that pin. If this is the case with your amp, sometimes you can take a tiny screw driver and bend the socket contacts back into place. If you're technically inclined, the other option would be to replace the socket itself.
     
  4. Randy

    Randy Member

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    OK - thanks guys, I'll take a look at it. Hopefully I won't have to send it back to Sweden! :eek:

    It just seems so odd that 2 of the same tube didn't work while all my others did. It just occured to me that maybe the pins are just slightly smaller on the GE's and are not making good contact. I'll put a micrometer on them and check it out.
     
  5. TheAmpNerd

    TheAmpNerd Member

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    It always amazes me....

    Well It would be the other way around, if anything.
    The chinese or other pins would be slightly larger.

    HOWEVER

    I you heed the advise given here, you can fix your
    problem. If you ignore their advise, you can go on
    through life thinking that old GE made smaller pins than
    new fangled chinese pins.

    AND

    Don't be the ostridge, either tighten the pins as
    explained or check the internals as explained,

    OR

    Stick your head in the sand.



    :horse
     
  6. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    This may not have to do with tube pins and sockets at all.

    I've had this kind of problem when one of the two triodes goes bad. There are two circuits inside a 12AX7 (dual triode). Seems the Marshall doesn't run through the bad side of your GEs, so enjoy them in that circuit, and find another 12AX7 for the Eastwood.

    - T
     
  7. Randy

    Randy Member

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    Thanks for the help....:rolleyes:
     
  8. AL30

    AL30 Member

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    There are some differences in those tubes.

    http://tdsl.duncanamps.com/show.php?des=12AX7

    The WA is listed down the page under "Different rating or performance"

    This could be your problem OR you may have gotten some bad tubes.

    AL
     
  9. VaughnC

    VaughnC Supporting Member

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    If only one of the two filaments lights, the performance ratings won't mean anything because the cathode has to first be heated by the filament for ANY tube to work. Even if one section of the tube isn't used in an amp, the filaments should still light up.
     
  10. VacuumVoodoo

    VacuumVoodoo Member

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    Not neccessarily. Each section of a double triode tube has its own heater. If only one section is used by virtue of the amp's design the other section's heater might not be connected at all.

    It doesn't seem to be so in this particular case.
     
  11. Randy

    Randy Member

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    Both sides of the GE's lit up in the Marshall, and they've also worked fine in other amps I've tried them in previously.

    Thanks for the comments and suggestions everyone. I'm sure I'll figure it out someday, but for now I'm just sticking with one of the new Tungsols in V1, which by the way is a great sounding tube.
     
  12. VaughnC

    VaughnC Supporting Member

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    True, but he stated that both sections of some tubes light up in the same socket....which means it has to be contact related.
     
  13. JohnnyL

    JohnnyL Member

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    I have a Telefunken ECC83 smoothplate that performs the same as this discussion. One side of it is inactive. Not to worry in my case. I use it in V2 of a Marshall 4210 and get all the goods from this tube. However, I can't switch to 'clean' when using it, but clean is not why it's in the amp in the first place ;)

    Peace,

    JohnnyL
     

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