Differences between 12AU7 vs 12AX7?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by sniffin, Jun 12, 2008.

  1. sniffin

    sniffin Member

    Messages:
    384
    Joined:
    Aug 3, 2006
    Location:
    Italy
    I've just dismantled an old Italian tube amp which has 4 x EL84's, 1 x 12AT, 2 x 12AU7's, 2 x 12AX7's and one last tube that I can't figure out. All the tubes are mullards branded Brimar (not sure about the EL84's) anyways the mystery tube is a mullard too, but there aren't any writings on it so I was wondering if there is any way to tell whether it's a 12AU7 or a 12AX7 simply by looking at it?

    here are some pics:

    http://www.flickr.com/photos/73835524@N00/2567388602/

    http://www.flickr.com/photos/73835524@N00/2567388604/

    thanks
     
  2. rockon1

    rockon1 Member

    Messages:
    11,479
    Joined:
    Aug 12, 2005
    Location:
    Stamford CT

    If in fact it is a Mullard hopefully the etch code will tell you. Many 12AU7's appear identical inside. Heres a link. Bob

    http://www.tube-classics.de/TC/Tubes/Valvo ECC82/ECC82.htm


    ....

    http://www.tube-classics.de/TC/Tubes/Valvo ECC82/ECC82.htm
     
  3. sniffin

    sniffin Member

    Messages:
    384
    Joined:
    Aug 3, 2006
    Location:
    Italy
    thanks for the reply, there is no code whatsoever on the tube, that's probably why it's so hard to recognize, are there any visible differences inside the tube that would tell me whether it's a 12AU7 or a 12AX7?
     
  4. SatelliteAmps

    SatelliteAmps Member

    Messages:
    6,185
    Joined:
    Oct 19, 2007
    Location:
    Normal Heights, Calif
    Hard to say, but in the pictures it looks like a 7 pin tube, which would be neither.
     
  5. Blue Strat

    Blue Strat Member

    Messages:
    30,036
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Yeah, I only count 7 pins too but I suspect that it's the position of the tube, pins doubling up so you can't see them all.

    No, there's no way to ID a 12AU7 from a 12AX7 unless you know for certain what the brand is AND you know that that particular brand used different plates for the different types.

    The photo kinda looks like an RCA something or other.
     
  6. sniffin

    sniffin Member

    Messages:
    384
    Joined:
    Aug 3, 2006
    Location:
    Italy
    I didn't notice that, but actually one ho the tube pins has snapped off so it actually has 8 pins. Anyways I've resolved the mystery by looking at the grid wires through the small holes in the plates.... the 12AU's have thicker grid wires and seem to be speced out wider.... also the shafts that have the grid twisted around them are made of copper.

    On the 12AX the grid wires are thinner and closer together and the grid shafts are made of steel or another plated metal but not copper... so that makes this mystery tube a 12AX7!!
    thanks for all the help!
     

Share This Page