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12AU7 instesd of 12AX7

Discussion in 'Amps and Cabs [BG]' started by brock29609, Jul 16, 2004.


  1. brock29609

    brock29609 4 strings, 2 wheels

    May 11, 2003
    Greenville, SC
    Is it just me, or does a 12AU7 tube make amps sound FAT as hell. Why don't more bass players us them? They're interchangeable with the 12AX7, right?

    Agree? Anyone have any 12AU7 tube experiences?

    Since they're low output, do they work better with active basses than passive?
     
  2. They're not really interchangeable, much lower gain for the 12AU7....they will "work" sort of but may not be enough gain for that particular circuit.
     
  3. If you want more headroom with less gain I would go with a 5751 and yes it may sound "fatter".
     
  4. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    The 12AU7 isn't very common in tube guitar amps. It is (occasionally) used in the phase inverter position, for instance the Marshall Major needs one to drive all those 6550's. But it's rarely used in the preamp section. More common would be the 12AT7 and 12AY7. The 12AY7 in particular is a very nice sounding tube. But, it depends on the amp. What amp are you using it in? And what exactly do you mean by "fat", more low end, with maybe a little less sparkle?
     
  5. What I mean by "not interchangeable" is that you wouldn't want to sub a 12AX7 for a 12AU7, it'll possibly be far more gain than the circuit was designed for. Some circuits may tolerate it, some can't. Going the other way--subbing a 12AU7 for the 12AX7--is more likely to be tolerated by the amp since there's less gain. But it's not a direct interchange, the gain would be so much less.

    Kinda like putting a 25 watt light bulb in an overhead light where there was a 100 watt bulb. Sure the bulb screws in, and it produces light, but it's a lot less light and you might not be able to see very well.

    There's really not much rhyme or reason to tube naming conventions. The 12 means the heater filament is 12 volts, so a 12 AX7 and 12AU7 and 12AT7 all use the same heater voltage and they all fit the same base. But beyond that... it's not like saying that the 12 AX7 is a heavy gauge string and the 12AU7 is a light gauge string...More like the 12AU7 is a violin string and the 12AU7 is a bass string, sorta similar but different.
     
  6. brock29609

    brock29609 4 strings, 2 wheels

    May 11, 2003
    Greenville, SC
    I'm using it in a Genz-Benz GBE-600. It has both a FET and a tube pre than can be blended together. The 12AU7 does have less sparkle and it's more round sounding. Blended with a little FET pre it makes for a nice combination. Am I OK using this tube in this head? I'm using an active bass with more gain, plus the tube pre has it's own volume knob so I can boost the tube side of the two preamps.
     
  7. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Yes, electrically speaking, you're perfectly safe. If you're into experimenting with tubes, the 5751 that people mentioned might well be worth a checkout. Also the Mullard CV4024. But the 12AU7 will work fine too, if you're happy with the sound that's what counts. :)
     
  8. ........in a recent thread here, someone was saying they got good results putting 12AU7's in the pre-amp section of their vintage Fender bassman head. Not too much loss of volume and a fatter sound.
    My only experience with 12AU7's is in a rare Guyatone valve bass compressor pedal that I have. I replaced the 12AX7 in this pedal with a 12AU7 ( the recommended valve ) and the difference was remarkable. It no longer had any hiss at high settings and the sound was rich and fat.
    I imagine that the 12AU7 would handle active basses with hot outputs in a lot cleaner manner than the higher gain 12AX7's.

    As has been said: if it works for you, then go with it!
    :bassist:
     
  9. brock29609

    brock29609 4 strings, 2 wheels

    May 11, 2003
    Greenville, SC
    A 5751 might be worth checking out. Thanks.