12AX7 substitutes

Discussion in 'Amps and Cabs [BG]' started by MarkMcCombs, Jan 11, 2001.

  1. My amp/head has 2-independent channels (preamps), both with a 12AX7 in them. I love the distortion I get from both channels, but would like to have one of the channels run a bit cleaner. Some other boards I've seen mention replacing this tube with a 12AU7 or 12AT7 or some others that have less gain. Would this give me a cleaner sound? Or would it just require me to drive the power amp harder and thus pick up some distortion there? (the power amp section is 4 7027s).

  2. bassmonkeee

    bassmonkeee Supporting Member

    Sep 13, 2000
    Decatur, GA
    Hey, I had an SWR400 that I replaced the 12AX7 with a 12AT7. You definitely get a cleaner sound. I never had any problem with power amp distorting, either. I had to turn the input gain up a little, but that was about it. Granted, the SWR had a solid state power amp, but I don't know if that should make a difference. If you have two preamp sections, I say go for it--you can always put the 12AX7 back. Of course, if you haven't ever changed a tube before, I suggest you have a tech do it. Good luck!
  3. I don't think changing to a 12AT7A will get you what you want, but it's worth a try. You may have a bad 12AX7, in which case a good 12Ax7 will clean up the sound. On the V4B, if you're turning up the volume past about 11 o'clock, then it's going to distort. If it's distorting before you turn the volume up to that point, something's not right.

  4. Hey Chris,

    yes, it's a v4-B, good catch! It does distort more than I'd like after about 11, definitely 12, o'clock. I'm after a way to have one channel cleaner....any ideas? Some posts from long ago mentioned changing power tubes to 6550's, but that would cost quite a bit just for tubes, then add an amp tech's time to that for biasing, resistors, etc, and I'm probably out $300. Can't do it.
  5. I don't think you can get any more clean power out of that thing. Your amps ok if it gets dirty between 11 and 12 o'clock. That's normal. I think you are really hitting 100 watts output at that point, so the growl you are hearing is from the power amp (7027A's), not the just the pre-amp (12AX7's). Putting in 6550's is not gonna change that much at all. Long story short, if you haven't got enough volume with the amp at 11 o'clock, you either need a different cabinet or a different amp. I dig that V4B at 11 sound, and I use huge cabinets, so I've got plenty of volume. Not much more you can do with your amp to get more clean volume, though. If you decide to sell it, tell me, I'd love to have another one. :D You may be one of those folks who has to have 400Watts of solid state power to get a very clean loud sound that is to your liking. I love distortion, and wouldn't play through anything else but tubes, myself.