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12AX7 vs. 12DW7 : Tone, Reliability

Discussion in 'Amps and Cabs [BG]' started by Bron-Y-AurStomp, Mar 21, 2004.


  1. Bron-Y-AurStomp

    Bron-Y-AurStomp Lazy

    Nov 30, 2003
    Plainfield, IL
    Just wondering what the differences are. There's an SVT I've been keeping an eye on that was modified to take 12AX7s instead, apparently. Just wondering what tone implications etc. it may have, or how the modification is even possible. Any info would be awesome, thanks.
    -Russ
     
  2. bassmantele

    bassmantele

    Jul 22, 2003
    Boston MA USA
    These tubes are dual triodes - actually two tubes in one. Each "side" of the tube does a different job in the amp, like two separate tubes. In most tubes, like the 12AX7, the two sides are identical. The 12DW7 has two different sides, one with a gain factor of 100 like a 12AX7, and one with a low gain factor of about 20, like a 12AU7.
    The pinout of the two tubes is the same, so there's no harm done substituting one for the other. It may be that the circuit was modded just to go with the more commonly available AX7. The mod doesn't make much sense to me, but I doubt it was for tone.
     
  3. bassmantele covered the differences between the two tube types in general. In specific to the SVT, one with 12AX7's will overdrive easier and won't have the same amount of clean volume. It's not a significant difference, but it is there and I much prefer the sound of DW's. Also, MOST SVT's I've seen whose owners claim have been modded have in fact just had 12AX7's stuck in place of 12DW7's and while they work, they just don't sound as good and lose low end.

    The SVT's preamp uses the 12DW7 (half 12AX7, half 12AU7) with the "12AX7" side as a gain stage and the "12AU7" side as a cathode follower. A 12AU7 can take a much higher amount of plate current than a 12AX7 so by substituting the "12AU7" side, you increase the gain and lower the headroom, and an SVT already has TONS of gain.

    The only reason the amps have usually been modded in the first place is because the 12DW7 is nowhere near as common as the 12AX7. Aspen Pittman, the owner of Groove Tubes, also recommends the mod in his Tube Amp Book, yet another thing I have against him.
     
    Martizmo likes this.
  4. Passinwind

    Passinwind I Know Nothing Supporting Member

    he SVT's preamp uses the 12DW7 (half 12AX7, half 12AU7) with the "12AX7" side as a gain stage and the "12AU7" side as a cathode follower. A 12AU7 can take a much higher amount of plate current than a 12AX7 so by substituting the "12AU7" side, you increase the gain and lower the headroom, and an SVT already has TONS of gain.

    Psycho,

    In practice, how much more gain can 12AX7 provide in a cathode follower circuit like the SVT's? The circuit itself precludes even unity gain, right?

    In any case, I also prefer 12AU7 for cathode followers, so the 12DW7 seems like a better choice to me too. They're not that hard to come by either.
     
  5. I have two late 70's SVT's one with 12DW7's and on that has the 12AX7 mod and found that the one with 12DW7's overdrives earlier and that the one with 12AX7's seemed to have a little more volume (there may be other factors that I am not aware of affecting this). If you want that true signature midrange growl you will want to go with the 12DW7's but I don't mind the sound of the 12AX7's the sound of that amp reminds of a really powerfull Portaflex. The 6550's can also affect the tone as well (I have GE 6550's with the 12DW7's and Sovtek 6550WE's with the 12AX7's). I have also been playing around with the 12BH7's and lately been alternating between the Tungsol 12BH7A Blackplates and the Brimar 12BH7 Blackplates although the differences are pretty subtle.
     
  6. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The original Mod to use 12AX7 is something either from late Magnavox or early MTI engineering.

    For a cathode follower, no tube gets close to unity gain, because of the grid volts to plate current relationship. But a 12AX7 is inherently not as good a follower if you are driving low impedance loads.

    It will overload differently, at a different input and output voltage. it will have a slightly different transfer function (output vs input relationship). Hence it will sound different.

    If their old mod had simply used a 12AU7 split between and used as the "second halves" of the 12DW7s, it would have been a much closer match to the original.
    We did that on the new SVP-CL. I think its better.
     
  7. Bron-Y-AurStomp

    Bron-Y-AurStomp Lazy

    Nov 30, 2003
    Plainfield, IL
    So is there any real mod that alters the SVT to accept 12AX7's, or are they all just plugged in straight. Does pittman reccomend just plugging in 12AX7s, or does he suggest a mod for them? Thanks for the info, Psycho, your insights have added a lot to my tube knowledge, which didn't even exist barely until I started here. So, um, yea, thanks.
     
  8. Mine is the 4.1 edition. On page 278 he says:

    "Courtesy of the MTI company when they owned Ampeg.

    Most of the common Ampeg bass and guitar amps used 12DW7 preamp tubes, which makes things a little sticky since the tube was discontinued. Have no fear, however, because we have the official Ampeg changeover operation and it's pretty simple to change over and use the very common 12AX7 tube.
    For Models VT-22, VT-40, V-4, V4-B and V-2 simply plug in the 12AX7 in place of the single 12DW7 in the preamp section.
    For the SVT bass amp, change Resistor R-25 to a 4.7K, and Resistors R-7 and R-6 to 220K. Then simply plug in the 12AX7 tube (also fine is ECC83 and 7025)."
     
  9. Bron-Y-AurStomp

    Bron-Y-AurStomp Lazy

    Nov 30, 2003
    Plainfield, IL
    Alright guys, thanks. Now for my final question, if said amp was modified by changing those resistors, would it be expansive/worth it to replace the resistors back to the originals? Would common resistors be used or do special, discontinued ones need to be used? Or, if the resistors were changed, does the change effect using 12DW7's at all? Thanks again for the great input.
    -Russ
     
  10. It's an easy mod. The hardest part is taking the amp apart to do it. It's pretty labor intensive, but not expensive parts-wise.

    They're just plain ol' resistors. Any kind should work fine, although I recommend metal film over carbon comp for noise reasons.

    Yes; if you attempt to use 12DW7's once the resistors have been changed, you'll have a significant loss of volume.