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12AX7 Question

Discussion in 'Amps and Cabs [BG]' started by hdracer, Mar 7, 2013.

  1. hdracer


    Feb 15, 2009
    Elk River, MN.
    Do Matched Triodes (Balanced) matter in pre amp tubes?

    I know the power tubes should be matched but is it worth the extra money to have the matched triodes? It isn't much but is it worth it?
  2. Hi.

    IME only on PI if even there.
    OTOH harmonica players for example get that wonderful distortion by using an asymmetrical PI.

    Most of the pre-amp stages that use dual triodes use the sides differently anyway, so the advantage is neglible.
    Just IME again, others will disagre I'm sure ;).

  3. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    No they don't for musical instrument pre-amps.

    In some amps, where the triode is in a power amp's phase inverter, a balanced tube could make a difference. Amps like a vintage SVT or an SVT-VR have a pot to adjust the balance.
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Rather than putting cash into balanced pre-amp tubes, buying a low noise tube for the first tube (often called V1) such as a 7025, can be worth the extra cost in some amps. It makes the amp quieter.
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Very true and a slight imbalance in the PI can give an edge to any amp that you want to run with a bit of dirt.

    Many NOS tubes were built to such a high spec that the individual triodes come very closely balanced. Some tube resellers offer this just to make a little more money on the tubes.
  6. hdracer


    Feb 15, 2009
    Elk River, MN.
  7. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    It might help in the PI if you have a balanced matched everything else in the PI circuit, but its probably made out of 5% tolerance resistors, and whatever tolerance caps, and a power section made of valves 'matched' to an entirely unknown tolerance.
  8. chadds


    Mar 18, 2000
    Position in a pre can matter. The characteristics of say one tube may shine as V2 over V1.

    For touring heavy gigging types you may want what's available and consistent. If you an hear the difference and can carry spares or are recording you may find an old Sylvania is best at say V2 and your black plate RCA shines as V1. Just an arbitrary example.
  9. BasturdBlaster


    Feb 19, 2012
    Crandon WI
    I am going to replace the preamp tubes in my svt4pro, should I just replace the V1 and call it good maybe? Or should I order all three? Ive never replaced them, the amp is over ten years old, its weak on gain it would seem, so thats why Im gunna replace them but I ask about the single tube replacement since I have a few new single tubes laying around.
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I'd only change the tubes that need changing. It never hurts to have spares. You can try swapping in the tubes that you have and see how it sounds. Then when you order your new tubes, you'll know what you need.

    With an amp that is 10 years old, it is possible that your power supply caps need to be changed. A tech can tell you if this is necessary. Certainly changing the tubes is the most cost effective first step.
  11. SirMjac28

    SirMjac28 Patiently Waiting For The Next British Invasion Gold Supporting Member

    Aug 25, 2010
    The Great Midwest
    +1 I would recommend the RCA Black Plate or one of my favorites a Vintage Raytheon USA 12AX7A Tall Black Plate. I got into tube rolling and I find it's a expensive hobby when you first start out but gets cheaper as you dig through the hype and find better values.
  12. Matching triodes in a 12AX7 for a MI amp waste of money. Most MI amps use one triode as a voltage gain stage and the second as a cathode fallower to the tone control section. This not a balanced use.

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