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Tube Gurus - matched preamp tubes?

Discussion in 'Amps and Cabs [BG]' started by marc40a, Feb 1, 2004.

  1. marc40a


    Mar 20, 2002
    Boston MA
    Hey guys,

    From what I understand, preamp tubes for guitar or bass don't have to be matched. Is this true?

    Are there any advantages to running a matched set?

    For what kind of application would one need a matched pair of preamp tubes?
  2. notanaggie

    notanaggie Guest

    Sep 30, 2003
    No, I have never heard of any reason to do that.

    Is someone touting it?

    Matching means something when two (or more) tubes work in combination, usually from one bias source. That would be in most any (push-pull) tube power amplifier other than little 5W wonders.

    But in a preamp they are independent, so matching isn't useful for anything I can think of.

    Besides, most preamp tubes come as two sections in one glass enclosure...do they match those in one tube, or two complete tubes, or what is being claimed?
  3. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Nope, totally meaningless IMO. Matching is important in the output section of a push-pull amp, and even there, there are ways of making it "less necessary". But IMO matched sets of output tubes are a very good idea, and in some designs they're downright essential. But not for the preamp, not even debatably for anything before the PI (phase inverter) stage.

    In fact, those of us who've been working with tube amps for a while will recognize that the variability of circuit componentry is one of the key elements of a good sounding design. That may sound funny, but look at it this way. Typically the electronic components in a tube amp have tolerances of 10% (resistors) or 20% (capacitors and pots). When you're talking about matching two tubes, the usual description of what is considered "matched" is "within five percent". When you translate to in-circuit idle current, that would mean they're running within a couple of mils of each other. That means, the tubes are more closely matched than any of the other component tolerances in the circuit.

    Another thing I'd point out, is that even in the PI stage, tubes with "matched halves" are largely meaningless. The typical phase inverter in, say, a Fender tube amp, has deliberately unbalanced plate load resistors. Usually they're 82k and 100k, so around 20% apart. Many people think this makes for better tone. It's debatable, but it's a common design point, that most tube amps are deliberately unbalanced, 'cause they "sound better that way". From an electronic standpoint, in that particular circuit, you can balance either for plate voltage or signal level, but not both. The circuit is designed in such a way that it is inherently impossible to match the behavior of the two halves of the PI to within a range that is any closer than 20%. Some people try to "split the difference" by using 91k and 100k, but all that means is that BOTH the plate voltage and the signal voltage will be unbalanced!

    I can't think of any reason at all why matched preamp tubes would be advantageous in a tube amp. Unless you're talking about a mission-critical PA system, where all of the eight input channels (or whatever) should be "exactly identical" for calibration reasons. But not in y'r average musical instrument amplifier, where the two channels are usually radically different to begin with.

    In my amps, I select the best sounding tube for use in each position. For instance, my favorite V1 tube is a 60s/70s Mullard 12ax7 (or ecc83 in the European designation). But my favorite V2 (or driver) tube is an Amperex Bugle Boy. In my '65 Bassman head, I use an RCA 12ax7 in the clean channel, and a Mullard in the (modified) bass channel. In my Twin, I use a JJ for the driver stage, and a Mullard CV4024 in the phase inverter. Each amp wants to see something different to bring out the best in the tone, and it's even more specific than that. Each "stage" of the amp may have a tube (or tubes) that it responds well to, so it sounds best.

    Anyway, that's my opinion based on 40 years as tube amp designer and tech. You'll always find people who'll claim that preamp tube matching is a good thing. If you give me five minutes with their amp and some tubes of my choice, I'll bet you I could change their minds. And besides, many of those people are out there selling tubes. :)
  4. TheAmpNerd


    Apr 25, 2004
    Dallas, Texas

    You do not need matches triode sections. These are not
    output tubes, they are wired in series with typically a coupling cap, pot, resistor, or any combination with multiples of each between sections.

    Save your money for burned in and matched output tubes if
  5. Make this vote #3 for no such thing as "matched preamp tubes."
  6. Well theyed certainly have fun with my Trace, 7 12aX7s. There is a lot of crap talked about valves, sod numbers and names, simply use ones ears. I personally have no gripes with the sovteks in the V4.
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Matched preamp tube is a bunch of hooey.

    In fact, I would NOT match, I'd use a goodie for the first stage, and then whatever you like the sound of in later stages. That first one is for lower noise and so forth.

    Sound? Ok, use what brand you like. No problem.

    The only place matched tubes might make a difference is in certain amps where the phase splitter is not a single tube. In such a case, it "might" make a measurable difference, maybe. You won't have that problem, most likely.

    Tube sales folks want you to think tubes are some sort of magical thing requiring their own sort of "pixie dust". Of course their pixie dust is also more expensive.

    A lot of folks think more expensive or more "precision matching" is always better. But it ain't always true. In fact it usually isn't true, aside from the power tubes, and it may not be particularly crucial there..
  8. geshel


    Oct 2, 2001
    In a stereo amp, maybe. :p
  9. Garey

    Garey Supporting Member Commercial User

    Apr 23, 2003
    Northern California
    Artist Relations/Product Specialist: Mesa Boogie
    Its really only in issue in home stereo amps where you want each channel to be as similar as possible, and when the tube is used a phase inverter, a dual triode, where each side is matched in the SAME tube. Even then....its not critical, and you shouldn't pay for a matched preamp tube, or matched preamp tube pair.

    Matching is for power tubes, so that they operate under the same conditions, to eliminate crossover distortion.

    Aloha, Garey
  10. As a phase inverter/splitter in most vintage svt's the 12DW7/7247 presents a problem because it is a dissimilar triode (half of a 12AX7 and half of a 12AU7) so matching is impossible. Although Terry Buddingh recomends using matched pairs for the 12BH7 drive tubes: triode 1 should match triode 1 of the other tube in transconductance and cathode current. Triode 2's should also match, because it is a push/pull amp.