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  • #1

    Tone Stacks

    G'day from Oz (Downunder),
    I'm a new forum member and I wanted to get some feedback on tone stacks.
    I'm about to embark on the build of my 5th Guitar Amp (I've designed and built more than a dozen Tube HiFi Amps) and am considering using Baxandall Tone Stacks instead of the "usual" FVM (Fender/Vox/Marshall) stack.

    Preamp is to be the Kevin O'Connor London Power Standard Preamp (LPSP) but with 6SL7 substituted for the 12AX7s (I've done this before in 2 of the amps and loved the result).

    Power Amp is to be Push Pull 6V6 with another 6SL7 as a Common Cathode Amp and Concertina Splitter (I think the Concertina Splitter is one of those "secrets" of good guitar power amp design which seems to have been lost).

    All tube Effects loop (1 more 6SL7) is sandwiched between the preamp and power amp.

    What I'm contemplating is ditching the anode driven FVM tone stacks for Baxandall stacks.

    This is purely to do with predicabilty of the stack response, I'm an average guitarist and one thing I've often struggled with is the tone controls (hey I mostly play acoustic). I'm familiar with the theory that you need to treat the Treble Control as a Volume Control and its setting determines the level of response of the Bass and Mid Controls, somehow knowing the theory doesn't seem to help in driving them, they just seem too interactive.

    Anyone care to comment on the advisabilty or otherwise of the Tone Stack change. I don't want to do it just to be different, I'm looking for improved usability.

    Thanks,
    Ian
    Tags: None
  • #2
    Your perception seems spot on to me. I like the standard tone stack because I'm use to it. Same goes for most players. Your not most players. IIRC Dr.Z uses a Bax type tone stack in most of their amps. Seems to work for Brad Paisley!?! Lots of guys here have built with a Bax. Maybe someone will key in to offer some sage advice. There is a Bax SPICE simulator labeled "James" in the Duncan TSC program. It's the non feedback version of the Bax. You can download it here:

    TSC

    Sounds like a neat amp.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      Thanks Chuck H,
      Another alternative is the supposedly leass iteractive Hiwatt style as shown here:
      http://www.webphix.com/schematic%20h...watt_sa412.pdf
      That is my circuit trace and draw by the way - I provided it to the guy who brought me the amp for a restoration. It was a bit weird to suddenly see it pop up on sites all over.
      I had all of Duncans Simulators, use PSUDII extensively for both Guitar and HiFi Amp Power Supply design
      Cheers,
      Ian

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      • #4
        FWIW there was a recent discussion here about adding a mid control to the Bax stack. A search should bring it up. I've never tried the HiWatt stack.

        In a quest to isolate the conrols I've been drawing up a tone stack based on T filters. I think Traynor did it with a later version of the Bassmaster. I had one of those amps and the tone stack worked great. A lot of loss in the Traynor stack though. What I'm working on should have less loss but compromise on interaction a little.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

        Comment

        • #5
          I've recently used a baxandall tone stack in a stupidly high gain preamp. There are a couple major differences, most notably that it tends to be FAR more sensitive then your 3 pot FMV. Literally a couple mm in the wrong direction can turn your sound into something totally different. Seems to have more cut/boost in relation to the middle frequency as opposed to the FMV. This can be useful if you're trying some unconventional designs, as it allows you to dial in a sound that doesn't make your ears bleed from horribleness, in contrast to the FMV which is relatively fixed in what it does.

          The other difference (well similarity I guess..) is that the mid frequency (the static frequency) tends to change a bit depending on the position of the tone controls. If you have a relatively balanced amp this doesn't really pose a problem (ie, if you like the controls somewhere around neutral), but the more your settings approach the extremes, the more the mid frequency will change, making it a tad interactive. The mid-shift control comes in handy here. In fact, the mid-shift control comes in handy pretty much all the time

          Just replace the 180k resistor you normally see between the treble/bass pots with a variable resistor potentiometer arrangement.

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