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Another build idea...

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  • Another build idea...

    Well, it seems to me after the last year and some that many players are getting back to the business of making music. But I've noticed, at least around here there's a slight change. There are more acoustic ensembles and less larger bands with drummers and the clubs and venues featuring live music seem to confirm this. Consequently, I'm playing alot more acoustic guitar and am currently playing out with two such acoustic groups.

    So now, I could really use an acoustic guitar dedicated amp. I could buy one but I think I'd rather try to build one for myself (I have more time than money these days and it's fun to boot). As it stands, I have a bunch of Champ and VibroChamps around (5 or 6) and except for a couple, they are doing very little except taking up space. I do have one I've built: a 2x 6L6 amp w/ a 10" speaker inside and I use that one alot (kinda like a Bandmaster, without tremolo). I have one that has a Vox preamp in it and it sounds great but very different from Fender and, of course, I built an amp from that Glass Audio article 'The Making of a Champ' and it's a good amp but not a great amp but it gets used. So, I thought I'd make something dedicated to making my acoustic guitar sound really good. I don't need alot of power and/ or size, just a really good preamp circuit that is not a distortion generating device, like most guitar amps. I'm good at designing a very clean power amp and I'll probably opt for 2x 6V6 and a solid state rectifier.
    Has anyone come across any acoustic guitar-specific preamp circuits using tubes? Any and all ideas, help and suggestions are very appreciated.
    Bob M.

  • #2
    Caveat: I have no experience with acoustic guitar amps, other than to know that they are clean, and more hi-fi than electric guitar amps. So these are just my speculations...

    1. If your acoustic has an on-board preamp, it takes the onus off of the amplifier to 'play nice' with the input impedance (normally very high for a piezo or such, as you likely know). If not, you'd need to address the >=10MOhm impedance match.
    2. I imagine that the EQ section would be tailored for a different set of boosts and cuts, a wider overall bandwidth.
    2a. I'm sure speaker choice plays a big part too.
    3. The preamp may be called to reduce the dynamic spikes of the DI guitar without sounding clipped.

    I hope to hear reports on your adventure!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


    • #3
      Agree with the above.

      You want a clean, wideband amp and a full range speaker (maybe using an additional tweeter).
      Transducer/PU type matters for input sensitivity and impedance.
      An equalizer before, looped in or built into the amp would be useful to adapt frequency response to room acoustics and to kill acoustic feedback.

      Don't underestimate required power for clean sound and dynamics.

      But most of all, there is absolutely no benefit from using a tube amp.
      - Own Opinions Only -


      • #4
        Acoustic amp is different from guitar amps. It is an absolute clean amp, with specific tone controls and desirable automatic feedback elimination (feedback killer), because acoustic guitars are subject to acoustic feedback.
        Acoustic amp there are two channels, one for an acoustic instrument (piezo pickup), the other for a mic.

        At DIY variant everything that applies for the harp amp, can also be applied to the acoustic amp.
        When all the requirements are put on paper, the construction becomes complex and comes down to solid state solutions.
        If you search IE you will see that the economic variant comes down to post-war amps due to clean sound and simplicity .

        Useful links
        Tube guitar amp for acoustic guitar
        EL34 PP acoustic guitar amp

        From the factory acoustic amps you can see the solutions Fender, Marshall, Fishman.

        Acoustasonic Junior

        AG200 / AG300 Acoustic Combo

        Marshall AS50R.pdf
        Last edited by vintagekiki; 07-30-2021, 10:24 PM. Reason: 1)
        It's All Over Now


        • #5
          Agree with the above. A lot of build decisions would depend on whether your acoustic has a built in preamp or not. As Helmholtz stated, there's no benefit to using a tube amp for acoustic guitar. I'd probably just buy a cheaper powered monitor if it were me- likely not much more than the cost of build materials. You are going to want some type of HF driver.
          "Yeah, well, you know, that's just, like, your opinion, man."


          • #6
            Originally posted by The Dude View Post
            I'd probably just buy a cheaper powered monitor if it were me- likely not much more than the cost of build materials.
            Exactly what I was going to suggest.
            - Own Opinions Only -


            • #7
              I prefer to play acoustic thru the PA system, with a good preamp to shape the sound, plus reverb and a little compression.

              But it certainly depends on your style. I'm looking at you, Monte Montgomery!


              I build and repair guitar amps


              • #8
                Originally posted by Helmholtz View Post
                But most of all, there is absolutely no benefit from using a tube amp.
                I'll wager you've never heard vocal harmonies live through a tube PA amp. Pure heaven. If you can't hear the difference a tube will make, you're doing it wrong

                WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                REMEMBER: Everybody knows that smokin' ain't allowed in school !


                • #10
                  Not sure how many watts you need. I've been working on a simple tube circuit for home audio that CAD's pretty well. You'd still need a preamp. If your guitar doesn't have one built in then they can be purchased pretty reasonably.

                  The circuit I'm working on should be shiny clean and flat to about 15W using a pair of el84's. If you build a double (for stereo effects) you'd have 30W. As mentioned, proper speakers for reproduction rather than "electric guitar" speakers would be necessary.
                  "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

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A


                  • #12
                    I used one of these coax speakers recently for a portable PA. Inexpensive and good to 20Khz.

                    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


                    • #13
                      When choosing speakers for the acoustic amp everything that applies to the PA system, can also be applied to the acoustic amp.
                      In other words, 2-way speakers, woofer 10 "or 12" higher efficiency with hard suspension and horn + crossover.

                      It's All Over Now