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Acoustic 370 Power Amp Sim Pedal

Discussion in 'Effects [BG]' started by johnk_10, Mar 15, 2016.


  1. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    after successfully working with Frank Appleton (aka boomertech) in designing/voicing the GK800 amp sim pedal, I decided to work on designing another power amp (simulator) that has been one of my favorite amps of all time, the vintage Acoustic 370 (power amp section).
    so far, with Frank's help, I have drawn up a PCB for an exact clone of the power amp but scaled down the output to pedal level. I just need to complete a gain circuit that will adjust the AGC in the original amp, make up an input buffer for it and probably make a few adjustments.

    here's what the enclosure (125B with all top jacks) will look like:

    370powerampPedalEnclosure600.jpg
     
    Last edited: Mar 15, 2016
  2. nshuman

    nshuman Supporting Member

    Sep 4, 2012
    Montreal, Qc, Canada
    WWWJWWW
    Ooooooooohhhhhhhhhhhh This is going to be great...
     
  3. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    This may be really, really cool. The AGC protection circuit in the 370 has me really intrigued as to how it affects the tone.

    :thumbsup:

    -Frank
     
    blastbass, andrew and blindrabbit like this.
  4. kbakerde

    kbakerde Supporting Member

    Jan 24, 2012
    Gimme gimme gimme gimme!!!! Haha
     
    LiquidMidnight and blindrabbit like this.
  5. sprag

    sprag

    Sep 15, 2011
    Melb Australia
    looks sweeeeet
     
    blindrabbit likes this.
  6. Oh my.
     
  7. andrew

    andrew

    May 20, 2000
    Vancouver BC/Pacific Northwest
    Endorsing Artist: Aguilar Amplification, Spector, Regenerate Guitar Works, Tech 21 NYC
    Very cool!
     
    blindrabbit likes this.
  8. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    we're still working on it, but it may or may not have two more external controls. an input level and an AGC gain level, but they could also just be internal trimpots.
    fingers crossed I hope to have a working prototype by this weekend.
     
  9. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    The 370's AGC protection circuit uses a FET attenuation at the power amp's input signal. The 1176 compressor uses very similar FET attenuation on its input signal for compression. Very cool!!

    -Frank
     
  10. chris merrill

    chris merrill Supporting Member

    Dec 14, 2012
    Chicago, IL
    Endorsing Artist for: GHS Strings, Lakland Basses
    Sub'd
     
  11. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    I'm thinking that you will find a sweet setting for the AGC where you can replace its adjustment with fixed resistors. Possibly put a toggle switch in for that momentary AGC defeat switch. This way you can have two settings.

    -Frank
     
    blindrabbit likes this.
  12. Basshappi

    Basshappi

    Feb 12, 2007
    Tucson,AZ
    Ohhh Yeahhhh!!
     
  13. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    i'm putting a trimmer for the AGC gain in it for now.
    I also added a mini momentary push button switch for S301 (AGC bypass) just like the original amp has. personally, I don't think that a toggle is what i'd want on the original amp or the pedal version of it.
    R319 is a 680R in the original amp, and it sets the amp's overall bias ( & AC current draw). since I've rebuilt a LOT of vintage 370's, I found that putting a 1K trimmer in its place allowed me to adjust the amp to it's best sound so i'm adding this to the pedal as well. the later Acoustic 320's also have the trimmer instead of a fixed bias resistor.
     
  14. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    Do you have the manufacturing directions for calibrating R304 with switch S301?

    -Frank
     
    andrew likes this.
  15. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    yes. R304 is the offset adjust, you turn it so the output is exactly half the power supply voltage.
    the 370 Schematic, Note 6 says: "ADJ R304 FOR SYMMETRICAL CLIPPING WITH A 2Ω LOAD AND S301 CLOSED (AGC DEFEAT)"
     
    andrew, boomertech and blindrabbit like this.
  16. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    BTW, i'm etching the first prototype PCB for it right now. :)
     
    blastbass, rhythmwiz, odin70 and 4 others like this.
  17. 40Hz

    40Hz Supporting Member

    Maybe I missed it somewhere. But is this something that's going to be sold, or is this just a personal project you're doing?
     
  18. chris merrill

    chris merrill Supporting Member

    Dec 14, 2012
    Chicago, IL
    Endorsing Artist for: GHS Strings, Lakland Basses
    I'd buy one
     
  19. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    it's just a personal project at the moment, but if it sounds like I think that it will, I could probably build a few of them for others.
     
    OEW3, vincent69, boomertech and 4 others like this.
  20. mrwood4

    mrwood4 Supporting Member

    Apr 17, 2014
    Southern Illinois
    Yessssss......want want want....
     
    blindrabbit, MCF and nshuman like this.

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