New build : VOX AC4 from old radio receiver : help with trem

Discussion in 'Shock Brother's DIY Amps' started by nasdak, Aug 24, 2017.

  1. nasdak

    nasdak Tele-Meister

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    Hello!

    I picked up a old radio receiver (schneider romance 57) and used it as a base for a AC4 build (based on the old 1960 schematics)
    here's the radio :
    [​IMG]

    [​IMG]

    i kept the housing, the chassis, power transformer, the rectifier (EZ80), and some pots / resistors / capacitors.
    the rest had to go!

    here it is with the EZ80 and the 2 new valve (nos philips EF86 and EL84)
    [​IMG]

    of course a 3-prong power chord was fitted (safety first) and everything grounded.
    i used an (big) old output transformer from another old tape recorder
    I was happy enough to use the original pot for on/off and volume, and a existing hole for the guitar input.
    Speaker is a 8" alnico weber 8A125

    Here it is now :

    [​IMG]

    [​IMG]

    I have to say it sounds great !
    I made some changed to old schematics :
    [​IMG]

    EF86 part :
    R8 changed to 1M (as in the AC15)

    Power supply :
    C5 changed to 32uF
    grid leak resistor moved to the input jack (a la fender)
    grid stopper changed to 33k

    EL84 :
    screen grid resistor added (2K 2w)
    OT connected to first filter capacitor (before the R3)
    (this way the screen grid voltage is just some volts under the plate voltage)

    And i did not wire the tremolo circuit.


    I have to say i'm pleased with the result !
    I get a little overdrive with HB and a nice grit with a strat. All it misses is a nice reverb.

    Here is a sample with some reverb :
    http://picosong.com/wsV8D/

    I hope you enjoyed my post, fell free to comment / ask some questions !
     
    Last edited: Aug 24, 2017
  2. nasdak

    nasdak Tele-Meister

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    Because a build is never over :
    i'd like to add a tremolo :)
    but i think using a full 12AX7 for that sole purpose would be a waste of tube.
    is there a way to make a oscillator with something like a diode ?
    thanks ;)
     
  3. Marcelo R

    Marcelo R Tele-Afflicted

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    Cool! Many years ago, I thought of making an amplifier out of an old radio, but to be honest, I don't think I had the skills or even the patience to do that. But who knows, now that I'm an older guy maybe I'll give it a try someday.

    Best regards
     
  4. nasdak

    nasdak Tele-Meister

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    Hello,
    i'm still learning about pentodes with this project, and i found some strange readings on my EF86 :

    My B+ is 245V
    Va : 138V
    Ra : 265k
    so Ia = 0,4ma

    Vc : 0,68V
    Rc : 1.5k
    So Ic = 0,45ma

    Vg2 : 31V
    Rg2 : 3.2m
    so Ig2 = 0,06ma

    Vg2 seems fairly low and all curents seem also very low.
    Note that I have no trem so no connection to a 12AX7 via R9

    I tried with 2 EF86, same readings.
    Is something wrong ?
     
  5. nasdak

    nasdak Tele-Meister

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    Hello,
    i'd just like to share my opinions : this amp ROCKS.
    It can be cranked at moderate volume, and really sings.
    The EF86 gets really great with any pedal (fuzz or TS) and i think i also get EL84 distorsion.
    Damn, i really like this one !
     
  6. Tonetele

    Tonetele Poster Extraordinaire

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    What would be the RMS output in Watts?
    Seems like an awful lot of tubes ( valves) in there. You could make a completely new amp using all those parts and still house it in that original radio cabinet.
     
  7. nasdak

    nasdak Tele-Meister

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    Around 5 watts
    It's a completly new amp!
     
  8. gusfinley

    gusfinley Tele-Holic

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    Check out Vox AC15C1 Schematics for their single MOSFET Based Tremelo circuit. It is derived from the tube tremelo used in the early 2000's AC30HW and runs off the high voltage feed. I'm trying to find a safe way to put at least part of this circuit into a pedal so I can apply some bias wiggle to my vintage-spec AC15!
     
  9. vkinetic

    vkinetic Tele-Meister

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    Fantastic!
     
  10. nasdak

    nasdak Tele-Meister

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    just made a little vid :)
     
    Mhat and dan40 like this.
  11. Wharfcreek

    Wharfcreek TDPRI Member

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    Nas, don't know if you're still around or if you'll get this.....I know its 6 years old.

    Question about your build here, and how you 'eliminated' the Trem part of the circuit. The Trem 'feeds' from the cathode and G3 of the EF86. I presume you left the connection between K and G3 in tact, and just omitted the feed from that junction down to the 100K on the K of the 12AX7? And, what about the Return cutoff point? There is a C5 going to ground after the plate of the EF86, then a 10M in series with a 1M all going to ground. Where did you decide to cut off that part of the circuit? Did you leave the 10 and 1 M all in and still connected to GRound......or did you make your cut off prior to that....or what? Thanks, Tom
     
  12. nasdak

    nasdak Tele-Meister

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    Hey! Yes i'm still here
    Just disconnect r9 and r18 :)

    Envoyé de mon Redmi 4X en utilisant Tapatalk
     
  13. Wharfcreek

    Wharfcreek TDPRI Member

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    Perfect! Thank you. I think I'm going to try to build one myself. Never used this EF86 tube as a 'front end' before. So, interested it give it a try. Thanks again, Tom D.
     
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