Marshall 3005 preamp into 12AU7 Poweramp

Discussion in 'Shock Brother's DIY Amps' started by Rock_Glenn, Dec 30, 2018.

  1. Rock_Glenn

    Rock_Glenn TDPRI Member

    Posts:
    75
    Joined:
    Aug 10, 2014
    Location:
    Western NY
    I have a Vox Cambridge 30 PT kicking around, I tried to sell it on ebay with no takers. I've been looking for something to build with it, tagboardeffects posted a layout of the 3005, and I've thought it could sound good into a 12A*7 poweramp, a la one of Rob Rob's micro poweramps.

    The Vox has a HV winding, ~286V CT, and a LV winding ~45V. The HV powered a preamp 12AX7.

    I drew the following schematic up quick, and I have some concerns.

    Do I need to get my B+2 to the grid somehow?
    Coupling Capacitor necessary?
    Grid Stopper resistor sizing.
    Can the 3005 drive the Power Amp section?
    Would I better off using a Push Pull on the 12AU7? If so what to do about a phase inverter?
    Would the DC heater circuit work as I have it drawn?
    Is my bipolar power supply correct?

    https://imgur.com/SvRAgLQ


    [​IMG]
     
  2. Zepfan

    Zepfan Poster Extraordinaire

    Posts:
    9,329
    Joined:
    Nov 30, 2013
    Location:
    Horn Lake, MS
    You might want to look into the FireFly amp. It uses a 12AX7 preamp tube and the 12AU7 in a Phase Inverter/power tube role.
    [​IMG]I've also seen a schematic using 2 12AU7's in the Phase Inverter/power tube role for more output wattage.
     
    radiocaster likes this.
  3. Rock_Glenn

    Rock_Glenn TDPRI Member

    Posts:
    75
    Joined:
    Aug 10, 2014
    Location:
    Western NY
    I've thought about adding the second 12a*7 tube into this circuit as it would give me a phase inverter and a tube to drive the power amp.

    I'm concerned that the PT can't handle a second tube though.

    I'd like to use the 3005 preamp, my main concern is connecting the ss preamp into the tube power amp. Also, this will be my first bipolar power supply, but its something id like to tackle.
     
    Last edited: Dec 30, 2018
  4. Rock_Glenn

    Rock_Glenn TDPRI Member

    Posts:
    75
    Joined:
    Aug 10, 2014
    Location:
    Western NY
    The firefly circuit could work great. Thanks for posting. I think I'm going to try this. I just need to figure out the voltages.
     
    Zepfan likes this.
  5. Zepfan

    Zepfan Poster Extraordinaire

    Posts:
    9,329
    Joined:
    Nov 30, 2013
    Location:
    Horn Lake, MS
    [​IMG]Here's the whole schematic and there's a good one from the AX84 site.
    [​IMG]
     
  6. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    7,612
    Joined:
    Dec 29, 2012
    Location:
    United States
    I built a Firefly a long time ago and I didn't care for it's tone. It's one of the things that inspired me to design the Micro tweed amps.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.