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Bassman Micro Transformers

Discussion in 'Shock Brother's DIY Amps' started by dunner84, Jan 11, 2019 at 8:57 PM.

  1. dunner84

    dunner84 Tele-Meister

    278
    Jul 15, 2014
    Victoria, bc
    I have a few projects starting up in the next few weeks, and one of them is going to be a bassman micro inspired build. I have a surplus of 12bh7 tubes, a few really "cute" chassis that would be perfect, and an itch for some micro amps.

    Does anyone know the current requirements for 1 or 2 12bh7, phase inverter, and 1 or 2 tube preamp? I am not worried about heater current, just everything else. I would really like to use a hammond 229 series for the HV, and the two I am looking at can only supply 25mA, and 50mA respectively, and I am not sure if either are sufficient. I am leaning towards these transformers for size reasons. The chassis I want to use would be very cluttered with standard transformers.

    Thanks,
     
  2. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    I cant answer that question but, I beleive the pt rob recommends is 58ma you should be good with the 50ma.(240-0-240)
     
    Last edited: Jan 11, 2019 at 9:31 PM
  3. dunner84

    dunner84 Tele-Meister

    278
    Jul 15, 2014
    Victoria, bc
    Thanks, I suppose I could have worked backwards like that too!o_O

    So that begs another question, which happens to be a hole in my tube knowledge.

    With a tube data sheet, and a schematic what am I looking at to calculate the current draw on the HV tap? Is it the current going through the first dropping resistor in the power supply? Or is it the sum the current at each cathode?

    Without doing a full circuit analysis, how would you ballpark this from an initial design point of view?
     
  4. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Each of the V1 and V2 triodes are pulling about 1.5ma at idle and the 12AU7 power tube is pulling around 18ma so it's pulling around 24ma total with the 12AU7. The 12BH7 will only pull 1 or 2 more ma so total high voltage draw for the amp would be 25 or 26ma with the 12BH7.

    You normally do these calculations backwards. If it's a new design you can compare similar circuits from other amps. You can use the voltage drop across a power resistor to calculate the total power node draw or the plate or cathode resistor to calculate the triode current.
     
    D'tar and sds1 like this.
  5. sds1

    sds1 Tele-Holic

    Age:
    42
    809
    May 4, 2017
    Orlando, FL, USA
    You can ballpark off the datasheets, but drawing load lines would be a more accurate way to derive idle current draw for a given design.

    https://robrobinette.com/Drawing_Tube_Load_Lines.htm
     
    robrob likes this.
  6. dunner84

    dunner84 Tele-Meister

    278
    Jul 15, 2014
    Victoria, bc
    Thanks everyone!

    Whenever I come to a new problem with tube amp building, I come hear and ask, and have a read through the valve wizard, and london power websites, or the rest of google. I am never disappointed, so thank you for your help!

    This is a good article hear:

    http://www.valvewizard.co.uk/smoothing.html

    This photo is from the above article, and it helped me sort it out a bit. I have two more questions to help clarify a couple of things.
    In this photo, why are the EL34s only drawing 19mA, should each draw this much? or is it because only 1 tube is working at a time?


    [​IMG]
     
  7. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    39
    113
    Sep 18, 2018
    The Netherlands
    Screen grid current is 19 mA. Plate current is shown to be 125mA.
     
    robrob likes this.
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