Wiring a tube 12AX7 Preamp

Discussion in 'General Electronics Chat' started by tomlryan, Apr 5, 2016.

  1. tomlryan

    Thread Starter New Member

    Apr 5, 2016
    Hey everyone! This is my first post, so sorry in advance if I am 1: in the wrong place, or 2: at a low level of electrical "competence!"

    So my question is how I would go about wiring a 12ax7 tube preamp with minimal current? I'd like to eventually figure out (since all I'm doing is guessing at this moment) if it's possible to have 5v(usb) or a 9v(bat) supply for a 2ch stereo preamp that's output is around 2x50w currently. I had one of my close friends suggest that all I had to do with switch my w100k potentiometers with a50k's...but...I have high doubts about that method. Mainly because it has nothing/little to do with my power supply issue.

    If anything I just said makes no sense, PLEASE let me know. It's better for me to learn the right way than to continue guessing.

  2. crutschow


    Mar 14, 2008
    I'm afraid it makes little sense (at least to me). :confused:
    I suggest you read up on tubes. They can't be powered by low voltages like transistors.
    A 12AX7 normally requires over 100V (typically 250V) for the plate voltage at about 1.2mA plate current (300mW @ 250V) so you'd have to generate that from the 5V or 9V source.
    It also requires a heater current of 150mA @ 12V.
    And a 9V battery would likely only last a few minutes generating that much power.

    And what is generating the 2x50w outputs (?), certainly not the 12AX7's. :rolleyes:
  3. tcmtech

    Well-Known Member

    Nov 4, 2013
    Volts times amps equals watts. A USB port has 5 volts @ 1 amp available continuous duty.

    A good quality 9 volt battery has about 1 Ah or ~9 usable watts total. You need ~100+ watts for extended periods.

    What do you think?
  4. #12


    Nov 30, 2010
    I've seen a 12AX7 run at 500 microamps on the plate but you still have to deal with that filament power.
    If you had 100 volts, that would be 0.05 watts in the gain circuit but
    6.3 volts at 300 ma is 1.89 watts just for the heaters.
    There is no free lunch with a vacuum tube.
    That's why transistors were invented.

    You can do it either way. I don't care which.
    Just, you must deal with the power budget.
  5. Veracohr

    Well-Known Member

    Jan 3, 2011
    I have a tube distortion pedal that uses a 12AX7 running on 9V. So if you want to distort your signal you can certainly use 9V, but not if you want a clean signal.​
  6. #12


    Nov 30, 2010
    True. I've seen some very low voltage circuits with a 12AX7 but they are not about clean, hi-fidelity signals. They are always about intentional distortion.
    Engineers older than I often think this is just wrong, but I am also a musician, and anything goes in the world of tone and effects.
  7. crutschow


    Mar 14, 2008
    I find it curious that most people who listen to music want no distortion and some who generate music crave it and both go to great lengths to achieve it. :rolleyes:
  8. Zoltan Markella

    New Member

    Mar 16, 2016
    I've built this headphone amp, which uses 12V plate voltage for a 12AU7 tube.
    It's not a valve-only amp as it uses a voltage regulator and a MOSFET, but it's my first stab at building a valve amp.

    It sounds great with my turntable (at least to me).
    absf likes this.
  9. #12


    Nov 30, 2010
    True. The artist side of a person doesn't care a bit about what an engineer believes, as long as the engineer believes he can achieve distortion, or lack of it, according to the job at hand. Wanna go nuts? Think about how hard the artful person works to create distortion and how much the listener wants to hear that distortion, as precisely as possible. Precision distortion. Better to not think about that.:D
    It's amazing that we can go from a milliamp or two to 1/8th of an amp in about 3 parts. Be grateful for the way the electronics industry has achieved this level of versatility compared to what we had to go through to do this 50 years ago!
  10. DGElder


    Apr 3, 2016
    Hey I resemble that remark!
    #12 likes this.