tube mic heater wiring

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erikx

Member
2014-09-21 9:55 pm
Chicago, IL
  • #1
    I'm building a G7-ish mic from the (single pattern) Aurycle kit. A few things are different, e.g., like the G7, it uses an EF86. I am however using the Aurycle PCB. (The stock Aurycle uses a 12A*7 but the circuit is very similar to that of the G7.)

    About heater wiring: The Aurycle (schematic) puts the +6.3V to socket pins 4 and 5, with a capacitor (C11) connecting that line to ground. Heater current returns via the center tap on pin 9, and connects to ground.

    Two questions about this arrangement:

    1. What is the purpose of C11? Noise filtration, I assume?

    2. The trace running the +6.3V is in the middle of the board; the current is returned to one of the ground buses on the side of the board, with everything else. (Here's the board layout) I'm certainly no genius with this stuff, but isn't this a potential noise source? I looked on the G7 PCB, and both + and - heater connections are wired with their own thick, parallel traces---the heater ground is maintained as a separate connection back to the plug.

    I was considering skipping C11 and running the heater wires in a twisted pair directly from the plug to the tube socket. Thoughts?
     
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    erikx

    Member
    2014-09-21 9:55 pm
    Chicago, IL
  • #3
    Good idea---turns out it already has a DC heater! I realized I had no idea what it supplied, because the Aurycle PSU ships as a completed unit, so no schematic. I looked inside the supply and saw what looked like two different bridge rectifiers. Then I saw this:

    "The PSU shipped in the DIY kit supplies two DC power sources to the microphone. One is the low voltage about 6.3V to the filament of the tube to heat the cathode, and the other is the high voltage about 200V to the anode of the tube."
    Click to expand...

    Aurycle.com - Manufacturer Direct Pricing on professional studio microphones, usb microphones and microphone kits

    (Method: Dissect unit first, then read FAQ.)

    So: Is there any point in running separate heater wires, if the heater current is DC? (I assume ripple is low enough to be ignored?) And C11 is then there to bleed any AC off to ground, right?

    Thanks.
     
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    Enzo

    Member
    2004-09-25 8:17 am
    Lansing, Michigan
  • #4
    C11 is there as a filter cap to insure the DC heater supply is ripple free. it is DC so is not going to radiate a bunch of hum.

    Why not build it stock, and then IF you have hum issues, consider other wiring options.

    If you make sure the 6vDC is well filtered on the PS unit itself, then there should be little or no ripple current in the ground trace on the circuit board.
     
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