Tube Preamp Build Doing Evil Things - Any Thoughts??

Discussion in 'DIY' started by willyrover, Feb 13, 2018.

  1. willyrover

    willyrover Super Member

    Messages:
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    Hamilton
    Over the past several months I have been building a tube preamp using "kit modules" sourced from eBay. It's finally assembled - but it's doing some evil things that I think are related to grounding.

    IMG_0264.jpg


    The board in lower left is power supply board. This board has no connection to chassis. It has 12V regulated DC for the heaters and approx. 300VDC B+.

    The main board is obviously the preamp board. Again, no direct connection to chassis.

    The upper right board is the input selector. Chassis ground is through the upper left standoff - including the signal ground. It has it's own 12VDC regulated supply.

    RCA inputs and outputs are otherwise isolated from the chassis.

    As it is there is a hum (120hz?). The hum is loudest at min. and max. on the volume control and reduces greatly as the control gets closer to center.

    Connecting a ground wire between the heater negative and chassis eliminates the hum - BUT - the heaters on one side of the tubes glow brighter than the other - and the "Dark" side stops passing signal. Removing the ground wire causes the the dead channel to fade back in, but the hum returns.


    Puzzled....!
     
  2. elnaldo

    elnaldo Addicted Member

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    From my limited experience, you need the Power Supply ground connected to chassis at only one point.

    I'd connect the PS board ground (where all negative terminals of the main capacitors go) to chassis, just at one point, next to those capacitors. Then, measure continuity from all the other boards ground to chassis, to check all the grounding is interconnected.
     
  3. willyrover

    willyrover Super Member

    Messages:
    2,898
    Location:
    Hamilton
    So I may have figured it out. The connection from heater supply ground to chassis was correct, as per the schematic, even though it killed the right channel.

    The tubes were doing evil things because one of the regulators for the DC heater supply was bad and not outputting anything. (There are two regulators in parallel) The single regulator was not supplying enough current and the voltage was being pulled down to about 9 VDC. Since the two heaters in the 12AX7/12AU7 were wired in series, the first heater in the series was not getting hot enough and its resistance was staying high. The second heater in the series was not even working enough to start passing signal - again resistance staying high and pulling down the heater supply voltage.

    The preamp seems to work fine with the heater voltage provided by an external supply.

    Once I get the regulators replaced, I will have to see if any noise/hum remains.
     
  4. elnaldo

    elnaldo Addicted Member

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    Double check that DC supply. If you have 2 12ax7 in series you need 24 V DC out of that circuit.
     
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  5. willyrover

    willyrover Super Member

    Messages:
    2,898
    Location:
    Hamilton
    The two heater sections within each tube are in series. (Pin 4 and 5) The two tubes are parallel.
     
  6. elnaldo

    elnaldo Addicted Member

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    OK, I understand. 12ax7 tubes can be used with 6 or 12V.

    I thought tubes in series since I was working on a unit with four 12ax7 in series, 44V feeding the 4 heaters in series (Fisher 500-S), 11V at each tube.
     
  7. mv213

    mv213 Active Member

    @willyrover

    I ran into a similar issue using that source selector board...there is a 2.2 ohm resistor (just to the rear of the black heat sink of it’s power supply) that connects the power ground to the signal ground. Clip that resistor out.

    That solved the problem in my preamp and allows the signal ground to be independent of the power ground. Hope this helps.

    EDIT: In your picture, that resistor will be “above” the heat sink, at the back edge of the board...on the opposite side of board from the flat cable plug for the selector knob.
     
    Last edited: Feb 14, 2018 at 10:49 AM

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