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Bleeding resistor + led

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  • Bleeding resistor + led


    I'm building a phono tube preamp from Valvewizard:

    With this power supply:

    I'm using the last schematic, without the voltage doubler. But I've notice that the 100k 2w resistor it's getting very hot.

    My B+ voltage is 350V

    And I use a switch with 12V led:

    So, If my maths are right. I need a 33k 5w resistor?


  • #2
    That 100K is a 2W rating because it is expected to get very hot. Without knowing the current draw of the switch LED, you can't really figure out the external resistor value that you need.
    If you are not using the doubler, then you need to include R2 and R3 from the second schematic.
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


    • #3
      A single blue LED is typically 20mA maximum and high brightness types will run on a lot less - say, 10mA. To my thinking then, to run the LED at 12v you need to drop 338v (350v-12v) across the resistor at 10mA. So R=338/0.01 or 33.8K Ohm. The dissipation would then be 3.38W.

      But that's without knowing what the LED actually needs.


      • #4
        Feed the switch LED from 6.3V heater winding.

        Your "12V LED" is most probably a regular Red Led (1.9V forward drop) and a series resistor to limit current, and will most probably work with 6.3VAC, which you may even rectify with a single 1N4002 and a 100uF cap to get about 8-9V DC.

        Remember this voltage is **floating** , neither positive nor negative connect to ground, except through filament balancing 470 ohm pot or dual 100 ohm resistors.

        I find it TERRIBLE they do not supply pinout and Led connections on the page you linked, also dangerous that both Mains and Led contacts live so close to each other and they do no suggest any safety measures.

        Be certain to heat shrink any and all terminals there.

        EDIT: Blue Leds drop between 3 and 3.3V , all other considerations apply.
        Also valvewizard calls AC filament terminals "+" and "-" ..... he should correct that.
        Last edited by J M Fahey; 08-23-2021, 02:19 PM.
        Juan Manuel Fahey