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B+ voltage question?

Discussion in 'Amp Tech Center' started by JSMac, Mar 1, 2021.

  1. Mar 1, 2021 #1
    JSMac

    JSMac Tele-Holic

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    I'm working on converting an old mono amp to a guitar amp. The Transformer reads 346vac. The B+ is 396vdc with nothing connected after the 5Y3. When I connect a 22uf filter cap to the b+ and ground the other end the voltage goes up to 476vdc. Can someone tell my why this is?
     
  2. Mar 1, 2021 #2
    Paul G.

    Paul G. Friend of Leo's

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    Here is a picture of what you are measuring with and without filtering. You can see why your filtered average voltage measures higher than unfiltered.


    full-wave-rectified-DC-voltage.jpg
     
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  3. Mar 1, 2021 #3
    Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    ^^^^
    This.

    Adding a capacitor to the output of a half wave or full wave rectifier increases the measurement to the average DC voltage.
     
    Last edited: Mar 1, 2021
  4. Mar 1, 2021 #4
    JSMac

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    I see. But that gives me a pretty high B+1 voltage then. In the circuit that I'm working on It should be about 390vdc to the OPT and first dropping resistor.
     
  5. Mar 1, 2021 #5
    J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    The voltage will drop with the full load of the amp circuit. Right now you have an unloaded power supply.
     
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  6. Mar 1, 2021 #6
    JSMac

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    OK. So after I finish building the power supply and get some tubes in there the voltages should fall in line.
     
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  7. Mar 1, 2021 #7
    dan40

    dan40 Friend of Leo's

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    Yes...voltage will fall once the tubes are drawing current. Just be sure not use use more tubes or a different power tube type than the original amp used without knowing the PT's current capacity. Using 6l6's in an amplifier that was designed for 6v6's may overload the PT if the current rating is not high enough.
     
  8. Mar 1, 2021 #8
    JSMac

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    This amp will have the same tube compliment as the original.

    I just installed the B+ Filter caps and resistors and with the 6v6s installed the voltage didn’t change. I haven’t completed anything in the circuit beyond that.

    I must have something wrong. Earlier I had gotten to this point and installed the cathode bias cap and resistor on the 6v6s and 5 watt resistor got very hot.
     
  9. Mar 1, 2021 #9
    2L man

    2L man Tele-Meister

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    If circuit is not ready and you power it it can begin to oscillate and generate power and voltage spikes that can damage components.

    If you did not set grid voltage right possibly full anode current did run?
     
  10. Mar 1, 2021 #10
    J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    You should complete the circuit first. Then before powering it up go through the circuit part for part and compare it to your schematic. Correct any errors you find. Then power up with a lightbulb current limiter. If that passes check your voltages and a adjust what needs to be adjusted. Take your time and play it safe.
     
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  11. Mar 1, 2021 #11
    JSMac

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    Good advice and I should probably know that even in my limited experience. I do use a DIY dim bulb tester in conjunction with a variac when I power up.

    I guess I was trying to take shortcuts to avoid redoing my work. These conversions are labor intensive. I need to learn to exercise patience.
     
    Last edited: Mar 1, 2021
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