Voltage too high

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by AL1, Dec 29, 2004.

  1. AL1

    AL1 Member

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    After blackfacing a 1978 princeton reverb and putting a standby switch on it, when I hit the power switch the voltage is going over 500 volts.What can I do to lower the voltage? It uses a 5u4GB rectifier. I am using a Weber copper cap.
     
  2. drbob1

    drbob1 Silver Supporting Member

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    Use it with a Variac to get 110v from the wall.

    Use a 5Y3 rectifier.

    Add a resistor to reduce the B+ voltage.
     
  3. Blue Strat

    Blue Strat Member

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    If this is with 6V6s installed and properly biased, that's WAY high. Without 6V6s installed B+ voltage will always be high.
     
  4. AL1

    AL1 Member

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    Hi Mike, no it has not been biased yet and it is with the new jj 6v6s. I shut it down as soon as I see it go over 500v. I just tried again with the original 5u4gb and it still goes over 500v. I have it hooked up to a current limiter box and when I shut the limiter off that is when it goes over 500 volts. The bias control is set at 22k if that helps. I have not turned the amp all the way on yet.
     
  5. Wakarusa

    Wakarusa Member

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    If "haven't turned the amp all the way on yet" means never been out of standby then 500V on the DC output of the PT isn't all that surprising.

    According to the BFPR schematic the PT secondary is supposed to develop 340VAC which translates to a no-load DC out of the rectifier of around 480VDC -- and that schematic probably assumed 115V at the wall.

    At 125VAC from the wall the secondary will rise to around 370VAC which will rectify to around 522VDC. A tube rectifier will drop this down a bit (where the copper cap will not).


    Install the 5U4, set the bias to somewhere between -35VDC and -40VDC, chuck the JJs in it and see where B+ settles out with a load. Insert an appropriately rated power resistor between the rectifier and the PT primary center tap if you want it lower from here.


    Edit: Note that the tube rectifier won't drop B+ "down a bit" until there is a load drawing current. With the amp in standby you get full no-load B+ regardless of the rectifier used
     
  6. tweber

    tweber Member

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    oooops... yes it will. it has power resistors and other components in it to emulate the plate resistance of the vacuum tube rectifier it is emulating. The copper cap WU4GB or the WY3GT will drop as much or more as its vacuum tube counterpart.
     
  7. Wakarusa

    Wakarusa Member

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    My bad. I always assume they're using a straight diode thing as opposed to one of the ones with the nifty tube emulating circuit.
     
  8. tweber

    tweber Member

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    We do have the straight (no sag) copper cap. That is the WS1, and has just diodes and an inrush current limiter.
    Otherwise, they all have some drop and sag.

    We also have the Z-Cap, which has zeners in it to drop a specific amount, and then sag from there like the tube rectifier it is emulating.
     
  9. AL1

    AL1 Member

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    Would it matter whether the amp is biased or not since it is still in stand by . I hooked it up to a varic at 115 volts and it went up to 481 volts. Is this still too high since it is over the caps rating of 475? I took out the old 450v one.
     
  10. Wakarusa

    Wakarusa Member

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    Bias setting has no effect on no load voltage. See my previous post. 481 is almost exactly the predicted no-load B+ with 115V from the wall.

    Like I said, set the bias to a reasonable range (-35VDC to -40VDC) stick the power tubes in and take the amp off standby and then tell us what you see on the power tube plates.
     

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