power transformer output voltage

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by AL1, Apr 22, 2005.

  1. AL1

    AL1 Member

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    I just put in a new Mercury Magnetics Axiom FBFPP power transformer in my blackfaced Princeton Reverb and the vdc from pin 8 of the GZ34 rectifier is only 320. It should be about 420. VAC from wall is 120.5. Anyone have any ideas why the voltage is so low? Thanks.
     
  2. John Phillips

    John Phillips Member

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    Do you have the first filter cap connected to the rectifier? Disconnecting the cap drastically lowers the measured voltage because the meter tries to average the voltage in the half-wave ripple. Once the cap is connected, the voltage stabilises at the full peak voltage.

    Just a thought! If you haven't disconnected the cap (or fitted a standby switch) that isn't it...
     
  3. AL1

    AL1 Member

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    John, everything is connected. The only thing I changed was the transformer.
     
  4. AL1

    AL1 Member

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    John, I didn't put the2- 100 ohm resistors that are by the lamp. Would this make a difference. A friend of mine has a 1964 Princeton Reverb and it doesn't have these 100 ohm resistors.
     
  5. John Phillips

    John Phillips Member

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    No, that shouldn't make a difference. Those are the 'virtual center tap' resistors for the filament winding, which on the original transformer doesn't have a real one. If the new one does, those resistors should be removed. (Even some of the original amps without a center tap don't have them, either one side of the winding is grounded via the lampholder, or it's done elsewhere.)

    Check the AC voltage between ground and the plates (pins 4 and 6) of the rectifier tube. It should be around 340VAC on each.

    Also check the ground connection to the HT secondary center tap - if you've reconnected it to one of the big solder points on the chassis, it can sometimes be hard to get a good connection there since the chassis is a very good heat sink.

    If the center-tap connection is good, and the AC voltage at the rectifier plates is right, the problem is elsewhere (bad rectifier tube maybe?); if the AC voltage is low, it's the transformer.
     
  6. AL1

    AL1 Member

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    John, do I put the black tester lead on say pin 4 and the red one on pin 6? Not sure how to test for the 340 VAC.
     

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