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Fender PA 135 - weak output

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  • Fender PA 135 - weak output

    Hi, I'm new to the forum...

    I have a Fender PA 135, worked fine for a long time but now producing only very anemic output - maybe a watt or two. I've been over it a few times looking for anything obvious, but nothing is visibly fried, melted, leaking, etc

    I checked all the voltages, but unfortunately the only schematic I can find does not show much in the way of typical voltages at test points.

    Schematic: Prowess Amplifiers - Fender - Schematics - pa135

    V1A Plate:200, Cath: 1.3
    V1B Plate: 206, Cath:*1.3
    V2A Plate: 218, Cath:*1.3
    V2B Plate: 206, Cath:*1.3
    V3A Plate: 138, Cath: 1.7, 160mvDC on grid
    V3B Plate: 383, Cath: 4.6
    V4A Plate: 217, Cath: 1.3
    V4B Plate:*285, Cath: 2.1
    V5A Plate: 241,*Cath: 1.9
    V5B Plate: 203,*Cath: 1.4, 144mvDC on grid
    V6A Plate: 249, Cath: 108, 85v on grid
    V6B Plate: 263, Cath: 108, 85v on grid

    V7-10: Pin 3 ~ 475, Pin 4 ~ 485, Pin 5 ~ -52

    I get 485v on the standby sw.

    From my limited knowledge, these all seem reasonable except for 6L6 screen voltages being about 10v higher than the plates. Could this cause no power output? Also, should I have 85v on the PI grids (V6)?

    Thanks in advance!

  • #2
    Did you try a different speaker and cord first?

    Turn the amp on, turn the reverb up midway, they rock the amp to crash the reverb springs. Does that come out loud or is it also diminished?


    Your screens come from ultralinear taps on the OT and so naturally have a slightly higher voltage.

    And you don;t really have 85v on your PI grids, that is meter loading. I am sure the voltages are fine, but if you want to know what is on those grids, instead of measuring to ground, measure from the cathode direct to grid. Then the volt or two you get you can subtract from the cathode voltage to get the grid voltage. The grid voltage there should be about the same as the voltage at the bottom end of that 470 ohm cathode resistor.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      You were on the right track, Sir! Turns out there were two (2) malfunctioning speaker jacks. Neither one was disengaging the shorting contact anymore with the plug inserted. Weird, since I never move it or disconnect the speakers...

      But I'm still curious about the screen voltages. Shouldn't the 470 ohm resistors drop the screen volts below the plate volts? Or is the transformer impedance from the tap to the plate end greater than 470 ohm?

      Comment


      • #4
        Ohm's Law: the screen resistors only drop voltage when current is flowing through them.

        Check it yourself. With the amp on, measure the voltage drop across one of those resistors at idle.


        And in this case, any reference in the OT would be to its resistance, not impedance.
        Education is what you're left with after you have forgotten what you have learned.

        Comment


        • #5
          Resistance, since we're talking about DC, gotcha...

          Thanks for the info & help!

          Comment

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