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AB763 Oscillator Problem

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  • AB763 Oscillator Problem

    I have a new Deluxe Reverb build with a problem I can't figure out. This is the AB673 version with the two channels joined together a la Rob Robinette to provide reverb and tremolo on both channels. The amp works, sounds good in limited testing, and biases normally. Voltages look very close to spec except for one area. The oscillator (V5) is completely whacked. I'm getting ~400V on the plates and zero volts on the cathodes. I've tried several known-good tubes in this spot with no change. I've double-checked connections, layout, part values, and can't see the problem. At this point I'm stuck. What could cause this condition and what should I be looking for?

    V5 voltages:
    Pin 1 - 408
    Pin 3 - 0
    Pin 6 - 357
    Pin 8 - 0

    AB763 schematic

    Robinette Normal channel mod:

  • #2
    I guess you've not got a footswitch connected?
    The footswitch RCA jack needs a shorting link for the oscillator to work.
    My band:-


    • #3
      Originally posted by Fletcher Munson View Post
      Pin 1 - 408
      Pin 3 - 0
      Pin 6 - 357
      Pin 8 - 0
      Means that triodes are not conducting. Apart from open FS as mentioned above, two more possible reasons:

      1) Heaters: Check if both filaments are glowing. If yes..
      2) Open cathode circuits. Measure resistances between pin 3/ pin 8 and ground.
      Last edited by Helmholtz; 12-28-2020, 06:07 PM.
      - Own Opinions Only -


      • #4
        Is trem working and sounding good? If so, you have no problem, just measurement issues mentioned above.
        "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


        • #5
          When the trem is OFF the tube is turned off and so the plate voltage goes up to B+. WHen it is running, voltages drop into place.

          I am with g1 here, if the trem works, then it is right.
          Education is what you're left with after you have forgotten what you have learned.


          • #6
            The heaters checked out fine. When I shorted the foot switch jack and re-checked plate voltage, I could see the circuit was oscillating. So I went ahead and plugged in a signal generator and found that the tremolo was working on both channels, with Speed and Intensity controls working as expected.

            So it seems I just misinterpreted the readings. I didn't understand that when the trem is off, the tube is off.


            • #7
              One follow up question: Helmholtz mentioned measuring resistance from the cathodes to ground. When I checked pin 3 to ground I got ~2.7k as expected. But when I measure pin 8 to ground the multimeter jumps around and never settles on a value. Why is that?


              • #8
                FDR set to stby (DC voltages are off) Multimeter range set to 200k and measure resistance from pin 8 to ground.
                Who does not know and knows that he does not know - teach him Confucius)
                Who knows and does not know that he knows - wake him Confucius)


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