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Princeton Reverb Reissue Tremolo issue

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  • Princeton Reverb Reissue Tremolo issue

    I haven't got into this one too much, but the trem is not working. This is the single triode bias modulating type . Shares glass with the cathodyne PI.
    Changed the V4 tube and tried different footswitches( stereo 1/4" type). Reverb is working with the f/s.
    Ill need to dig in and get some voltages, but the one thing I noticed right off the bat is that regardless of footswitch position, when intensity knob is at "0" the negative voltage to the grids is around -37VDC which seems fine. When turned it, the negV decreases to around -6VDC when on "10". There certainly isn't any fluctation in the bias volatge, thus apparently no oscillation.

    Any know issues with this amp or any time saving ideas?

  • #2
    Forget "known issues" and just find out what is wrong.

    For example the intensity pot. Is there a stable -6v on the end of the pot regardless of setting? SO is C22 leaky?

    The FS is a kill switch for both reverb and trem. Make sure that FS jack contact is not shorted to ground.

    V4 pins 1,2,3. Your DC voltages agree with the print? No oscillation at intensity pot, but any on the plate of the tube?

    Joint of C21, C6 goes to ground through the speed pot. Measure resistance to ground from that cap joint and see if the speed control varies the reading from 3M to 100k.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      With the FS engaged(trem off), voltages at tube are steady and match schematic. When trem is engaged, oscillation does occur at the tube, as the plate voltage swings greatly, as does the cathode. But the oscillation is not making it to the pot...

      With trem off, I'm getting -6VDC at one end of the intensity pot, and -37 at the other. Mostly steady regardless of pot position. wiper of pot puts out a range of -6 to -37 depending on pot position, so it's acting as a bias adjustment when the trem is off.
      With trem on, I'm getting the same measurements at the int pot, but it's a bit erratic at the -6v end , and barely oscillating by a volt or 2, but not swinging

      I can't get to the junction of C21,C6, but from the in of the speed pot to ground varies from 2M8 to 100K with the pot rotation
      Last edited by LarBal; 08-14-2018, 10:28 PM.

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      • #4
        With FS on to kill trem, we should have steady DC in the circuit. I am still concerned with your -6v. There is nothing to form a voltage divider with the intensity pot. So I am led to suspect a leaky C22. B+ from the trem tube plate leaking through and countering teh -40v.

        C12 and C22 may not be accessible, but R14 is. What is on each end?
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Well, with the trem off vis the FS, when the intensity knob is at 0 I have 257VDC at the plate side of R14, and 233VDC on the other side.
          With the trem knob on 10 I have 171VDC on the plate side, and 160 on the other.

          I flew another .1uf from the non plate side of R14 to the in of the intensity pot, in effect paralleling C22(without pulling the board), to see if C22 was leaky, and the same wacky voltage readings remain, and I've still got -6V on the pot side of that now .2uf cap

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          • #6
            You can't parallel to check for a leaky cap, the leakage will still be there. You have to disconnect at least one leg of suspected leaker.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


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            • #7
              Allright, well I pulled the board and installed a new C22 to no avail.
              What is happening is that the intensity has turned into a straight bias adjust, as opposed to swinging it. I put a probe on it and when the intensity knob is turned to 10, the 6V6s are pulling like 120mA...
              So I'm thinking voltage is going to ground somewhere. Maybe it's through a leaky C12?

              Comment


              • #8
                You skipped a step really. Instead of a new C22, what happens when C22 is just removed? That tells us if the DC is coming through that position or not. If it is now new, we will assume it is not leaky.

                If C22 is good, as in new, then C12 is isolated from it anyway. C12 if leaky would ground off the voltage at R14. But we have DC on the other end of C22 for some reason.

                You have a couple 6-pin connectors along the control board with cables back to the main board. Those are straight one to one connections. ANy chance the cables are crossed or is one off a pin to the side?
                Education is what you're left with after you have forgotten what you have learned.

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                • #9
                  I'm not clear on why the problem disappears when the footswitch turns off the trem, but I think it's a good clue.
                  The DC is sitting on plate side of C22 even with footswitch off, so that must not be the path.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #10
                    Well the problem doesnt really disappear when the trem is off. There is -6V on one side of the pot with the trem off, and with trem on, my matter kinda flashes on and off between 0 and -6, but not like you would get with a swinging V reading.
                    Whetehr the trem is on or off, turning the intensity pot is still producing -36 to -6 VDC at the 220K resistors, depending on where the pot is in rotation. -6V when the pot is on "10", with no voltage swing ala tremolo.

                    The 6pin connectors are properly seated. Beyond rebuilding the whole circuit, pot included. I'm at a loss.

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                    • #11
                      One thing I haven't seen mention of is the intensity pot itself. Worth a look as far as checking or replacing.
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


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                      • #12
                        A good a place as any. We clearly have a DC gradient across the pot. It is coming from somewhere. Pull the pot, now measure voltage at the three points wher the pot legs used to go.
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment


                        • #13
                          I was thinking rather that it may not be coming from anywhere, just not getting through from the bias supply, but either/eyether/or.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


                          Comment


                          • #14
                            Well, if there is a gradient across the pot, there must be current through it. And the bias goes on to the power tube grids which ought not have any DC current flow, and even if that were the path, then the far end of the pot would not be lower than the grids (wiper).
                            Education is what you're left with after you have forgotten what you have learned.

                            Comment


                            • #15
                              It was totally the pot. I ended up hardwiring in a mini pot and all was fine. Also biased new 6V6s. I took it to rehearsal(the owner was in the band), and played it for 3hrs, and after about 2 hrs the amp started sounding kinda crappy/volume loss/slight distorted undertones, etc. I said "Do you hear that"? He says , "Yea that's what that amp does after about 2 hours, it's a preamp tube". Of course he never mentioned that issue, just the vibrato issue. My goodness.
                              Nonetheless, thanks for the help on this, I appreciate it. I'm sure I'll see it again when he realizes its not a preamp tube.

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