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Plush P1000S (AB763) Tremolo Question

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  • Plush P1000S (AB763) Tremolo Question

    I've got a Plush P1000S, which is supposedly an AB763 Twin Reverb clone in a Kustom-ish vinyl sparkle cabinet. I have not been able to find schematics specific to this amp, everyone just says it is an AB763 (mostly). The tremolo on mine is pretty awful, engaging it loads down the signal to the point of being unusable even if the speed is set to 0. I took the shrink tube off the roach and the neon bulb seems to glow all the time with at least 1/2 of full intensity, and when oscillating gets somewhat brighter but not super bright.. I was expecting the bulb to go from full off to full on (whatever brightness that is), but maybe that is an incorrect assumption. I have not scoped the voltage yet to see what the voltages are, what should these values be? Should the bulb be on when there is no oscillation, and should it go full off at some point? I don't know if this could be a tube issue instead of a roach issue.

    Thanks in advance,
    Greg

  • #2
    The 'bulb' is a neon and should be off with no oscillation. If it is on then there is an issue with the bias to the driver valve.
    Straight forward to fault find, with a schematic that is.
    The AB763 uses a 100k in the cathode to ensure the valve is off when not oscillating. Click image for larger version

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    Last edited by Jon Snell; 06-15-2020, 06:34 PM. Reason: More information added
    Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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    • #3
      When the trem is off, -ve 50 volts biases the ECC83 off, when the pedal is on, the -ve 50volts disappears and allows the Colpitts oscillator to start up.
      Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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      • #4
        So I verified that with the tremolo switched off via pedal the bulb is off. Whenever it is turned on the bulb glows, with only a small amount of increased brightness during an oscillation. Is this what is expected?

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        • #5
          The neon should go between dim and bright.Neons require about 90volts to strike. If the neon is out at the off position, is there full volume? If there is, has the intensity pot been wired correctly.
          If it is just poor trem, what is the wave form and size on the anode feeding the 10M Ohm and neon?
          Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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          • #6
            Got distracted on this but getting back to it now. Took a closer look and I do not believe the tremolo circuit actually is a clone of the AB763. Here is a picture.

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            Instead of (bulb in series with 100k) in parallel with 10M resistor, it is (10M in parallel with neon bulb) in series with 100k. I think it is more like this then.

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            I have 467VDC at B, 250 after going through the 100k, and about 180 at the tube anode. It is hard to measure as the numbers are jumping around some, and going to different oscillation speeds changes the voltages.

            Wondering if I should just convert this to the AB763 circuit or if there was some reason why they changed this .

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            • #7
              It's a job to tell with a 'cloned' chassis. The 10M is there to stop the neon from striking randomly or staying lit under very low current conditions. As I said, the neon strikes at around 90volts so the 10M will hold the voltage off slightly.
              Is the Opto-Isolator a Fender part? There are many clone parts that are neon and Opto-Transistor, the original is a neon and LDR and hard to find.
              That schematic will work. It would be easy to pin point the issue using an oscilloscope. Do you have access to one?
              Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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              • #8
                Originally posted by Jon Snell View Post
                It's a job to tell with a 'cloned' chassis. The 10M is there to stop the neon from striking randomly or staying lit under very low current conditions. As I said, the neon strikes at around 90volts so the 10M will hold the voltage off slightly.
                Is the Opto-Isolator a Fender part? There are many clone parts that are neon and Opto-Transistor, the original is a neon and LDR and hard to find.
                That schematic will work. It would be easy to pin point the issue using an oscilloscope. Do you have access to one?
                I do have a scope, and this was a good excuse to break out the high voltage probe I recently got for it. Voltages were 476VDC at point B, about 280VDC after the 100k, and about 205VDC at the tube anode. The oscillation was about 12Vpp at lowest and highest speeds, and about 110Vpp around the middle of the speed range.

                Are Fender opto-isolators labeled as such? BTW, the LDR in the picture is not the original, that is what I put in to replace the original which seemed to test low resistance. Was going to put in an LED to replace the neon bulb as well (some say it reduces ticking and makes the trem deeper), but wanted to make sure the circuit itself didn't have a problem.

                I think part of the issue may also my lack of familiarity with trems like this. I was under the belief that with the trem on but speed all the way down that you would essentially get full volume, and that it would modulate the volume down from there as the speed was turned up, but since the light is on dimly for that condition (speed at 0) there will be some loading down of the signal.

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                • #9
                  Originally posted by glebert View Post

                  I do have a scope, and this was a good excuse to break out the high voltage probe I recently got for it. Voltages were 476VDC at point B, about 280VDC after the 100k, and about 205VDC at the tube anode. The oscillation was about 12Vpp at lowest and highest speeds, and about 110Vpp around the middle of the speed range.

                  Are Fender opto-isolators labeled as such? BTW, the LDR in the picture is not the original, that is what I put in to replace the original which seemed to test low resistance. Was going to put in an LED to replace the neon bulb as well (some say it reduces ticking and makes the trem deeper), but wanted to make sure the circuit itself didn't have a problem.

                  I think part of the issue may also my lack of familiarity with trems like this. I was under the belief that with the trem on but speed all the way down that you would essentially get full volume, and that it would modulate the volume down from there as the speed was turned up, but since the light is on dimly for that condition (speed at 0) there will be some loading down of the signal.
                  If Fender had been a little more technical, a jFet would have done nicely in there. If the jFet is biased off until the vibrato sends its signal, the drain (N channel) would mimic the wave form and shunt the signal, as they do with mute circuits in HiFi amplifiers. The amount of mute can be adjusted with the pot as you do now.
                  12volts is not enough to set off the neon! More like 50volts is needed. I wonder what could cause that. Is the cathode stable with no vibrato modulation visible?
                  Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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                  • #10
                    Originally posted by Jon Snell View Post

                    12volts is not enough to set off the neon! More like 50volts is needed. I wonder what could cause that. Is the cathode stable with no vibrato modulation visible?
                    There is about 75V of DC across the neon always(280-205), the 12V is the pk-pk of the oscillation at the low and high end.

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                    • #11
                      Originally posted by glebert View Post

                      There is about 75V of DC across the neon always(280-205), the 12V is the pk-pk of the oscillation at the low and high end.
                      Sounds like the neon is on all the time. try placing a 100k resistor across the neon, that may help with a sweet spot and vary the brightness more, seems like placing the 10M across the neon was an attempt to achieve just that.. I would expect about 50volts of low frequency signal across the neon and anode load resistor.
                      Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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                      • #12
                        Found something that might explain the poor performance. Was trying to figure out why the anode voltage was so low, implying much more current through the 100k + neon than expected. Measured the cathode voltage and got 0V instead of 17V-ish. I think that capacitor may be shorted out.

                        edit: swapped in the closest cap I had (33uF) and now I am getting a much better off/on cycle. Thanks for the help Jon!
                        Last edited by glebert; Yesterday, 12:57 AM.

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