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Motorboating vibrato circuit

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  • Motorboating vibrato circuit

    Im trying to install a Blackface Vibro Champ vibrato circuit into my amp(its cathode biased, which is why I chose this circuit) and Im getting decent results with some problems though. First and foremost, motorboating. When I turn both speed and depth up to 3 or above I get motorboating. Even with one up and one down I get it. I guess its interactive with the controls. I did change some component values from the original circuit because I didnt have all the original values lying around. I changed the intensity control from 25K to 50K, the speed control from 3M to 1.5M. The bypass resistor on the first stage is changed from 4k7 to 2k2. And the .01uF cap that hits the ungrounded side of the speed control was changed to .022uF. Also, the Vibro Champ doenst have a phase inverter like my amp does, and I noticed that other Fenders in push-pull feed their vibrato circuits into the phase inverter coupling cap. This is what I did with mine. Could any of these changes have this affect?

  • #2
    All the controls seem to function properly, and all the voltages are correct.

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    • #3
      i know this might sound kinda weird but i had a similar problem not to long ago

      i was recieving motorboating on the low freq. so after searching and searching came up with the idea that it might've been a grounding issue.....

      if you pull the pots out the chasis take a pair of needle nose pliers and tighten the gorund on the pots (they look like little tabs if you have no idea what im talking about) then put everything back together..

      might do the trick
      "Success!!! Only three leftover parts!!!" -Mr. Flavinger after putting his daughters bass back together

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      • #4
        Don't know if this is related or not, but are you using an anti-log pot for the speed control? It also might pay to go higher in value for this pot than lower.
        Building a better world (one tube amp at a time)

        "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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        • #5
          Originally posted by EETStudent View Post
          Also, the Vibro Champ doenst have a phase inverter like my amp does, and I noticed that other Fenders in push-pull feed their vibrato circuits into the phase inverter coupling cap. This is what I did with mine.

          I think of the signal as coming "from" the preamp(s) and "feeding into the phase
          inverter coupling cap" from where it then goes on to the power tube grids
          (generally left to right in a Fender schematic). The only Fender I can find
          that would fit your description of the vibrato signal being applied "before" a
          coupling cap is the 5E9-A Tremolux but then there are two coupling caps.

          Push-pull Fenders usually have the vibrato circuit modulating the bias to the
          power tube grids. These grids have a DC bias voltage on them coming from
          a separate bias supply (I haven't been able to find a Fender schematic of a
          cathode biased push-pull amp with vibrato). The vibrato signal added to the
          steady bias voltage causes the bias to go up and own, making the power tubes
          conduct more and less, producing the wobbly variation in the output volume.

          Can you provide a schematic of your amp so we can see where you have
          injected your vibrato signal ?

          Paul P

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          • #6
            Can't motorboaring also be the result of not enough filter capacitance?
            Building a better world (one tube amp at a time)

            "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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            • #7
              Eh lets see how I can best respond to the replies. Ajisjesus, I will surely try your suggestion. Tubeswell, I took both of the pots from the original vibrato circuit Im working(Gibson Mercury). I checked their markings for an indication of reverse audio or not, but it didnt say either way. I would assume though that the particular pots are fitting for a vibrato circuit. I believe I did go higher. The Fender schemo calls for a 25k pot, and I used a 50K. Paul P, from my understanding of a standard Blackface vibrato circuit, the signal leaves the intensity knob and hits just before that 220K resistor(mixing resistor I believe) and that resistor hits the .001 coupling cap. I knew how fixed bias amps produce vibrato, but since my amp is cathode bias I went with the Brown Pro circuit. That amp is single-ended cathode biased, however mine is push-pull cathode biased. Unfortunately I cant provide a schematic. Ive basically taken an old, two-channed Gibson amp and completely stripped, recreating a jcm800 preamp in one channel and I was going to try using the other side as space for a vibrato circuit. And I dont think lack of filter capacitance would be the issue here. I just replaced all the filter caps with beefier Spragues.

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              • #8
                And by Brown Pro I mean Blackface Vibro Champ

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                • #9
                  keep us posted on how everything goes man!!!!
                  "Success!!! Only three leftover parts!!!" -Mr. Flavinger after putting his daughters bass back together

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                  • #10
                    Originally posted by EETStudent View Post
                    the signal leaves the intensity knob and hits just before that 220K resistor
                    (mixing resistor I believe) and that resistor hits the .001 coupling cap.
                    The AA764 Vibro Champ :This is a circuit I hadn't seen before. It has DC on the intensity pot which I've
                    heard causes noise when operating the control but perhaps in this case the
                    voltage is low enough ?

                    Here is a AA763 Deluxe :Maybe you could try hooking your vibrato signal in this way (just move to
                    the other side of the 220K resistor).

                    You have a coupling cap at the output of the preamp in front of the 220k
                    resistor I presume ?

                    Do you have any idea of the voltage swing coming out of your vibrato circuit ?

                    Does the motorboating frequency change with the speed control ?

                    Paul P
                    Last edited by Paul P; 04-12-2008, 03:24 AM.

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