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Big Hum (+ distortion) with volume pot. Help.

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  • #16
    Ok, I drew up some schematics. I couldn't put the whole thing on one page so I did it in sections that interconnect. I might try to tape 4 blank pages together to see if i could get the whole thing in one place but for now this is what I got. Sorry for the messy drawings, i'll try to clean them up tomorrow. I don't see any mods and I don't really get the photo cell vibrato circuit and how it works with the 2nd channel preamp section. maybe some one here could explain it to me. Also, I don't see how the reverb circuit blends with the 2nd channel. maybe someone who is familiar with reverb circuits could explain that one. Phase inverter looks normal as does the power section/ power supply. Anyways, I got rid of the motor boating . Put shielded wires on the 2nd channel input and replaced a couple of the brown .05uf caps off the plates but the hum is still there. Not sure if that photo cell is the cause of it or not? Maybe some kind of ground loop Any ideas would be appreciated.

    supro vibra verb 1965 schematic in sections

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    • #17
      Did you verify that the heater CT is connected to the cathodes of the power tubes?
      I'm sure not keen on heater voltage over 7VAC.
      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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      • #18
        No CT on the filaments. I guess I could tie a couple of 100ohm resistors together to create a virtual CT and connect it to the cathode of the power tubes.

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        • #19
          I think the trem is mis-drawn, and that is why it is mysterious. The pin 6 side of the trem tube is the oscillator, but you have no feedback path. I think from pin 6 will be another connetion, probably to that 470k to left of the intensity control. And that line down to ground from the 470k doesn;t belong there. That would create an oscillator like a zillion other amps.

          The pin 1 side also looks wrong, there is no path for cathode current. Perhaps the grounded 47k goes to pin 9 instead of across the bulb?


          The input stage looks funny to me also. V1a has no grid return resistor. Are you sure that 6.7M resistor is not there? Yellow and blue photocell wires, are they not connected to ground? Those tube cathodes need a ground path. In fact, if I can use my imagination, on the V1b, I might expect the 3.3k to be a cathode resistor.

          I susspect that 500pf cap feeding the reverb should come off the plate of the tube, not the B+ end of the plate resistor. And as to how they blend, I se no dry signal path, I suspect a connection from the plate of V2b has another connetion
          Education is what you're left with after you have forgotten what you have learned.

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          • #20
            Thanks for the input enzo. I'll retrace the circuit again tomorrow and double check the layout. Thanks again.

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            • #21
              If you are absolutely sure there is no connection between the heaters and a 24VDC source, then add the 100R's for virtual centre tap. Might even cure your hum .
              Most likely source for DC heater reference would be power tube cathodes, pull the power tubes and check if there is still DC on heaters. If it is gone, then they are connected somewhere.
              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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              • #22
                Ok, I redrew the circuit right. Here is the schematic. Supro vibra verb vibrato circuit schematic. Not sure how I miss drew the old one Must have been a late night. This one seems ok though. Let me know what you think. I'll look over the preamp section again, where the photo cell joins with the circuit. Strange that the cathode there is not grounded to the chassis Let's see...

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                • #23
                  Originally posted by g-one View Post
                  If you are absolutely sure there is no connection between the heaters and a 24VDC source, then add the 100R's for virtual centre tap. Might even cure your hum .
                  Most likely source for DC heater reference would be power tube cathodes, pull the power tubes and check if there is still DC on heaters. If it is gone, then they are connected somewhere.
                  I measured without the tubes and got about 1.4vdc on the filaments. the reading wasn't stable and was drifting downward. with power tubes the reading was a stable +22vdc. AC across the filaments was a constant 7.3VAC using a wall voltage of about 127vac.

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                  • #24
                    I should also say that besides the heavy hum, at times I also get a high pitched wine sound as well as a ray gun sound. kind of like a synthesizer sound. not all the time but if i start banging around and such the amp will start to wine. i also took tubes out one by one and the only one tube that got rid of the hum was V1. I guess that is the hum is originating.

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                    • #25
                      Here is the redrawn schematic for the second channel. There is indeed a dry signal that bypasses the reverb. Also, I can't figure out why none of the cathodes in V! are not grounded

                      supro vibra verb 2nd channel schematic

                      i also redrew the 1st channel. very low gain. only half a 12ax7 driving it. no wonder it sounds anemic.

                      Supro Vibra Verb 1st channel schematic

                      let me know if you need a higher res image.

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                      • #26
                        I think i've narrowed it down to the 1st preamp tube section on the 2nd channel just before the volume pot. The hum varies with the volume pot just on that 2nd channel. Removing that 1st preamp tube also illuminates the hum. The problem is that 1st preamp tube is also the one that is connected to the photo cell in that weird arrangement. Not sure how to tackle that one?? Could I just disconnect the photocell from the preamp tube?? Tempted to try...

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                        • #27
                          Case solved!! V1 plate resistors are suppose to be 10K (not 270k). Now the amp is dead quiet. I bumped them to 15k just to get a bit more out of that channel. Now the amp roars the way it should. I'll post the revised schematics over at valco forum for those Supro Vibra Verb owners out there. What a mystery tour this amp was. Glad it's over.

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                          • #28
                            I know this is an older post Cluster but I just rescued a 1965 Supro Vibra-Verb from a dumpster. I can't locate a schematic for this amp. Did you ever have any luck?
                            Also, thank you for your work, hand drawing the circuits. It will be very helpful to me.
                            The amp I have is in rough shape but everything this there. I have to buy new tubes, recap, and speakers and possibly a new output transformer. Any info will be helpful as I start on this project. Thanks

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