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

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

    Ok, I got this old Supro Vibra Verb that I've been trying to fix. The thing about this amp (ch2-trem/reverb ch) is that when the volume is at 0 there is NO hum but as I increase the volume the hum just goes through the roof (big hum here). The higher I go on the volume, the higher the hum. Also, the amp distorts at 2 (way too early- using a strat for pete's sake). Bit of hum on ch1, but not as much (still there though). Ch1 (non trem/reverb) also sounds anemic (not much volume here at all). The weird thing about this circuit is that it uses a photo sensor/bulb to get the vibrato and the sensor is wired to the same preamp tube that drives the 2nd channel. I don't have a way to check filter caps but is it possible that the filter caps are shot?? Any ideas where this hum/distortion could be coming from??

    Thanks

    Joseph

  • #2
    Since both the gain and the hum seem high on channel 2 perhaps there is a mistake on one of the gain stages. Bypass caps? Dividers? I know this amps circuit has been messed with (from another thread). Look for errors that don't jive with the schematic. Once the two channels are working at what seems like the correct gain and function it will be easier to troubleshoot the hum, which is probably a ground placement issue, a noisy preamp tube or old, grossly mismatched power tubes.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      one thing i just found out is that most of the hum is coming from the vibrato intensity knob going to ground. When I turn the Vibrato Intensity knob to ground (down/off) the hum comes on strong. Any idea what could cause that?? It could also be that as I turn the vibrato intensity up then the hum gets chopped up and seems less intense.

      Thanks

      Comment


      • #4
        Originally posted by cluster View Post
        It could also be that as I turn the vibrato intensity up then the hum gets chopped up and seems less intense.
        Ground the oscillator and then adjust the intensity knob. That should answer your quandary.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

        Comment


        • #5
          not sure what you mean by that. This amp has a photo cell vibrato circuit with a bulb.

          one thing here is weird is that I measured the heat voltage and besides the 6.3 VAC which is normal, I'm also seeing 21DCV to ground on the filaments. Is that normal?? Could that be a source of the hum. I thought filaments were suppose to be only AC in these amps.

          Comment


          • #6
            here is a photo of the preamp section showing the photo-cell circuit.

            supro_vibra-verb_1965_preamp - clusterstudios's library

            Comment


            • #7
              i traced the DC on the filaments back to the power transformer. it seems that the PT secondary filament leads carry about 24VDC to ground and about 6.3VAC between them. I thought transformers weren't suppose to have DC

              Comment


              • #8
                Originally posted by cluster View Post
                i traced the DC on the filaments back to the power transformer. it seems that the PT secondary filament leads carry about 24VDC to ground and about 6.3VAC between them. I thought transformers weren't suppose to have DC
                If the filament windings do not use a center tap then they usually use a pair of 100 ohm resistors connected to ground. However you can reduce hum by connecting those two resistors to a DC voltage source. (Bruce at Mission Amps will connect them to the cathodes of a cathode biased amp in his 5E3 kit.) You didn't mention if the DC voltage was positive or negative with respect to ground.

                If you can you ought to restore this amp to its original circuit before attempting any modifications. You might want to draw up a schematic for how the amp is wired up at this time.

                Good luck!

                Steve Ahola
                The Blue Guitar
                www.blueguitar.org
                Some recordings:
                https://soundcloud.com/sssteeve/sets...e-blue-guitar/
                .

                Comment


                • #9
                  The transformer didn;t MAKE the DC, it is providing the 6vAC for the heaters. Now that whole thing is also connected to the +24v somewhere to "elevate" the heaters. Very common.


                  The exsception might be is that was not the intention and the 24v is coming from a shorted tube.

                  ANy chance you can link us to a schematic?
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
                    What is the output section configured as?
                    Push pull or cathode biased?

                    A number of Supro amps used the cathode voltage to run the first preamp tube heater.

                    Comment


                    • #11
                      I've looked long and hard with no luck finding a schematic. The one proposed in the other thread is obviously not correct. It may be close in the macro sense, but so far anything that has come up has been different between the amp and that schematic. I think service of this amp would need to be a labor of love by a tube amp electronics experienced individual. If it's been modified, and there's no schematic, the only way through it is to know enough about tube circuits to reinvent the amp.

                      Another way, and a difficult one, would be to accurately draw a schematic of the amp as it is and post it here for evaluation.
                      "Take two placebos, works twice as well." Enzo

                      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                      "If you're not interested in opinions and the experience of others, why even start a thread?
                      You can't just expect consent." Helmholtz

                      Comment


                      • #12
                        I'm tracing a schematic for this amp, but it takes time. I've kind of visually scoped out the circuit but now I need to drawn it out on paper. I have the power section drawn out and I'm winding my way backwards from the 6DR7, which by the way, drives the photo-cell vibrato. Reverb drive and recovery are done by 12ax7's.

                        Well, I though I might have solved the hum problem this evening when I was tracing back the circuit and discovered a bad bass input jack (shorted to ground). i replaced the jack, thinking that I solved the problem only to find that now, not only does it still hum, it also motor boats as well I notice that the wires to the photo-cell run along the back of the chassis just behind the input jacks and messing around in there with my chop sticks causes the hum to swoosh around. The inputs aren't shielded, which might be part of the problem. Anyways, I'll take another stab it tomorrow. In the meantime, does anyone know of a good schematic drawing app out there??

                        thanks

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                        • #13
                          Just draw it by hand and scan it. You shouldn't need it to be fancy. Just neat and functional Most schematic programs I know of take longer than drawing by hand. Use graph paper. It helps.
                          "Take two placebos, works twice as well." Enzo

                          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                          "If you're not interested in opinions and the experience of others, why even start a thread?
                          You can't just expect consent." Helmholtz

                          Comment


                          • #14
                            Still working on the schematic but the fact that the hum is ONLY in the 2ch does rule out the PS filter caps? Yes or no?? Thanks

                            Comment


                            • #15
                              I'll bet that in that amp the filter nodes are shared on both channels, so yes. I think the filters are absolved.
                              "Take two placebos, works twice as well." Enzo

                              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                              "If you're not interested in opinions and the experience of others, why even start a thread?
                              You can't just expect consent." Helmholtz

                              Comment

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