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My supro is silent... dead silent

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  • My supro is silent... dead silent

    This is a 1966 Supro 6497 "combo tremolo" which is a 2X6L6GC amp, 5U4 rectifier, 3X12ax7 in the preamp one of which is used as a trem tube (both sides).

    Originally I thought the OT was bad as I was getting weird readings, however I'm not experienced troubleshooting transformer issues and I may have been making a mistake. I had an extra OT that works perfectly and I stuck that on there, but still... nothing.

    Problem: the amp makes NO sound. No hiss, static, nothing. Power on, and I can hear the PT humming very faintly - AC power in is fine, indicator light works, I have all new caps throughout the amp (filter, signal and otherwise), conversion to 3 prong, all grounds test good, rolled all different tubes, used different speakers, disconnected standby (which is just a PI ground to silence it) - still, nothing. Absolutely no sound. ALL voltages everywhere in the amp are where they ought to be - no funky readings anywhere. I tested all the pots and they all seem fine. This is a two channel/4 input amp, no sound no way no how neither channel. It sounds just like the PI is grounded out, but I've checked it a dozen times and it all seems fine. Tubes are not redplating, transformers not getting hot. To ward off parasitics, I put 2.2K stoppers on the leads from the PI (pin 5 power tubes) and I put a 56pf cap across the PI plates. No difference. This amp is cathode bias, no NFB, no choke. I've repeatedly checked for cold joints, continuity etc. I can't get a pop or crackle or hiss out of this amp to save my life no matter what I touch or jumper etc.

    THIS IS DRIVING ME CRAZY!!!!! Anyone have any ideas?

  • #2
    I did not see anything about a different speaker.
    O/k. The amp passes static Vdc test. Right?
    Try testing it while it is doing something.
    The preamp plates should show a signal on them.
    The grids of the output tubes should show a signal on them.
    No scope?
    Use your meter set on Vac.
    Track it down.

    Comment


    • #3
      The original speaker was garbage (as in, punctures in the cone) so I have been hooking up different speakers - nothing works, and they are all good speakers.

      Static VDC tests all seem absolutely perfect. I can't get it to do *anything* as it will not make a peep. Correct, I am scope-less (hangs head in shame).

      I will go set the meter on AC and check preamp through power amp, see what happens.......

      Comment


      • #4
        Amp on, nothing plugged in and no sound (all inputs are live, however - these are not grounded/switched inputs)

        BTW, this is single gain stage each channel into PI, like a 50s tweed pro.

        V1 12ax7: pin 1(grid leak bias, channel 1) 220VAC, pin 6(cathode bias, channel 2): 556 VAC

        V2 12ax7 paraphase PI: pin 1 385 VAC, pin 6 471 VAC

        Power tubes have 925 VAC on plates (pin 3), @ 914 on pin 4, nothing (0 VAC) on pin 5 input from PI.

        0 VAC on speaker hot from OT.

        Does this mean anything?

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        • #5
          If I hook my LCR meter into the input of either channel and inject a 120 hz or 1 khz signal, and completely dime the volume knob, I can hear it very faintly coming through the speaker. Very quiet, like it's a mile away. When I do this, the VAC readings on the output tubes drop a bit. Both channels.

          Somehow, I feel that this might be a PI problem. ??

          Comment


          • #6
            Originally posted by EFK View Post
            Amp on, nothing plugged in and no sound (all inputs are live, however - these are not grounded/switched inputs)

            BTW, this is single gain stage each channel into PI, like a 50s tweed pro.

            V1 12ax7: pin 1(grid leak bias, channel 1) 220VAC, pin 6(cathode bias, channel 2): 556 VAC

            V2 12ax7 paraphase PI: pin 1 385 VAC, pin 6 471 VAC

            Power tubes have 925 VAC on plates (pin 3), @ 914 on pin 4, nothing (0 VAC) on pin 5 input from PI.

            0 VAC on speaker hot from OT.

            Does this mean anything?
            These measurements are meaningless.
            How can you have 925 Vac on the 6L6 plates?
            What I was hoping for, as you worked your way from the input to the 6L6 grids, is a gradual raising of the signal from input to output.
            It is an amplifier.
            A 50 watt Fender 6L6 will show a signal of around 90Vac, amp dimed, on grid pin #5.
            That is with a 100mv signal in to the amp.
            See what I mean. 100mv to 90V.
            Amplified.
            Last edited by Jazz P Bass; 12-09-2010, 10:48 PM.

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            • #7
              I'm with JPB here. If you can hear the signal from your LCR meter faintly through the speaker, the signal is not getting through somewhere. Bad coupling cap would be my guess.

              What ac voltage do you read coming out of your LCR meter? Follow that voltage through the first stage. What ac reading do you get at the input of the next stage? Keep tracing the signal stage by stage until it dies out, then investigate why.

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              • #8
                Ooooooohhhhhh! I got you. Thanks very much for explaining that! I'll measure the LCR AC voltage and trace it through. The coupling caps block DC, but should allow AC to pass, correct?

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                • #9
                  Originally posted by EFK View Post
                  The coupling caps block DC, but should allow AC to pass, correct?
                  Yes, that's why they are sometimes called blocking caps.

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                  • #10
                    You also should check your VDC idle voltages on the plates, cathodes, (and screens) and B+. (VAC readings are meaningless).

                    Check coupling caps for leakage which might be affecting the bias of a following stage etc

                    You could also check your signal chain for continuity with your Rmeter.

                    You could also (carefully) do the 'pop' test on each stage's grid.
                    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

                    Comment


                    • #11
                      Oh yea....if the original paper/wax coupling caps are still in there....replace them. ALL of them.
                      The farmer takes a wife, the barber takes a pole....

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                      • #12
                        Well still working on it. I have replaced all of the old "standard" .05 poly caps, however those are not used for coupling in Supros but rather they are actually filter caps hanging off the junction of the 100K/270K series plate resistors, as Merlin explained a month ago give or take. All of the coupling/signal caps in these are ceramic disc, and so far in all the Valco amps I've worked on, probably 20, I haven't found a single leaker yet. Heven't found one in this one yet. The mix resistors for the two channels had drifted pretty far, I replaced them but it made no difference. I'm going to check out the volume pots next, but really, it does seem strange that BOTH channels / all 4 inputs do the exact same thing. If it was a problem in one channel, assuming the mix resistors are good, it shouldn't affect the signal in the other channel I wouldn't think. I get about 8 VAC out of the channels before hitting the mix resistors with .1 VAC going into the inputs. I'm getting about 48 and 55 VAC (these phase inverters are supposed to be notoriously unbalanced) out of the phase inverter to pin 5 of each output tube, with .1 VAC going into the inputs - shouldn't that make more than just a very faint sound w/ volumes at 10?

                        I'm drawing out a schematic - badly, but readable.

                        Comment


                        • #13
                          With about 55 VAC on the output tubes (pins 5) that's probably about right for this amp as these 6L6 Supros likely only make about 25 watts. This is driving me nuts!!!!!!! I suspect an oscillation problem, somehow, but don;t know where to look. Given that it is identical on both channels, I feel that it has to be PI through power amp. I rebuilt the PI in entirety a little while ago - no difference. I've jumpered an extra whopping 500 pf across the PI plates - no difference. 2.2 K stoppers on pins 5 - no difference. I'm going to try to clip in much bigger resistors, like 15K, just to see if it makes a difference.

                          Anyone have any ideas? Yes this is really how I am spending Saturday night.

                          Comment


                          • #14
                            Just tried 2200 pf snubber caps on the power tube grids ala silverface amps - no difference. I've tried clipping-in 500 pf snubber caps all over the place too - didn't do anything.

                            What is siphoning off my output?

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                            • #15
                              I do not see anything about your bias voltage on pin #5 of the 6L6 tubes.
                              I did take note that you have a 55Vac signal to the grids.
                              If the bias voltage is there, I am leaning towards a bad OPT.

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