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My supro is silent... dead silent

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  • #31
    No, it's a 5U4 rectifier. I'm going to go through all the grounds first, maybe try to semi-"star" it as best as one can in one of these 2-chassis things.....

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    • #32
      Improved some grounds to an extent but didn't make any difference. *sigh*

      This is definitely an oscillation issue: if I change the signal that I'm injecting from 120 hz to 1Khz, it doesn't change the problem however at that point I can very noticeably hear a much higher pitch - I don't know how high but definitely up there - coming from the power tubes themselves, pretty loud. I don't need to put my ear right up near the tubes to hear it, it's that noticeable, and it's definitely coming from the power tubes and not through the speaker.

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      • #33
        I've been worried - after reading a little bit about modular amp sections etc - that when I tried running it through the other amp there may have been some major ground loops present being as I was not lifting ground on one or the other. Anyway, many Supros usually utilize the split amp sections (preamp in top of cab, power amp down in the bottom) and fortunately the cable connections/sockets were somewhat standardized. So, I opening up one of my 1690T amps and tried this again - doing it this way avoided any ground issues, and it worked. The preamp of the "problem" amp made perfect sound into the 1690 power amp, but the 1690 preamp into the problem power amp resulted in the same identical oscillation. So, it definitely appears the problem is in the power amp.

        I have rolled tubes, changed speakers, changed the OT (although the original is apparently good so I'll probably put it back on there eventually) and all of the filter caps. The tube sockets were pretty beat so now it has new power and rectifier sockets too. The cathode resistor and the dropping resistors are new. I have moved wiring around to death.

        The only thing I haven't swapped is the power transformer. I kind of understand what tubeswell is saying above, but since this is a 5U4 rectifier, there are no diodes. Can the power transformer itself be causing this? I guess there's only one way to find out....

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        • #34
          Has the amp got global NFB? (If so, try disconnecting it)
          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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          • #35
            Nope, no NFB of any kind.

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            • #36
              Have you started testing the OT (for shorts etc)?

              (And did you test the coupling cap(s) to the output stage for any leakage? - sorry of that's going over old ground)
              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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              • #37
                Yeah I've really hit a wall here. It has to be something going on with the PT. I'm almost embarrassed to admit it, but after all this work, when I finally got to hear it through the 1690t power amp (which is almost identical, less about 30 VDC), it didn't sound all that great. Not horrible, but nothing to write home about. The grid leak channel is kind of splatty and nasty (as some can be when they're not octals) and the normal channel is just kind of boring. I hate to just give up on it, but at the same time, I don't think it's going to be worth it in the end to keep wasting time on it so what I'll likely do is gut it out and maybe make myself a tweed pro or something similar. I have a beautiful 1962 p12n already in the cab (which is damn near 1" thick ply) and lots of new parts so I'll likely save the trem section (whcih works perfectly - it oscillates the oscillation beautifully - how ironic is that!!!!) and quick bang out a 5E- series with a trem. I appreciate you hanging in there - I think I might have a hair left, myself. Hope it's as nice in NZ as your little pic b/c right now it's 11 degrees F and about a 25 mph wind here, and it;s not pleasant!

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                • #38
                  FWIW the 5E9A or 5G9 would sound good. (I did a 5G9 with a 5k Pr Z OT and it sounds great). The weather here has been stinkin' humid for the last 3 days (but it was really nice and fine in Nov - here's hoping there is some more of that to come). If it turns to crap I've got a whole pile of home brew amp projects to retreat to in my garage.
                  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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                  • #39
                    You need a scope!

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                    • #40
                      I know it! I have no idea how to use one though.

                      FWIW, if anyone is still interested, I didn't gut it yet as it kills me to walk away from something in defeat. So it's just been sitting around. Had a little spare time and quick temporarily wired in a spare JTM45 PT which is pretty close, spec wise - and it still makes no difference!!!!

                      Something interesting I did notice - with *no* signal input at all, if I turn the volume knob/s all the way up, and meanwhile stick a .1 cap inline with my meter hot probe, I can watch as the power tube grid (seems to do it on one tube only) goes into oscillation - AC signal jumps up to around 30V and it squeals at me, then randomly, it drops back down to 0 VAC. It seems to do this completely randomly, although when I touch it w. the probe more often than not it causes it to do this. Don't know if it means anything.

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                      • #41
                        What AC sinal jumps to 30V.
                        The grid?
                        Or the output?
                        This post is so long I cannot remember...
                        Did you change out (not test, change out) the screen resistors?

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                        • #42
                          I would completly forget about the preamp until you verify whether or not the power amp is working. With the amp powered up, just touching various points on the sockets of the power tubes and phase inverter tube with your DVM probe will tell you alot about what is going on. When you touch the terminal for the plate on a power tube, in this case pin 3, you should get a little squeak or frizt (those are technical terms, try to keep up...). If not you probably have a problem somewhere btwn pin3 and the spkr. When you touch pin 5 (the grid) you should get a slight pop. If not then you have a problem somewhere downstream of pin 5, which could be the tube, the socket, the OT, the spkr, etc. Likewize on the phase inverter, you should get a louder pop on the plates and especially the grids, because now you are amplifying the disturbance caused by touching the connection. have you verified that your pwr tubes are actually pulling current? A bad ground can keep a tube from working. Just some thoughts.
                          Last edited by Billy R aka DynaFreak; 12-21-2010, 12:39 AM.

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                          • #43
                            Yeah this is a long thread for sure. Look at it this way - I have almost completely rebuilt the entire amp. Every cap in it is new. I have not deviated from the original layout at all. The 30V that jumps up wth NO signal applied on the one tube is pin 5, the grid. I know what the problem is - it is a severe HF oscillation that is draining virtually all sound out of the amp. Unfortunately, I'm not experienced enough to figure out what is causing it. I have swapped tubes, new caps, resistors, swapped transformers, filters, all new sockets, it's basically a new amp - and yet, nothing makes any difference. I have chopsticked it to death and moved wires around ad infinitum. I get good voltage readings on all the tubes, good cathode bias, good heaters etc. All grounds check out and I've semi-star grounded it as best as one can in a 2-chassis amp. I've gone up to 12K grid resistors on the power tubes. I can follow AC signal right through the phase inverter to the power tube grids, although the random signal jumping in on the grid with no signal is a little puzzling. Just drawing a blank at this point and hoping someone somewhere has an "ah ha" moment!

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                            • #44
                              Couple quickies if anyone is still laughing at me!

                              (1) Should I get any AC voltage on the 6L6 plates? I have 5.1 VAC on the 6L6 pin 3 (both tubes). Nothing on screens - I'm guessing this is just residual AC ripple from the first filter cap, just wanted to make sure.

                              (2) Is there any chance that fairly unbalanced AC signals sent from the PI to the power tubes could cause oscillations? Supros have notoriously unbalanced phase inverters. On this one, there is about a 10-12 VAC difference at any point of the AC signal in to each power tube pin 5, i.e. if I send a signal through, what little comes out may show 56 VAC on one 6L6 and maybe 68 VAC on the other. Can this be a problem?

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                              • #45
                                The Vac on the plates is o/k.
                                As long as the grid is not rippling, it should not be a problem.

                                I have seen Mesa Nomads with the same imbalance.
                                That should not be a problem.

                                What I don't get is...
                                (1) if you have 56Vac on the grids, why is nothing coming out.
                                (I know, that is the million dollar issue here)
                                (2) When you have Vac on the grids, did you look at the screens to see what they are doing Vac wise. Not Vdc.
                                (3) Are you sure the output transformer is hooked up correctly.
                                I know you said it is good.
                                Did you run a low voltage Vac through the secondary & measure the resulting Vac on the primary.
                                (4) Are the cathodes securely grounded.
                                Run Vac through the grids while monitoring the cathodes.
                                Last edited by Jazz P Bass; 12-23-2010, 01:25 PM.

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