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Deluxe Reverb strangeness

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  • #16
    Originally posted by Chuck H View Post
    Notice that the vibrato channel has like phase gain stages sharing a filter node. The normal channel doesn't.
    Originally posted by Randall View Post
    I'm not sure what you are saying here. It looks to me that both channels as well as the reverb tube recovery are served by the last filter cap D.
    The reverb doesn't generally ascribe to the same phase considerations as the main (dry) signal path because there are multiple phase angles in the reverberated signal and it's rendered quite dissimilar from the main signal.

    Both channels are not used by the same instrument simultaneously in this design. We can ignore unused gain stages because there's no signal on them. If you follow the phase inversions in each channel you'll see that the normal channel only has one inversion before the phase inverter (which is supplied by the C node). The vibrato channel has two inversions of the dry signal, so there are like phase gain stages filtered at the same node. If that node isn't filtering there can be like phase interaction and that can cause oscillation. In fact like phase interaction is how oscillator circuits are made.

    Attached Files
    Last edited by Chuck H; 04-03-2020, 01:50 AM.
    "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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    • #17
      I got the go ahead to replace the filter caps. Anyone care to chime in on where to put the 22uF? I'm thinking screen node.
      It's weird, because it WAS working fine.....

      Comment


      • #18
        Originally posted by Randall View Post
        I got the go ahead to replace the filter caps. Anyone care to chime in on where to put the 22uF? I'm thinking screen node.
        Sure. Why not.?. Though I don't think it matters. Back in the day those caps had a -20%/+80% value spec!!! Whatever modern part you're putting in will be spec with the original design regardless of where the 22uf goes. As to where it might benefit the circuit most? I dunno. I'd probably put the youngest cap in your collection in the D position in the hope that it will be the freshest and most viable. Not sure how long you store your electrolytics before using them.?.
        "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

        "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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        • #19
          That^^^

          If I had a bunch of values, I'd put the largest one first. But you are talking 16 versus 22, and that is such a tiny difference as to not matter.
          Education is what you're left with after you have forgotten what you have learned.

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          • #20
            Well, darn. I replaced the filter caps, and the oscillation is still there in channel 2. It hasn't gone in the weak snarly condition yet, but I expect it will.

            Now what?
            It's weird, because it WAS working fine.....

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            • #21
              Were all the filter cap ground lead terminations checked and/or redone? What year Deluxe Reverb is this?
              "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

              "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

              Comment


              • #22
                Checking the filter cap grounds now, they all look good, less than 1 ohm to chassis. It is a 1973 DR.
                Last edited by Randall; 04-04-2020, 03:58 AM.
                It's weird, because it WAS working fine.....

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                • #23
                  Ok. Any modifications. Did it get "blacfaced" or have the small power tube grid caps been cut out, etc? The DR didn't have the same problems as other models, but they still added those grid to ground caps to the design. Probably to mitigate oscillation in shipped amps because the older wiring wasn't being duplicated.?. But I don't really know.

                  I'm surprised the cap job didn't solve it. I still think it's an oscillation though. If the amp has been modified it's possible some grounds have been moved?

                  I'll have to give this another read through tomorrow.
                  "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

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                  Comment


                  • #24
                    Originally posted by Randall View Post
                    Well, darn. I replaced the filter caps, and the oscillation is still there in channel 2. It hasn't gone in the weak snarly condition yet, but I expect it will.

                    Now what?
                    Does pulling the reverb driver stop it?

                    If yes, clean all reverb related contacts: cable jacks, plugs, jacks to chassis. Same for tank side.
                    - Own Opinions Only -

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                    • #25
                      Not sure if has been modded, but indeed it does not have those output grid caps.

                      Pulling the reverb driver makes no difference.

                      I have powered it up and down between long waits maybe 8 times since I replaced the filter caps yesterday, and although I do see the wave on my scope in Vibrato channel only, it has not once done into failure mode. I need to get this thing figured out, it has been on my bench for a week now, and I have more jobs to do.

                      edit: Well, no sooner had I typed this and it powered up in failure mode. In fact, I powered it up with a couple of 0.002uF caps clipped in from the output grids to ground, and it had barely any output. Thinking I had done something wrong I unclipped them, but it made no difference. Swapped V1 and V2 for good measure and that caused it to reset to normal. I however find that in normal mode the caps reduce the wave to about 1/5 in size. Strange how it didn't seem to effect the output? The wave is there regardless of which input tube is in V2.
                      Last edited by Randall; 04-04-2020, 07:16 PM.
                      It's weird, because it WAS working fine.....

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                      • #26
                        Ok now use a 10:1 probe with your scope and trace back the oscillation proceeding from output to input.
                        Last edited by Helmholtz; 04-04-2020, 08:09 PM.
                        - Own Opinions Only -

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                        • #27
                          I'm sorry, I am an idiot. I am afraid I have led you guys down a primrose path. This "oscillation" is probably not the real issue. I have been testing with a strat lying on a table maybe 3 -4 feet away from the amp. I can reach over and brush the strings to tell if it is failure mode or not. I just figured out if I pick up the guitar and point it just right while touching the strings, the "oscillation" all but disappears. Why I don't see the same condition on the normal channel, I don't know, but I am now convinced this is not what is causing my problem.

                          Interestingly, while I was realizing this, I only had a 12AX7 in the vibrato channel, none in the normal side. While in this configuration with the amp acting normally, I plugged in an RCA 12AX7 that the customer is bent on using into the empty normal channel. Once it came to temp, the failure condition happened in the vibrato channel. I pulled it back out, the amp popped, and the condition went away. I put this tube in my tester, and it showed one triode right at the minimum mutual conductance value allowed, and about half of the other triode. I'm starting to think this tube may be the culprit. Hard to prove without many power cycles to try to get it to go into failure mode with a different tube.

                          If this is the fix, I am sorry to have wasted anyone's time.
                          It's weird, because it WAS working fine.....

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                          • #28
                            I have had many conundrums on my bench just as a designer/builder. I actually like it in a masochistic way when things get tricky. But I don't know if I have the constitution for real working mans repair work!?! Hat's off to you (and others) that dedicate themselves and always figure it out.
                            "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

                            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                            Comment


                            • #29
                              This thing is making me crazy!!

                              I last had the wave on the scope with the vibrato channel sounding good, suspected faulty tube out, fresh replacement in. Turned it off for about an hour, switched it back on with nothing changed, and now it still sounds fine, but the wave is gone, and nothing I do gets it back. Not that I want it back, but how can this be? What is going on here? Am I going mad?

                              Damn.

                              edit: And, I power down, turn back on 12 minutes later, and the damn wave is back, but it sounds fine.

                              edit again: I turned it off, waited another hour, and this time it powered up in fault mode, very little signal, but with NO WAVE on the scope! It stayed in fault mode until I put a probe on something, then it reset to normal, and still with no wave. ARRGG!!
                              Last edited by Randall; 04-05-2020, 12:26 AM.
                              It's weird, because it WAS working fine.....

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                              • #30
                                I hate to put you onto this... Have you stabbed the red probe into the board here and there to see if it's conducting? And there's still the possibility that the bottom board might be doing it around HV contacts even if the component board isn't.
                                "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

                                "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                                Comment

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