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Tracking down oscillation in a Krank Rev1

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  • Tracking down oscillation in a Krank Rev1

    So, I've had this hellbeast of an amp on my bench for months tracking down problem after problem. I'm down to one last problem that has stumped me. The gain channel is oscillating, and for the life of me I cannot figure out why. Here's a schematic:

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    Here's what I've deduced so far:
    1. The first point I can see it on my scope is at the plate output of V2B. Obviously it must be present prior, because Grounding V2B's grid input makes it go away, therefore I'm thinking it's between V2A and V2B that it's starting.
    2. On the note of the above, switching to the clean channel (which grounds V2A) also causes the oscillation to disappear.
    3. I'm aware there are no grid stoppers in this amp. Adding a 47k stopper at V2B made no difference, and doing so on V2A is more or less impossible (the trace is on the bottom of the preamp tube board, and I'm not keen on cutting traces if I don't have to).
    4. The cables from the input jack to the preamp tube board as well as the one from the main board after the relay between clean and dirty are shielded cables, and I've checked their integrity.
    5. The power supply is not the source of the issues - the supply side of the plate resistors is clean.
    6. All grid leaks check out both in terms of value and their connection to ground.
    7. The oscillation is not present on the clean channel, at all.

    So... what am I missing? My thought now is that it's just RFI at V2A, but if that were the case wouldn't it be present on the clean channel at V1A too?

  • #2
    Is the oscillation frequency in the audio range?

    Is this with a guitar plugged in as the 'input' ? If so, then does changing the 'input' to say a 100k resistor in a jack, or just inserting a new jack (with no lead connected to it) do anything ?

    Can you change any part values 'on the fly' in the V2A/V2B region, to see if the oscillation level and frequency changes in value? Eg. add in extra cathode bypass across C2 or C1 or R26, or across C27 or C25 or R30.

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    • #3
      I'm not convinced yet about the power supply. You can get oscillation due to poor decoupling yet the supply rail looks fine. Tacking a known good cap across C19 is still worth a try.
      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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      • #4
        Originally posted by trobbins View Post
        Is the oscillation frequency in the audio range?

        Is this with a guitar plugged in as the 'input' ? If so, then does changing the 'input' to say a 100k resistor in a jack, or just inserting a new jack (with no lead connected to it) do anything ?

        Can you change any part values 'on the fly' in the V2A/V2B region, to see if the oscillation level and frequency changes in value? Eg. add in extra cathode bypass across C2 or C1 or R26, or across C27 or C25 or R30.
        1. Yes and it changes with gain, treble and master volume.
        2. As long as something is breaking the shorting jack's short, yes.
        3. Sure, I can give stuff a shot.
        Originally posted by g1 View Post
        I'm not convinced yet about the power supply. You can get oscillation due to poor decoupling yet the supply rail looks fine. Tacking a known good cap across C19 is still worth a try.
        Aw, that means I'm gonna have to pull the mainboard again... Friggin' radial caps lol.

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        • #5
          The first point I can see it on my scope is at the plate output of V2B. Obviously it must be present prior, because Grounding V2B's grid input makes it go away, therefore I'm thinking it's between V2A and V2B that it's starting.
          Preamp oscillation in the audio range typically involves 2 inverting gain stages. So I suspect some unwanted coupling between V2A and V2B. This could be due to a bad C19 or a bad ground connection.
          Last edited by Helmholtz; 02-13-2020, 03:17 PM.
          - Own Opinions Only -

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          • #6
            The schematic shows that opposing phase stages are sharing the HV supply nodes. This arrangement tends to not oscillate with power supply decoupling cap failure. But do try paralleling another decoupling cap anyway. More likely a bad ground connection on a load resistor or ribbon connection (I think).
            "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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            • #7
              I think you could tie the parallel cap (for C19) to the top of one of the plate resistors if you want to avoid flipping the board?
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment


              • #8
                FWIW I've read a couple of threads elsewhere that indicate a Krank amp (a few different models) as prone to oscillation. One thread here and SGM (don't ask if you don't know) said "design flaw" and hinted that the problem was probably not a problem at most amp settings and the manufacturer gave it a pass. SGM was already on everybody's nerves here for the very attitude that made him say that. No experienced opinion, just rash judgment. So the other members here called "BS" and chastised him (as he most often deserved)... This may have been a knee jerk reaction that instantly biased the thread to where there was no possibility that a respected manufacturer would actually do that. But I often leaned his way WRT his opinion on modern amp design. It's possible he was not wrong.?. Ideals from the golden age of tubes have indeed been lost along the way and performance demands are very different. There absolutely are amps made by known manufacturers that have inherent problems, including being prone to oscillation at some settings. I'm not saying this is the case, but I wouldn't rule 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

                "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


                • #9
                  I have the same problem with Dimebag signature Krankenstein/Rev1 model (DIP switch to choose between the two). I mean, 4 gain stages with anode driven tonestack sounds like a recipe for oscillation...

                  Anyway, the amp is bone stock except for a replacement PT; had low volume that I sorted out with contact cleaner (a LOT of it). It only oscillates on the distortion channel with both gain and channel volume past 3 o'clock. Master volume doesn't affect it at all. If i remember correctly, it happens even without guitar plugged in.

                  I can see a lot of signal wires (shielded) routed all over the place. I'll try it with DIP switches set to Rev1 and report back.

                  The manual clearly states that for best performance, you should never put master volume lower than channel volume.

                  Edit: having looked at other similar high gain amps, I can see cap across V3A (in your case) anode resistor in 99% of designs.
                  Last edited by m1989jmp; 02-14-2020, 10:39 AM.

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                  • #10
                    Originally posted by Chuck H View Post
                    The schematic shows that opposing phase stages are sharing the HV supply nodes. This arrangement tends to not oscillate with power supply decoupling cap failure. But do try paralleling another decoupling cap anyway. More likely a bad ground connection on a load resistor or ribbon connection (I think).
                    So I did end up paralleling another decoupling cap (all I had around were 10uF axials) across C19 and while the oscillation didn't go away, it certainly changed, and for the better...

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                    • #11
                      Further update: Changed the cap. It's better, but still not solved. When I touch the gain pot shaft, the oscillation gets worse. Far as I can tell all grounds are good (measuring via continutiy test).

                      Comment


                      • #12
                        switching to the clean channel (which grounds V2A) also causes the oscillation to disappear.
                        Where does the "grounding of V2A" happen? Can't see it from the preamp schematic. The grids of V2A and V1A seem to be connected?

                        Does the oscillation at the plate of V2B stop when you pull any of the other tubes?
                        - Own Opinions Only -

                        Comment


                        • #13
                          Originally posted by Helmholtz View Post
                          Where does the "grounding of V2A" happen? Can't see it from the preamp schematic. The grids of V2A and V1A seem to be connected?

                          Does the oscillation at the plate of V2B stop when you pull any of the other tubes?
                          It's not noted on the schematic (that is not an official schematic, but it's the closest I've got as the designer himself didn't even have a copy), but there's a relay between them that switches one grid to ground and the other to the signal path.

                          As far as pulling other tubes, it actually gets worse if you pull V1. If you pull V3, it's unchanged.
                          Last edited by DFuller; 02-16-2020, 01:17 AM.

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                          • #14
                            If you pull V3, it's unchanged.
                            That seems to confirm that the problem is restricted to the V2/V1 section. As pulling V1 makes it worse, it seems to be somehow involved as well. What happens if you ground the V1 grid?

                            Did you only exchange C19? I would exchange C16 and C32 as well.
                            Also verify that cable shields are well grounded at one end.
                            Especially the input switching relais wiring is critical and needs good shielding.
                            Do the preamp tubes have metal shields?

                            I actually wonder if this is a repair or a redesign. Was the amp ever completely stable? Any signs of modifications or prior repairs?
                            - Own Opinions Only -

                            Comment


                            • #15
                              Originally posted by Helmholtz View Post
                              That seems to confirm that the problem is restricted to the V2/V1 section. As pulling V1 makes it worse, it seems to be somehow involved as well. What happens if you ground the V1 grid?

                              Did you only exchange C19? I would exchange C16 and C32 as well.
                              Also verify that cable shields are well grounded at one end.
                              Especially the input switching relais wiring is critical and needs good shielding.
                              Do the preamp tubes have metal shields?

                              I actually wonder if this is a repair or a redesign. Was the amp ever completely stable? Any signs of modifications or prior repairs?
                              A tech who was perhaps less than competent was definitely in here before me, but as far as I can tell, the preamp was untouched - all the component values match properly.

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