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JCM 800 2204 Clone PreGain oscillation after pot swap

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  • #1

    JCM 800 2204 Clone PreGain oscillation after pot swap

    Built a clone of the 2204 circuit. Schematic and layout below. When I built the original, I used 24mm Bourns pots for the controls however, over time, I grew tired of their flimsy rotation. I decided to swap all of the control pots with Alpha Taiwan pots.
    I successfully completed the surgery and after firing it up I noticed that I now get constant oscillation when the PreGain pot is set above 8 or so. Between 6 and 8, the oscillation comes on when I hit the strings hard.

    Tried the easy stuff first by swapping the V1 and V3 preamp tubes. Problem still there. Decided to replace both of the shielded cable runs coming from the PreGain pot. Problem still there. Now I am thinking I just got a bad pot. I ordered a new one and totally removed the suspected bad one from the circuit and it metered out at 1,03M between Lugs 1 and 3. Its a Log pot so tracking resistance (to see if there are any problems with the tracks) is difficult. If the NEW pot doesn't fix the issue then I will need to throw the original Bourns pot back in. If THAT doesn't work then I am completely out of ideas as nothing else in the circuit has been touched.

    Can anyone give me an idea of possible suspects other than the pot? Like I said, worked just fine right before the pot swaps.

    Cheers!

    Schematic:https://mhuss.com/MyJCM/JCM800_2204.gif

    ​​​
    Layout: https://ceriatoneforum.com/jcm-800-2...4139/#msg24139

    Note: The 100pF cap on V1 is not in the schematic but, the amp worked fine with it installed prior to the pot swap.
    Tags: None
  • #2
    I think the other pots could be suspects as well, as you changed them all. Somewhere in the circuit you are getting oscillation when the signal reaches a certain level, but it could be further on down the line than the gain pot. For example, if new treble pot is letting more highs through, it might begin oscillating with either new or old gain pot set to 8 or more.
    I would put back the original Bournes gain pot for now as a test.
    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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    • #3
      Did you move ANY of the wires around at all when replacing the pots? Depending on the circuit, you can get oscillations that way too. Some are particularly sensitive to lead dress; my AB165 Bassman was one.

      Hope you can get it sorted; I hate when that happens.

      Jusrin
      "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
      "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
      "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -
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      • #4
        How does the oscillation show?
        Is it an audible frequency?
        Does changing the presence setting make a difference?
        Do you have a scope?
        - Own Opinions Only -
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        • #5
          Originally posted by Justin Thomas View Post
          Did you move ANY of the wires around at all when replacing the pots? Depending on the circuit, you can get oscillations that way too. Some are particularly sensitive to lead dress; my AB165 Bassman was one.

          Hope you can get it sorted; I hate when that happens.

          Jusrin
          I am going to poke around with my chopstick and see if it is being induced by any of the wiring. I can only assume that some of it was moved during the pot changes.

          As for other parts of the circuit "down the line," The amp works great when plugged into the Low input. This basically takes everything out of consideration regarding the Tone stack and just about everything after the first gain stage and the Pre Gain pot itself.

          The oscillation presents itself as a high pitched, high frequency "scream." Almost like those high pitched fire alarms. The dogs certainly didn't like it!
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          • #6
            Originally posted by Govmnt_Lacky View Post
            As for other parts of the circuit "down the line," The amp works great when plugged into the Low input. This basically takes everything out of consideration regarding the Tone stack and just about everything after the first gain stage and the Pre Gain pot itself.
            I don't follow this logic but am not going to argue.


            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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            • #7
              Originally posted by Govmnt_Lacky View Post
              The amp works great when plugged into the Low input.
              Tis makes me suspect lead dress and or a possible ground issue.

              ​
              Originally posted by Govmnt_Lacky View Post
              This basically takes everything out of consideration regarding the Tone stack and just about everything after the first gain stage and the Pre Gain pot itself.
              Sort of. The thing to remember about oscillation is that it's often (if not usually) a positive feedback loop invisibly induced by component ot lead proximity or hiding in the voltage rail or ground scheme. So consider that when you are plugged into the low input that the phase of V1a is opposite from when you are plugged into the high gain input.

              Since everything following V1a is phase analogous V1b becomes suspect as part of the loop. Check lead dress for V1b. Make sure it's grounded correctly. Perhaps even eliminating any shared or daisy chained grounds with other like phase circuits. And check the lead dress for any circuits of opposing phase with V1b where they are in proximity to anything in the V1b circuit.
              "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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              • #8
                Originally posted by Chuck H View Post
                consider that when you are plugged into the low input that the phase of V1a is opposite from when you are plugged into the high gain input.
                This is something I hadn't even considered.
                I do hope it turns out to be the gain pot or lead dress. My point was about the signal level getting to other parts of the amp. Without knowing the signal level being used for testing, it's just speculation. But if some signal level 'down the line' is required to put the unit into oscillation, then to test with the 'low' input, you would have to jack up the input signal level so you still got the equivalent signal level at the V1A plate (compared to using 'hi' input).

                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                ​

                Comment

                • #9
                  Thanks all! Turns out it was a wiring issue. The lead going from the 68K High Input resistor (on the PCB) to V1 Pin 7 was a bad solder joint. I presume that moving the wire when I installed the new pots must have exposed it and caused the oscillations. I simply re-soldered the joint on the PCB turret and the oscillations went away
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