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Park 150 60cycle hum

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  • Park 150 60cycle hum

    I've got a Park 150 here, came in working , but with a hum. It has the UL OT and has around 600VDC on plates, loaded with 4xKT88. The filter caps had some bulges, so I replaced all the original capcans, and also moved the giant 27K 9W dropping resistor in the power supply off board(where it had burnt) and onto the chassis with a chassis mount 10W. The amp sounds fantastic, but has a 60cyc hum that is not affected by the volume.

    This OT is ultra linear, and the screens are fed by the screen taps. The screen resistors are these 4 large cylindrical 1K things, that also sit on top of two 40r resistors of the same size, and wired in series for 80r, that I believe were originally wired to the cathodes. There is a fixed bias arrangement also, which I restored with the cap job, and I'm getting a good range fromm -100VDC to -50VDC from it.
    As it is now(when I got it), the cathode resistors were disconnected from the cathodes, which were grounded, so this amp is solely running in fixed bias, as opposed to the dual bias method it had previously.

    I can't find a schematic for the power supply, and I've reached out to the tech from Psionic that has the video series on youtube, but haven't heard back. His amp had the same hum, but it was not an UL, so he just moved the screen supply to after the choke.
    This power section is nearly impossible to trace out, as every wire is sheathed in tubing, and neatly bundled in groups. I would literally have to disconnect everything and start doing tests to figure out what is what. I'd like to avoid that.

    I'm having to look at the Marshall Major schematics for clues. Something that I don't understand is that in this bridge rectifier setup, w/a CT PT, the CT is run to the junction of the first set of stacked filter caps, for balancing. It is a black wire. I understand that, and that also lines up with the color code for the Major. However, there is another black wire that also connects to the junction of the stacked caps, and I can't figure out what that is. That may neither be here nor there, but I'm trying to consider if my bias circuit or the heaters could be the culprit, and trying to get the big picture here first to get a plan together. There is another ground tab close by the first caps' ground, that has 2 wires grounded, that I believe to be the heater and bias supply center taps.

    Any thoughts are helpful, thanks! Here are 2 schematics, one from Marshall and another redrawn


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  • #2
    If it's actually 60Hz hum and not 120Hz, it can't be power supply ripple (assuming all rectifier diodes are good).
    Might be a bad bias filter cap. But only with half-wave bias rectification - unlike the Marshall. Make sure.

    Are output tubes balanced?

    This power section is nearly impossible to trace out, as every wire is sheathed in tubing, and neatly bundled in groups. I would literally have to disconnect everything and start doing tests to figure out what is what. I'd like to avoid that.
    C'mon, it's just a simple tube amp.
    Identify wires by measuring voltages at both wire ends or use your Ohmmeter.
    - Own Opinions Only -


    • #3
      It's definitely 60Hz. I replaced both caps in the bias supply when I did the cap job. I'll tack a couple more in just to make sure they're good. Seems like yesterday I was working under the pretense that even with all pre tubes pulled, it was humming.
      Now I'm getting odd behavior when pulling tubes....when pulling V4 (12AU7 driver) the hum is decreased considerably. When pulling V1 is get waaay louder?!?! Here's the preamp schematic. Seems all these amps were a little different. In my amp, the 47K plates of the driver and 68K bias feeds were reversed....likely a mistake at the factory, as the solder joints are untouched.

      Preamp schematic :
      Last edited by LarBal; 09-05-2021, 07:25 PM. Reason: forgot schematic


      • #4
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        • #5
          Mains hum is caused by a bad ground point. Nothing to do with smoothing or decoupling capacitors.
          Check all ground points.
          Support for Fender, Laney, Marshall, Mesa, VOX and many more.
          If you can't fix it, I probably can.


          • #6
            And... Are you sure all the power tubes are conducting? Something like an open screen resistor would remove one tube from one side of the push pull and reduce hum cancellation in the power amp.
            "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


            • #7
              All the tubes are pulling current, and somewhat matched. I've got 3 pulling 30mA and one at 34mA. All the screen 1Ks measure in spec, and I've got 600V on all screens. All grids have -70V after the 5K6 grid resistors. The power tube setup seems solid.

              As far as the increased noise with V1 pulled, the treble cap was bad, replacing it solved that problem. I started to rework the grounds, as they're pretty bad, everything connected to the pot buss, but wasn't convinced that was it. The hum remains... Also popped in a set of 6L6 and rebias, no change


              • #8
                Rewired the entire ground scheme, separated everything from the input to the driver from the power amp grounds. Drilled a hole by the input to do so. Ran a floating buss that I tied to said hole. Maybe 25% reduction in hum.


                • #9
                  Well, hum is not generic, you could have more than one source of it. Each source will require its own cure.
                  Education is what you're left with after you have forgotten what you have learned.


                  • #10
                    I've got this one back on the bench. I was able to knock the 60hz hum down a bit, but I feel it was all just bandaids. Shielded wire to the input grids helped, and shorter wire runs to the pots helped, but taking off the cathoide bypass cap on the 2nd stage helped a lot. Of course now the amp isn't nearly as loud as it should be. Also of note, the 2nd stage input (schematic in post #4) was factory wired with a 33K cathode and a 22n bypass?!?!? This is obviously not right, I imagine it should have been a 3K3/22u . I think I had mentioned there were some other values in the amp, that when compared to other amps I've seen, makes it look like the builder was drunk. Or it was Friday.

                    Anyways, the hum remains, and is awful if I bypass that 33K Rk, and unbearable if I reduce it down to something more practical like 3K3.

                    Here are some thoughts at this point:

                    The bias circuit :
                    It's a centertapped winding that feeds a 2W pot, then filtering and voltage divider. The schematic is in post #1 . In my previous efforts I added a 100uf right at the junction of the diodes and the pot. I thought that had cured it, by all it actually did was make the bias very essence making the negative voltage jump from -80 to -100 . Why would that happen? That's how I have it in the amp now still, but I adjusted the bias back to around 30mA

                    The heater circuit:
                    It also has a centertapped winding, with the CT firmly grounded, but could there by an issue with this winding that lifting the CT and adding two 100r to ground from each leg might help? elevated? My biggest concern in this whole mess is not smoking the PT so I'm being overly cautious.

                    Pulling tubes: The amp does not hum when power tubes are pulled, and pulling the 12au7 driver also quiets the hum. Can I rule out OT and choke issues?


                    • #11
                      Is it still definitely 60Hz hum?

                      Both power supply and bias ripple would be 120Hz fundamental - except one of the diodes is bad.
                      - Own Opinions Only -


                      • #12
                        Yes it's 60Hz still. I replaced the 2 diodes in the power supply yesterday, haven't replaced the bridge rectifier in the power supply. There are 8 diodes there. What is the best way to check those?


                        • #13
                          Originally posted by LarBal View Post
                          What is the best way to check those?
                          Lift one end of the diode and proceed as described here:
                          - Own Opinions Only -


                          • #14
                            I'd be very iffy about trusting any of those schematics until you go through and verify what you have in front of you. The one in post #4 has 4 preamp tubes, the other schematics you have posted only have 3.


                            • #15
                              The Marshall Major circuit of post #1 has a design flaw that's hard to believe: It shows the tonestack within the global NFB loop.
                              That would make the tonestack rather ineffective as the NFB would counteract any change in tone settings.
                              Also bears a risk of instability caused by phase shift.
                              Last edited by Helmholtz; 09-16-2021, 03:15 PM.
                              - Own Opinions Only -