My Marshall is now a Theremin..parascitic oscillation

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pfrischmann, Jun 15, 2005.

  1. pfrischmann

    pfrischmann Member

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    Hi guys,
    I've been working on a super bass that I converted ti '69 SLP specs with a Metro amps board. This amp had an oscillation problem before the conversion and unfortunately, still has it.

    With the presence beyond 12:00 and the treble channel up past 10:00 I get a squeal. I can control the squeal by turning the presence knob up and down and/or by putting my hands near the tubes (ala theremin)

    I have so far replaced the circuit board, fitler caps and rewired a good portion of the preamp. I also added 5.6K swamp resistors.

    The problem still exists.

    I checked the layout to the bes of my abilities following the Ken Fisher tips in Gerald Webers book.

    Any ideas.

    I'm running out of places to look. I'm trying to see if I can figure it out before I spend money sending it to a pro.

    Thanks,
    Paul
     
  2. bruce egnater

    bruce egnater Member

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    With and/or without the guitar plugged in? Did you replace pots during the modification? Has the output transformer or transformer wiring been replaced or changed? Did it have any modifications before you changed it? Does the presence control work properly, does it get brighter as you turn up the presence control?
     
  3. pfrischmann

    pfrischmann Member

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    Hi Bruce. BTW cool modular preamp!

    Let's see if I can answer your questions.

     
  4. Swarty

    Swarty Member

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    Do you get it in both channels?
     
  5. John Phillips

    John Phillips Member

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    If it only does it with something plugged into the inputs, it's either a problem related to V1 (possibly grounding of the cathode network) or the jacks, or electromagnetic coupling between the OT and the guitar pickups. How close are you standing to the amp?

    Coupling when you put your hand near the power tubes (if you're wearing or holding the guitar) is quite normal, BTW.

    Does it still do it if you turn down the guitar?
     
  6. pfrischmann

    pfrischmann Member

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    I do get it into both channels but the treble channel is a lot worse.
     
  7. pfrischmann

    pfrischmann Member

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    Hi John,
    I can stand anywhere from 1-20ft away and it will oscillate, squeal, and do the buzzing thing under the notes.

    I'm not sure If I understand the "coupling when my hand is near the Power tubes" part. What seems to happen is that I can quiet down the oscillation or even control the pitch when I place my hands close to the tubes. It will still do it whether my hands are there or not. Does that make sense?

    Thanks,
    Paul
     
  8. Swarty

    Swarty Member

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    If you have a DMM I'd be curious as to the resistance value you get when you measure from the guitar cord tip to the chassis.
     
  9. John Phillips

    John Phillips Member

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    Yes, entirely - that's normal. Your body is modifying the electrical field, which is exactly how a theremin works too. You're possibly affecting the coupling between the OT and the pickups, which is why I wonder if it's that.

    Does it still do it with the guitar volume down?
     
  10. pfrischmann

    pfrischmann Member

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    I believe it does do it with the volume on the guitar down. I will have to verify tonight.
     
  11. pfrischmann

    pfrischmann Member

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    I just verified. It does squeal with the guitar volume off. It's a lot worse with the volume up however.

    Does that mean something?
     
  12. bruce egnater

    bruce egnater Member

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    Does the pitch of the oscillation change with the setting of any knobs or the guitar volume? Was the inside of the front panel clean when you replaced the pots? I have seen Marshalls become unstable when the panel becomes corroded and the ground is not solid.
     
  13. pfrischmann

    pfrischmann Member

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    The higher is set the presence or the volume the"deeper" the sound gets. It sounds like the amp is bearing down if that makes any sense.

    I didn't think to check the front panel.
     
  14. bruce egnater

    bruce egnater Member

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    This may be silly but it is assumed you tried different cables, guitars etc. Also, it only does it when you get past a certain volume setting, correct?
     
  15. pfrischmann

    pfrischmann Member

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    Hi Bruce.
    I did try different cables and guitars. It will do it with any cable and a guitar doesn't even have to be plugged in.

    It seems to always be there, just really pronounced as the volume and or presence goes up.

    If this is important, I can research it more.
     
  16. John Phillips

    John Phillips Member

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    Yes, it means that the problem is probably in the grounding of the input jacks and/or pot network. At least, that's where I'd look first. Check that the jacks are correctly connected to the busbar that runs along the back of the pots, and that the busbar itself is also grounded via a wire to somewhere in the power supply (the negative terminal of the first filter cap stack is ideal) - often they're not, and just rely on the contacts between the pot backs and the bushings, then to the panel, to ground the whole lot. Since the current from the preamp cathodes, and PI and feedback loop (via the presence pot) also goes to ground through this busbar, it's essential for it to be properly grounded. If there's any resistance between that and 'true' ground, it can set up a positive feedback loop.
     
  17. pfrischmann

    pfrischmann Member

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    Thanks John,
    Heres what I have two grounds going from each input to the buss bar on the back of the pots and one ground wire coming from the the negative terminal of the filtercap under the board. Using the method below, all the grounds have very low ohmage readings.

    should I still add a ground from the bussbar back to t filter cap in the power section?


    I was reading in Gerald Webers book about checking the grounds in the amp in general he suggests that I use an ohm meter, connect one end to the center tap ground (PT) and then check every ground in the amp, if we have any unusually high readings, we may have an issue.

    Any thoughts ?
     
  18. John Phillips

    John Phillips Member

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    Run the wire to the main filter negative anyway. A typical ohm-meter isn't quite sensitive enough to definitely eliminate a ground problem - although it wouldn't hurt to check first.

    This is one of the reasons many builders still prefer to use aluminum for the chassis BTW - it's a better conductor than steel and doesn't suffer rusting... which especially happens round the pot holes, just where you don't want it, on the steel ones.


    BTW, on a 100W Marshall the PT center tap isn't connected to ground, it's connected to the mid-point of the first filter cap stack; there's a bridge rectifier arrangement, so you will definitely find a lot of resistance between the center tap and ground :).
     
  19. pfrischmann

    pfrischmann Member

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    BTW, on a 100W Marshall the PT center tap isn't connected to ground, it's connected to the mid-point of the first filter cap stack; there's a bridge rectifier arrangement, so you will definitely find a lot of resistance between the center tap and ground :). [/B][/QUOTE]

    Good that will save me from having to find the PT color codes.

    BTW,
    does it matther where I attach my grounds on the buss wire on the back of the pots? (between treble and volume 1 as opposed to bass and mid pot for example)

    Thanks again,
    Paul
     
  20. pfrischmann

    pfrischmann Member

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    O.K. This is really interesting

    I'm not sure how to explain this. It seems like the inputs are acting like antennae. The shorter a cable I plug in, the worse the sound. I can plug un a 3' cabble and move areound the excess and hear difference things depending on where I put it.

    I did ground the pots. It seems to be making a small difference but not enough.

    *****I did notice something else*******
    I was looking at the OT. The marks on the outside I were told was just some mold or something is more likely score marks on the bell cover from arching. Could the OT be damaged and causing the oscillation as well as the amp to seem underpowered?
     

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