Amp squeal after mods

Discussion in 'Building the Classics' started by Tone_Chaser, Oct 25, 2021.

  1. Tone_Chaser

    Tone_Chaser New Member

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    A few years back I built a JTM45 combo. Recently I decided to convert it to a more modern amp like a famous amp based on an 800 preamp and a 45 power amp section.

    I did some research and found that it wouldn't take all that much to make the changes. I removed the parts that would no longer be used, including the PPIMV, put the new parts in and when I fired it up there was a loud squeal, sometimes it's more of a chirp sound.

    I pulled the wire from the impedance switch and it still did it. it squeals with nothing plugged in and the gain and MV are on 0. There is a gain switch which is in the lowest position.

    Can you check out the lead dress and see if you can make any suggestions?

    Thank you!

     
  2. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Start by cleaning up that sloppy wire dress and excessive lead length. And do something about that resistor running from the board to the back panel with naked, uninsulated leads.
     
  3. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    Your Orange and Green wires feeding the power tubes grids are backwards. The green wire goes to the first side of the phase inverter towards the preamp section and the orange wire to the second side by the feedback resistor on the board. They are correct at the tube sockets.
     
  4. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Any time I got the orange and green leads reversed, the amp squealed all the time, not intermittently.
     
    David Copeland likes this.
  5. Pete Farrington

    Pete Farrington Well-Known Member

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    Given that behaviour, the issues will probably be more deep rooted than lead dress, ie assuming the design is good, it probably hasn’t been correctly implemented.

    Hopefully the image is deceptive, but there looks to be a 220k resistor between a HT node turret and the hot speaker output :facepalm:
     
    Last edited: Oct 26, 2021
  6. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I see that 220k as being connected to the purple NFB line. So NFB is taking two paths... strange.

    The 220K is bypassed with a cap, and would seem to serve the purpose of providing NFB no matter which impedance tap
    is currently selected. But you get that anyway. NFB is active whether you've got a speaker connected to that tap or not.
    Where you tap NFB determines the amount of NFB, but it's not dependent on which tap is connected to a speaker.
    So that "mod" is unnecessary and completely nonsensical. Take it out.

    I see a number of resistors that were added but off the board. Some strange stuff going on there.

    This is a very non-standard build. Rules normally applied to a 50 watt Marshall may not apply.

    Not trying to be mean about it, but the workmanship is...let me diplomatic here...there's a lot of room for improvement.
    This looks like an amp that was built in a mad rush. Slow down, dress your wires neatly, do a neat job of wiring in the
    component leads, make it look professional, and it'll reward you with greater reliability, too.
     
    Pete Farrington likes this.
  7. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    Reverse the orange and green wires at the board.
     
    AndyD likes this.
  8. Tone_Chaser

    Tone_Chaser New Member

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    That was the problem. I keep forgetting about replacing the PPMIV when I was looking at this. I was checking all of the other mods and didn't think about this.

    Some of the things mentioned above are temporary while I'm waiting on a few parts. It will look better when I'm finished.

    Thank you William!
     
  9. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    I usually run same color wires to the grids and I have to check continuity but because you used the later coloring for a Marshall I spotted it quickly. The 82k side of the phase inverter goes to V4 and the 100k side to V5 since your primary plate wires are correct.
     
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  10. Nay

    Nay New Member

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    This is called motorboating, flipping the primary leads of the OT will stop it.
     
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  11. neikeel

    neikeel Well-Known Member

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    The chirping sounds like PO due to the temporary lead dress issues in the preamp wiring.
    Yes motor boating can be caused by reversed primaries (it can also cause a constant loud screech from the speaker and you have to turn off.
    The way to confirm the latter is to temporarily lift one leg of the NFB resistor (does not matter which - you are just interrupting the loop). If it cures the motor boating or squeal then you know to switch the primaries.
    The chirp is usually induced by turning the volume pot up.
    Avoid this by careful lead dress (usual rules, keep plate wires low and away from grid wires. Shield any long grid wires if you cannot keep the wires short.
    I cannot make out what mods you have done or if you have implemented them correctly. I would need a decent schem and some more pics (not video)
    Good Luck
     
  12. arthur.lowery

    arthur.lowery New Member

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    This is probably because you effectively have a 47K resistor for the feedback round the output stage. Yes, I can also see a 220K in parallel with a capacitor in series (from the 'speaker selector), but that will probably act like a short at he squeal frequencies (depending on the value of the capacitor). So, the net effect is that you have too much feedback* (this happened on a black flag plexi I looked at); the solution is to increase the 47K to 75K or more, as Marshall did in later builds, and maybe get rid of the 220K and its cap. This slightly changes the range of the Presence control.

    *Feedback around an amplifier (electronic - not via a guitar), is a complex (sic) subject, as negative feedback turns to positive feedback (and so oscillation) at higher frequencies due to stray capacitance everywhere in the circuit and the components (notably the OPT). The easiest solution is to reduce the amount of feedback ("gain compensation"). In a Marshall, it also depends on the Presence setting, and also seemed to be affected by the Treble (though simple theory would suggest that should not make a difference).
     
  13. obx351

    obx351 Active Member

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    That's what happens when you do the Ned Beatty mod....it SQUEEEEEEEELLLLLLSSSSS like a pig!
     

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