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2 Channel Amp Help with squeal on one channel

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  • #16
    Ok I am back to this amp that is gonna be the death of me. Sometimes you just have to walk away and think about it. I usually come up with some ideas but I am still dumbfounded. I do get some squeal with the chassis out of the box now but it is no where as bad as when in the box. When its out of the box if I just roll the treble back to 11 oclock the squeal goes away completely. I have swapped all the tubes with multiple new tubes and always the same result. This amp is like a living being changing on me daily. I have never had this problem and hopefully it gets worked out and I learn something that I will never forget. I am looking up transformer orientation and I get mostly threads about hum and noise. I separate all of my trannys so PT is at the opposite side as the OT. The choke is in the middle and I orientate the trannys the same way and to date I have never had a problem there. For the 90th time I rotated the chassis first trannys down and the innards exposed on top of the headshell and that is when I said the squeal was slightly present til I backed off the treble a wee bit and then I went trannys up and bam full on squealing like a pig. The only wires close to being outside the chassis (but aren't) are the tone control wires. All of my shielded cables are rock solid and not moving when I flip. Should I start hot gluing leads then flip and see what happens?

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    • #17
      For shits and giggles I once again swapped tubes from a 100w high gain amp that is dead quiet to this amp and lil piggy squealing. I tapped all the tubes and no change. The tubes are fine and I wont check them again. I can roll off the treble and kill the squeal as well as the pre gain. To me it boils down to lead dress or a signal too close to a high voltage field. Channel 1 is very high gain and does not squeal so I ruled out the power amp. Anything that travels more than a inch is shielded. This is one of the cleanest amps Ive ever built.

      I'm seriously one step away from destroying hours and hours of work because its driving me crazy. The guy has been asking for the amp for a few weeks and I have to eat shit and say I cant figure out the squeal and now all confidence is lost.

      Edit:
      Should I ground all the pots cases? I stopped doing that when I learned its not necessary.

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      • #18
        Seems everyone is stumped like me. I am just thinking about building a noise gate and put it inline. The mxr schematic seems like minimal parts and should be easy to fit in the amp. I know nothing of this gate and I hope it helps me.

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        • #19
          Solder a short pigtail , say 1 or 2 inches, not more to a .01uF cap x 400/600V, ground one end, make amp squeal and with others touch grids one by one, from input towards power stage until you find one which kills squeal.
          By the way, this is the one moment where grounding outer foil really does something.
          Post results.
          If possible, label parts (R1, R2 ... C1 , C2 .... etc.) so we clearly know what are we talking about.
          As in: "grid of V3a kills squeal"
          Juan Manuel Fahey

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          • #20
            First grid or the input from input jack to the lead channel. In the pic its the lead that runs to V3 with V1 and V2 being the tubes to the right that are for the crunch channel. I said earlier I moved that lead out from under the controls for the crunch channel which are all the pots on the far right. The squeal did go away when out of the head box but then came back. So as soon as I grounded the Mallory 150 which you will see in the pic on V3 pin2 the squeal went away completely.

            http://i587.photobucket.com/albums/s..._22_17_Pro.jpg

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            • #21
              After stopping the squeal the first time by moving the input jack lead I never looked at it again. Sure enough thanks to JM Fahey I knew where the problem was and I fixed it. So far!!! I had a bit too much of the resistor out the front of the shielded cable and I noticed I had a B+ line right beside it feeding the last 2 stages of the crunch channel. Moved that high voltage lead and shortened the input lead and bam the squeal is gone even in the headshell. So I hot glued it in place and it still is squeal free. So much time spent on this squeal I am so glad I didn't redo the whole amp because I was so disgusted. This amp took its toll on my ability to build amps and fix problems. I will never forget this one.

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