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Amp squeals at hi gain

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  • Amp squeals at hi gain

    Hi folks,

    I assume I have a problem with the wiring of my latest project.
    The amp is based on a deluxe reverb II or concert reverb II and sounds great in the clean channel (I added a switchable third gain stage here like in the champ II). Unfortunately the gain channel squeals at hi gain settings. If I set volume, gain and master at around 6 the sound is good. If I turn either pot higher it starts to squeal. I fumbled around with a chopstick and moved the wires around but there was no big change. (yes, I use shielded wire). (tubes are all brand new tung sol's and I even changed their places with one another).
    Only difference to the Deluxe Reverb II in the gain channel is the plate resistor of tube 4A. In the original schematic there are two resistors 82k and 22k and in between them is the lead to the coupling cap. I use a 100k instead like I do in my clean channel. Can somebody tell me why there are two resistors in the fenders?
    There is a .0005 cap from plate to cathode of that tube too. Can anybody tell me what that is for?
    I appreciate any help or comment.

    Thanx in advance

    Matt

  • #2
    Well, I found somewhat satisfying answers to both of my questions on the net (could have looked before, huh?). Will add a 500pF cap from plate to cathode and leave the 100k as it is. I'll see what happens after that.

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    • #3
      Depends where the coupling cap fits in the 82/22K split load combo - If that's 22K from the B+ supply to the coupling cap, with 82K between coupling cap and plate, that equates to a sizable amount of attenuation & a big difference compared to 100K as a non split load.

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      • #4
        That must be it.
        I changed what I said before and the amp reacts better but still squeals at (now higher) gain settings.
        I'll try the two resistors version and let you know.
        Thanks for the hint.

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        • #5
          That was it.
          Changed the 100k for a split load of 22k -- 0 -- 82k where the 0 stands for the coupling cap and the amp sounds killer now.
          Everything from sparkling clean to hard rock (not metal, though).
          Thanks MW.

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