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Pepco to Princeton conversion

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  • Pepco to Princeton conversion

    Hi Guys,

    I have this old Pepco Riviera 725 that I converted to a Princeton AA964 clone (added tremolo as well although it is too fast for my taste).

    Everything was fine until I added the feedback resistor (2K7). I had a high frequency squeal that would not go away. I then replaced the 2K7 with a pot and found that the squeal went away when the pot was at 10-12K. If I cranked the volume all the way up I could hear it coming back although not as loud as when the 2K7 was in there.

    After some research I found that a possible cause could be that instead of (-) feedback, I was sending (+) feedback. The cure for this (if this is the case) is to reverse the output wires to the OT. So I have a couple of questions.

    1) Is the (+) feedback a possible cause.
    2) Would reversing the output to OT cause any harm (as a diagnostic).
    3) Which .01uf cap (or both) should be changed to .02uf to slow down the tremolo speed.

    Thanks and Happy New Year to all.

  • #2
    Yes. I'd bet that you have + feedback. And it would cause the problem you have.

    probably the easiest way to reverse phase on the OT is to swap the plate leads on the power tubes.

    I believe you can change any .01 cap in the phase reversal string on the trem to .02 to slow it down. pick the one that looks easiest to replace.

    "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


    • #3

      Thanks for your help. I Finally got around to implementing the changes which cured the feeback issue but when I replace one of the .01 caps with a .02 I lost the tremolo completely. I also noticed turning up the rate and intensity would make the ouput tubes glow blue (but still no trem effect). I put back the original .01 and verified nothing was out of place and powered up, no trem again. Could swapping the plate leads cause the trem issue? Looking at the schematic, I can't see how this could be.

      I'll poke around again this weekend to be sure I didn't change something from it's original condition just to be sure.



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