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Problem with my Atma-sphere amplifier

Posted by Lew (A) on June 8, 2008 at 10:33:28

I've got highly modified MA240s, the old model that uses six 6C33C output tubes per monoblock, on MA2 chassis' with MA2 transformers. I just finished making several mods to the power supplies for the input and driver stages in one monoblock, aka the "high-voltage" supply that makes +/-400V. But I also changed the grid-stop resistors for the output tubes, and I added .01uF bypass caps across each hexfred in the output stage supplies. (There are two separate full-wave bridge rectifiers that feed the two separate 135V output supplies.) Those are the only things I did to the output supplies. When I turn on the filaments and 400V supplies, all is well. All voltages measure correctly, etc. However, when I then turn on the output supplies, the 5W/510-ohm resistors that float the output supplies above ground start to smoke, on both sides of the output supply. I measured >100V across each of these resistors (and rising before I shut the amp down), which of course would indicate way more than the 5W rating in terms of power. I've checked to see whether any of the hexfreds are shorted to ground, and that does not seem to be the case. I can't find anything else wrong, and I'm stumped.

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