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Odd fault found an a Fender Hot Rod Deluxe

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  • #1

    Odd fault found an a Fender Hot Rod Deluxe

    This morning, having restored the mod's to the Drive/More Drive circuit on a client's new Hot Rod Deluxe amp, while also scraping off all the solder mask on the original size solder pads for the power supply filter cap leads, where Fender's brilliant production team reduced the solder mask to tiny solder pads.....I powered the amp back up. All came up normal. Just using the internal noise of the system, in Normal Ch mode, I gave the tone knobs a spin, Treble being the first one. When I turned it fully CCW, the amp loudly objected, emitting a loud HUM at that setting, while backing away it went silent. ???! Never heard that one before. Continuing on, I found in the Drive & More Drive Modes, now having the lower gain installed with a 12AY7 in V2, I was still hearing too much tube noise.

    So, I swapped out V1 with the tube I had just removed from V2, and powered back up. Now quiet, and that odd loud HUM with the Treble control at fullly CCW was gone! I immediately tossed that tube that I had removed from V1 into the trash, though I've no idea WHAT was causing that loud hum. Any thoughts?
    Logic is an organized way of going wrong with confidence
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  • #2
    Possibly HF oscillation. The associated HF currents can be very high and thus load the power supply and produce audible ripple hum. Do you still have the "bad" tube? Maybe it's not really bad but just has exceptionally high gain. I would scope the power tube grids with the "bad" tube in. The OT may not be able to transfer very high frequencies (>>100kHz).
    Last edited by Helmholtz; 11-07-2019, 11:34 PM.
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    • #3
      Originally posted by Helmholtz View Post
      Possibly HF oscillation. The associated HF currents can be very high and thus load the power supply and produce audible ripple hum. Do you still have the "bad" tube? Maybe it's not really bad but just has exceptionally high gain. I would scope the power tube grids with the "bad" tube in. The OT may not be able to transfer very high frequencies (>>100kHz).
      The tube is still in my local trash box at the end of the bench, though will I be able to identify it from the others recently tossed in.....remains to be seen. I'll have a look. Thanks for the possible explanation.......Steven
      Logic is an organized way of going wrong with confidence

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