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Deluxe Reverb RI Bias Puzzle

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  • Deluxe Reverb RI Bias Puzzle

    I've got a Deluxe Reverb RI on my bench. Customer reports amp is blowing fuses. Suspecting the power tubes powered the amp up with them removed and the fuse held. Popped in a new set of tubes and the rectifier tube started up with internal fireworks. Put in a new rectifier and all seemed fine. I put the original powertubes back in and also got some internal fireworks. Back to the new tubes and powered it up. The tubes didn't redplate so I plugged a guitar in and played. It sounded OK. Attempting to bias I can't get it below 75 mA with the bias pot cranked.

    Open the amp up and traced the bias circuit (couldn't find the layout for the PCB version online and the bias circuit is somewhat different from the original). I found that the 22 ohm wirewound resistor in series between the bias diode and the pot was reading 200K.

    I went ahead and ordered a new one from mouser.

    A couple of questions.

    1. Why use a wirewound resistor there?

    2. Here's what I think may have happened, what do you think? Powertube shorts plate to grid, lot's of current flows through that wirewound resistor, opening it up. Increased current flowing through rectifier also causes it to fail, then fuse pops.

  • #2
    Originally posted by jcollins View Post
    1. Why use a wirewound resistor there?
    The schematic is marked FP for flameproof, but the parts list calls for a MF (metal film) fuse 22 ohm 1/4w. Fender has also marked this as a "DO NOT SUBSTITUTE" part.

    Originally posted by jcollins View Post
    2. Here's what I think may have happened, what do you think? Powertube shorts plate to grid, lot's of current flows through that wirewound resistor, opening it up. Increased current flowing through rectifier also causes it to fail, then fuse pops.
    Chicken or the egg...Resistor opens up, dropping the bias voltage, causing the output tubes to red-plate, drawing too much current, killing themselves and the rectifier, causing the fuse to blow.

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    • #3
      Thanks Bill. I see I made an incorrect assumption about the resistor being wirewound. I'll order the correct part.

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