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Fender Hot Rod Deluxe III BIAS Rising HELP

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  • #16
    OK, my thoughts so far:
    TP42,43 have nothing to do with bias and tube current.
    TP41 is your bias supply, and looks fine UNLESS it slowly drops as tube current increases.
    Let me make a plea for terminology: "Bias" is the negative voltage applied to pin 5 of the power tubes, the control grid. TP30 monitors power tube current. 88ma is 44ma per tube. That isn't bias, that is the resulting tube current controlled by the bias.

    What power tubes are installed? It is called a Deluxe, but it uses 6L6 power tubes. 6V6s might act like your issue.

    88ma, or 44ma per, and 430v B+ means about 18 watts dissipation, so not enough to red plate.

    How long does it take to ramp up to your 88ma? When the amp is cold and just turned on, what is the lowest tube current reading?

    Pin 4 is the screen, and has a 470 ohm resistor. power off, measure each resistor. CLose? Also, TP30 reads voltage drop across a 1 ohm resistor, R66. Is your R66 looking good and measure close to 1 ohm? DOn't forget your meter leads may add half an ohm or something.

    With power tubes removed, power on and measure voltage on pin 5 of each socket. What range of voltage does the bias control yield? Center the control. Let it run a while, does the voltage at the grid slowly drop or does it stay steady?
    Education is what you're left with after you have forgotten what you have learned.


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