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What does this schematic symbol mean?

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  • #1

    What does this schematic symbol mean?

    What does the black circle with vertical white bar mean? This symbol is on a 70's Fender Showman amp schematic.

    BTW, the 27K resistor is wrong. With the 27k resistor I could not get the bias voltage to go lower than -57v. At that voltage the amp sounded "squashed" or compressed (almost like a tube rectifier). I put the amp on a scope and there was a lot of crossover distortion.

    I tried a few different resistor values and settled on 10k.

    Also I changed out the filter capacitor to a 100uf/100v. Why would Fender put a 50v capacitor in a 52v circuit?
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  • #2
    Originally posted by Axtman View Post
    What does the black circle with vertical white bar mean? This symbol is on a 70's Fender Showman amp schematic.
    The symbol means that the potentiometer next to it is a trimmer rather than a front panel control. It's supposed to look like the head of a flat-blade adjustment screw.
    -tb

    "If you're the only person I irritate with my choice of words today I'll be surprised" Chuck H.

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    • #3
      Check that 470 ohm also.

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      • #4
        Originally posted by Axtman View Post
        Why would Fender put a 50v capacitor in a 52v circuit?
        For the same reason they seem to think they can derive 52V from a 48V source

        Probably someones copy of something else that's been redrawn after it was redrawn by someone else. Fender has a lot of faux pas like this in their schems. Especially the older ones. Use a 100V cap if you are building this circuit. Replace the cap with a 100V if you have one.
        "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

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

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        • #5
          Part of what's going on here has to do with the change in line voltage.
          I think if you plugged the amp into a variac and dropped it down to 115V things would look better. That would be around what the AC line was when this amp was built, or it might be spec'd for 117V.
          The resistor is not 'wrong', it's been working fine in this amp and all the other ones of the same model for around 50 years. Tubes at the time running on the lower line voltage may have biased up just fine.
          Many Fender amps will show crossover distortion on the scope with no negative effect on the audio. And I would not expect it to make it compressed or squashed sounding.
          Did the bias change solve that 'compressed sound' issue?
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          ​

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