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  • voltage at power tube cathode?

    I just finished building an amp based on a silvertone 1482 schematic, with some small mods to the mic input. All of the voltages are a little lower than the schem. except for the power tube cathodes. Voltage there is about 1.5v higher. I expected lower v throughout per the schem. for the radio that my PT came from. The amp sounds good, but I am wondering how I ended up with higher v there and should/can I do anything about it.
    Thankssilvertone1482.pdf
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  • #2
    It's a cathode biased amp and likely just the power tubes you selected. A different set of tubes would likely yield a different voltage. It's nothing to worry about. If you want to bias hotter or colder, change the value of the cathode resistor.
    "Yeah, well, you know, that's just, like, your opinion, man."

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    • #3
      Thanks Dude. I think it sounds pretty good as is. More clean headroom would be better, but I'm happy.
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      • #4
        Check that there’s no Vdc on the power tube control grids (terminal 5).
        Grid stoppers there would be good practice. Probably omitted from the original design to reduce cost.
        My band:- http://www.youtube.com/user/RedwingBand

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        • #5
          ...and lift one end of the 270R cathode resistor and measure resistance. Might have increased over time.
          - Own Opinions Only -

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          • #6
            Originally posted by pdf64 View Post
            Check that there’s no Vdc on the power tube control grids (terminal 5).
            Grid stoppers there would be good practice. Probably omitted from the original design to reduce cost.
            I get 0v on p5 of the tube that's fed by p3 of the PI and 3v on the tube fed by pin 1.
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            • #7
              Originally posted by Helmholtz View Post
              ...and lift one end of the 270R cathode resistor and measure resistance. Might have increased over time.
              It's all new.
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              • #8
                Those grid stoppers would be R23 & R25 in this schematic, right?
                silvertone1472.pdf
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                • #9
                  and 3v on the tube fed by pin 1.
                  That indicates a leaky 0.01µ coupling cap and would explain increased cathode current and voltage.
                  - Own Opinions Only -

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                  • #10
                    Originally posted by Helmholtz View Post
                    That indicates a leaky 0.01µ coupling cap and would explain increased cathode current and voltage.
                    yeah, I know the coupling caps are supposed to stop DC. I guess I got a defective part.
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                    • #11
                      Originally posted by dmartn149 View Post
                      yeah, I know the coupling caps are supposed to stop DC. I guess I got a defective part.
                      Just lift the coupling cap and measure grid voltage again.
                      - Own Opinions Only -

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                      • #12
                        Thanks, Helmholtz. I can't get to it now, but I'll do that asap
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                        • #13
                          Originally posted by dmartn149 View Post
                          Those grid stoppers would be R23 & R25 in this schematic, right?
                          [ATTACH=CONFIG]55525[/ATTACH]
                          Yes, but 330k is a far higher value than necessary.
                          My band:- http://www.youtube.com/user/RedwingBand

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                          • #14
                            Originally posted by pdf64 View Post
                            Yes, but 330k is a far higher value than necessary.
                            maybe 1.5k like a 5e3 deluxe?
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                            • #15
                              ok, I disconnected the coupling cap at pin 5 of the power tube. still getting an extra 1.5v at cathode AND still getting 3vdc at pin 5. So, is that a bad tube?
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