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Thread: voltage at power tube cathode?

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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.
    ThanksClick image for larger version. 

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    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.

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    Thanks Dude. I think it sounds pretty good as is. More clean headroom would be better, but I'm happy.

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    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.

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    ...and lift one end of the 270R cathode resistor and measure resistance. Might have increased over time.

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    Quote 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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    Quote 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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    Those grid stoppers would be R23 & R25 in this schematic, right?
    Click image for larger version. 

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    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.

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    Quote 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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    Quote 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.

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    Thanks, Helmholtz. I can't get to it now, but I'll do that asap

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    Quote Originally Posted by dmartn149 View Post
    Those grid stoppers would be R23 & R25 in this schematic, right?
    Click image for larger version. 

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    Yes, but 330k is a far higher value than necessary.

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    Quote 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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    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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    Quote Originally Posted by dmartn149 View Post
    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?
    If the increased grid voltage stays with the tube when you exchange power tubes, it should be the tube. If it stays with the socket, this could be the culprit.


    BTW, it just crossed my mind that cathode biasing with a common cathode resistor generally tends to emphasize asymmetry: If one of the tubes draws more idle current, cathode voltage increases. Increased cathode voltage in turn makes the "colder" tube run even colder. So using matched tubes seems much more desirable than with fixed bias (where also individual tube biasing is easy).
    Using individual cathode resistors could be a benefit.

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    Yes, a bad mismatch could result in the hot tube over dissipating, which increases grid current, which increases plate current and so on.
    With shared cathode bias (which Merlin advises can add even harmonics), a 1 ohm resistor in series with each cathode is handy to monitor the current in each tube.

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    which increases grid current
    Why grid current?

    Generally a cathode resistor counteracts thermal runaway. This is a benefit compared to fixed biasing.

    What I meant was that the hotter tube indirectly "steals" idle current from the cooler tube, thus increasing asymmetry.


    Additional individual cathode resistors are a good and logical proposal to allow for measuring individual cathode currents.

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    Last edited by Helmholtz; 10-13-2019 at 03:32 PM.
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    I replaced the tube that had dc at the grid. now both tubes have 0vdc at pin 5, but I'm still getting higher than expected voltage at the cathodes (about 1v more than what's on the schematic) If it's just mismatched tubes I'm ok with that. "it's just a guitar amp"

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    I replaced the tube that had dc at the grid. now both tubes have 0vdc at pin 5
    Seems you had a gassy tube that produces reverse (positive) grid current.

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    Quote Originally Posted by Helmholtz View Post
    Seems you had a gassy tube that produces reverse (positive) grid current.
    yes, it would seem so. Thanks for all the help, guys.

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