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Thread: 66 Vibro Champ bias keeps rising....

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    66 Vibro Champ bias keeps rising....

    ....and rising....
    Shame, everything else is stock.

    Filter cap can okay, cathode resistor measures 330 instead of 470 ohms, cathode cap is okay, but it looks like the output trans is bad.

    Tried several known good 6V6's, all the same.

    Can I use half of a push pull output trans just for testing to confirm that the OT is bad?

    That should work.

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    Quote Originally Posted by drewl View Post
    ....and rising....
    Shame, everything else is stock.

    Filter cap can okay, cathode resistor measures 330 instead of 470 ohms, cathode cap is okay, but it looks like the output trans is bad.

    Tried several known good 6V6's, all the same.

    Can I use half of a push pull output trans just for testing to confirm that the OT is bad?

    That should work.
    What makes you think the OT is involved? A defective OT won't cause idle current to rise. If the primary was open there would be no tube current. If there was a short, it would not influence tube current at all.
    There is a number of possible reasons for your problem. First exchange the cathode resistor for correct value. And check cathode bypass cap for leakage.

    ....and please post schematic.

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    Last edited by Helmholtz; 11-28-2019 at 11:20 PM.
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    What makes you think the OT is bad? The OT won't make the bias current rise. I'd suspect the control grid coupling capacitor. Short the control grid to ground. Does the bias current still rise? If the 470R cathode resistor is measuring 330R I'd replace it.

    Edit: Helmholtz beat me to it

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    DOH!
    Totally spaced on the input grid and didn't even think to check it.
    I figured a bad OT would cause excessive current draw.

    I had swapped out the 470 ohm cathode resistor and cap.

    So, the 220k to ground on the input grid was fine, some wild voltage fluctuation though....and...well look at that, the ground for the 220k popped off!


    Brain failure.
    Thanks guys.

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    Quote Originally Posted by Helmholtz View Post
    ... A defective OT won't cause idle current to rise. If the primary was open there would be no tube current...
    Screen grid current? With no voltage on the plate it may rise to a damaging level.

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    Quote Originally Posted by pdf64 View Post
    Screen grid current? With no voltage on the plate it may rise to a damaging level.
    You're right. I only thought of plate current, which is wrong. Do you have values for screen currents with zero plate voltage?

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    Last edited by Helmholtz; 11-29-2019 at 03:01 PM.
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    Quote Originally Posted by Helmholtz View Post
    You're right. I meant plate current. Do you have values for screen currents with zero plate voltage?
    No, but I accidentally connected things like that once and the screen grid resistors (470 ohm 1 watt, 6l6GC p-p amp) began releasing the magic smoke within a few seconds of the tubes warming up and conducting

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    Quote Originally Posted by Helmholtz View Post
    If the primary was open there would be no tube current.
    If the primary was open there would be no Ia but would be Ig2 current.

    ....and please post schematic.
    https://music-electronics-forum.com/showthread.php?t=19518

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    If the primary was open there would be no Ia but would be Ig2 current.
    Yes, see post #6.

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    Yes, just I saw post #5

    pdf64

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    Quote Originally Posted by pdf64 View Post
    No, but I accidentally connected things like that once and the screen grid resistors (470 ohm 1 watt, 6l6GC p-p amp) began releasing the magic smoke within a few seconds of the tubes warming up and conducting
    G2 assumes the function of the anode starts to light, and the fireworks start.

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    All this is true, but we are discussing a very rare situation. In my experience, a failed OT is more likely to short either within the windings or to frame, that to go open. They do fail, but OTs should be the last suspect on someone's list.

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    Quote Originally Posted by drewl View Post
    Can I use half of a push pull output trans just for testing to confirm that the OT is bad?
    As Enzo points out, an OT primary usually shorts when it fails. Most often this involves a single or small number of turns. A DMM will show you if a winding is open, but a single shorted turn makes a fraction of an ohm difference to the DCR and is not practically detectable by resistance measurement. A shorted turn anywhere in a transformer collapses the magnetic field and effectively kills the entire transformer. It doesn't just affect the shorted area. I have a DIY dedicated ring tester that gives an instant reading of a shorted or open turn, but a DMM + 'neon trick' test is all that you need, but as already established, the transformer isn't a suspect.

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    So it sounds like this might be solved (?) with the bad connection for the 220k grid bias resistor. Otherwise, all the above and I'll toss a possible leaky coupling cap into the fray.

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    Yes, the ground connection for the 220K grid resistor popped off, that was the problem.

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    Here's the schematic for posterity.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	champ_vibro_aa764_schem.gif 
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