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Thread: Positive voltage on grid of self-biased triode at idle

  1. #1
    Member mhuss's Avatar
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    Positive voltage on grid of self-biased triode at idle

    We're having an odd problem with V2 in a couple of recent builds. After a few minutes, a small positive voltage (~.5 - .8vDC) gradually builds up on the grid, which of course causes the stage to be biased very hot and incapable of passing an undistorted signal. The plate voltage is normally ~175v but gets dragged down to 140 or so at idle. Grounding the grid with a jumper brings things (at idle) back to normal. There is about 240k DCR from grid to ground (by inspection, and verified with a meter). Replaced the tube, measured all the neighborhood resistors, everything is within spec. Resistance between pin 1 and 2 measures 'open circuit.' Turning both input volumes to zero has little to no effect.

    I'm out of ideas - any suggestions?

    Schematic similar to this (but no cathode bypass cap): https://hiwatt.org/Schematics/DR_Pre4Input_v2.pdf

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    Maybe a bad batch of tubes?
    Try one from a completely different manufacturer.

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    Bent Member Chuck H's Avatar
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    Are there any other symptoms? Otherwise I agree that it could be a bad batch of tubes.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    I.ll look for severe high freq osscilation at that tube at a point.

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    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    What do you get if you pull the tube and measure voltage on the pin at the tube socket?

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    Put a grid stopper first, close to the socket.Change the ground reference point of shielded cable (if you used one). See if something changed.

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    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    Member mhuss's Avatar
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    Tried a known good tube from another amp - same thing.

    The preamp grounding is pretty 'good', grounded at the chassis next to the input jacks, with subsequent stages chained to that point. It's a low gain design, so we rarely have oscillation problems.

    We finally changed the (brand new) coupling cap connected to the plate(!) and somehow that made it come around.

    Thanks for the suggestions!

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    Don't forget the joker g1's Avatar
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    The coupling cap from V1 plate?
    You said the problem was in more than one build, so more bad caps?

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    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

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    Member mhuss's Avatar
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    Perhaps - or maybe the cap was a false flag(!) He hasn't gone back and checked the second one yet. (I'm troubleshooting remotely, sigh.)

    Given the tube is OK (based on working fine in an older amp) what else could cause significant positive voltage on the relatively-low-impedance-to-ground grid?

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    Old Timer J M Fahey's Avatar
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    Different possibilities to test:

    Leaky/contaminated board material.
    Same with socket.

    What do you use? Turrets/eyelets (mounted on what?)
    Fiberboard - pertinax - glass-epoxy - Formica?

    Please post close up pictures, both sides.

    Turn amp ON but with V2 unplugged, read voltage at all socket pins in this sequence: :OFF > just ON > 5/10/15/20/30 minutes later

    Now plug V2. all controls to 0, repeat voltage testing on all pins (except filaments that is).

    You are looking for the unexpected/abnormal/ "shouldn´t be this way".
    n

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    Juan Manuel Fahey

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