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    That's where I was going with my post. But often...

    That's where I was going with my post. But often a guitarists tech also maintains their amps. That doesn't necessarily mean they KNOW about electronics either. Especially WRT modern amplification....
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    Just to be clear, this guy knows about amplifier...

    Just to be clear, this guy knows about amplifier electronics? Not always the case. Confidence and vaguely related credentials aside,.. We are here if it comes up :thumbsup:
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    Good find. It seems that there are several models...

    Good find. It seems that there are several models though. The site is rather ambiguous about the actual circuitry in each model.

    It reads to me that the one they recommend for the Vibro Champ is...
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    ......

    ...
    https://music-electronics-forum.com/attachment.php?attachmentid=56267&d=1575735527
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    25k pots are redily available from many sources....

    25k pots are redily available from many sources. I'll try to post later with a layout that should make it easy.

    https://www.allparts.com/products/ep-0225-25k-push-pull-audio-pot
    ...
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    This is what I was talking about way back in post...

    This is what I was talking about way back in post #9. We don't even have a description of the problem. Just a second hand account that "there is a feedback problem" and that's all we know. I think...
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    I can get behind that. I don't mind the holiday...

    I can get behind that. I don't mind the holiday screen saver. I use to do it sometimes.

    For those that don't want to clutter their desktop and on the subject of "free". I've never had an OS that...
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    If the amp is unstable (parasitic oscillation)...

    If the amp is unstable (parasitic oscillation) your little speaker should be sufficient to reproduce the problem and you usually don't need to plug in a guitar since these typically do not depend on...
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    Right. And since guys are using these things in...

    Right. And since guys are using these things in their Marshall heads I think there also must be a DC blocking cap for the grid in there as well.
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    Well there's another option... You could use a...

    Well there's another option...

    You could use a 25k push/pull pot as the presence control (they don't come in 5k). Wiring like the later DC blocked Marshall design. Then use the pull switch to...
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    Perkinsman, You seem bent on using the scope...

    Perkinsman,

    You seem bent on using the scope for solving this problem no matter what. You're trying to solve the problem of "diagnosing this amps failure with a scope" rather than trying to...
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    Please see the edit in my post above :thumbsup:

    Please see the edit in my post above :thumbsup:
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    Actually... It doesn't work like that....

    Actually...

    It doesn't work like that. Increasing the cathode bias resistor would reduce current and RAISE voltage. Whatever your memory of how it went down I promise that you did not decrease...
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    I've biased el84's pretty hot with up to 400Vp...

    I've biased el84's pretty hot with up to 400Vp (but not over). They become noisy due to microphonics pretty fast like this, but they don't blow up and a shorter tube life isn't a deal breaker. I'd...
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    If only that schematic were accurate. I've never...

    If only that schematic were accurate. I've never measured less than 390Vp in a Vibro Champ and later models are worse and typically around 420Vp. That schematic may or may not be accurate to...
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    The Yellow Jacket socket is supposed to limit...

    The Yellow Jacket socket is supposed to limit screen current and correct the bias circumstances for the el84 tube. I'm not sure how "universal" this could be to any amp they may be used in since amps...
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    This is a good analogy. With the caveat that the...

    This is a good analogy. With the caveat that the scope could be damaged by high voltage at it's input. That's why the 10x probe is recommended. You'll want to keep your test points limited to the DC...
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    I'm providing a modification of your schematic...

    I'm providing a modification of your schematic showing your circuit (left) and the one I proposed (right) with the 4.7k resistor in place.

    I think you should consider the ratios for these...
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    It may. Though not necessarily a component...

    It may. Though not necessarily a component failure. It could be that some oxide is present more so than the other joints. I don't know what solder was used on that board, but it's probably rohs...
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    This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ You're...

    This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    You're going to need to replicate the problem if you're going to solve for it. If you need to talk with the amps owner to discover the exact circumstances...
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    That string trick was (no way) filmed one take ;)...

    That string trick was (no way) filmed one take ;) But it's clever. I like it.
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    They are serviceable. You have to remove...

    They are serviceable. You have to remove components to solder to them. This requires tools that reach into the f-hole but for near components you can sometimes secure them with your fingers enough to...
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    I don't understand this question. Are you saying...

    I don't understand this question. Are you saying that when you inject signal you get signal voltage at these locations? And what would be the criteria for preference? That information is a little...
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    The project might be getting ahead of itself. You...

    The project might be getting ahead of itself. You may not even need to hook up the scope. As you already deduced, the first thing to do is instigate the problem. After that you should tap on the...
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    This^^^^^^^^^^^^^^^^^^^^^^^^^ is the first go...

    This^^^^^^^^^^^^^^^^^^^^^^^^^

    is the first go to. Especially if the amp was known to work correctly at some point.

    If the amp has always had a "feedback" problem then I might suspect parasitic...
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    That IS an entirely different matter. Fortunately...

    That IS an entirely different matter. Fortunately the advice is still the same.
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    Oh, and... You really should hang a resistor...

    Oh, and...

    You really should hang a resistor from the downstream side of that preamp switch to ground. The arrangement as it is leaves the grid without bias. Only for a moment, I know, but it's...
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    Put 100mV @ 400Hz to 1000Hz into the input. Amp...

    Put 100mV @ 400Hz to 1000Hz into the input. Amp controls set as you would when playing through it and the problem occurs.

    You'll be probing along the signal chain starting from the input and...
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