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Thread: Control pots issue, help please

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    Control pots issue, help please

    Hello.
    I have some issue in my circuit regards volume/ treble control as follow:
    When keep volume pot to max position I can deal with treble and gain pot without problem to entire range.
    Turning down the volume pot little bit with treble in max position I cannot even open the gain pot without loud boom followed by loud noise and microphonic manifestation (ringing when I hit the chassis)
    Turning treble down a bit from max and I can deal with volume and gain without problem even max dimmed
    What I can do to get it stable no matter how I can deal with controls, please? Thanks.

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    Edit: my 12au7 seems to have its input extremely sensitive,

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    If that really is a 100uF cap in there it'll be leaking far too much current for this high impedance circuit. A 100nF film cap would be more like it.

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    Last edited by nickb; 07-13-2019 at 08:51 PM.
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    Hey Nick, thanks for answer. It is not any 100uF but 100nF instead, I made the sketch in a rush, sorry

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    Quote Originally Posted by catalin gramada View Post
    Hello.
    I have some issue in my circuit regards volume/ treble control as follow:
    When keep volume pot to max position I can deal with treble and gain pot without problem to entire range.
    Turning down the volume pot little bit with treble in max position I cannot even open the gain pot without loud boom followed by loud noise and microphonic manifestation (ringing when I hit the chassis)
    Turning treble down a bit from max and I can deal with volume and gain without problem even max dimmed
    What I can do to get it stable no matter how I can deal with controls, please?
    Any signs of RF oscillation (scope)?

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    I dropped the volume pot and I tied 100k screen resistor directly to treble wiper pot. I get loud hiss noise when put treble pot at the end of the track. It act like a switch when put treble at max. The pot have nothing wrong, I changed couple, the same thing...

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    Something seems wrong with firs pot, gain pot, heard a small pop noise when turn it up from 0 position even with tone pots down.Have to check

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    Quote Originally Posted by catalin gramada View Post
    Something seems wrong with firs pot, gain pot, heard a small pop noise when turn it up from 0 position even with tone pots down.Have to check
    Does the plate voltage of the second triode change with turning the gain pot?

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    Hey Helmholtz, no the plate voltage of second triode didn't change

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    Quote Originally Posted by catalin gramada View Post
    Hey Helmholtz, no the plate voltage of second triode didn't change
    Anyway, as the ground reference of the grids in your circuit depends on good pot wiper contact it is always a good idea to spend separate grid to ground resistors (1M).

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    I scoped the output of second triode, it flick a litle and come in back when it pops, is exactly the same

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    Okay, maybe it not worth to bother with the pop of gain control. How can I get rid of noise when put treble pot at max ? It horrible switching in loud noise at the end of the track.

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    When I keep the gain pot close I can put the treble pot at max. When I open the gain pot ,even just a little, I get loud noise when put the treble at max. The noise didn't come progressive, just switch suddenly into noise mode.

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    I just thinking, maybe circuits are not isolated enough due to PS scheme, or grounding, and interfere at a point...

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    Please note my posts #4 and #9.

    How does the noise sound? Like hiss or power supply ripple or 50Hz hum or a combination?

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    In this moment the circuit looks like this:

    Click image for larger version. 

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    I suprimed the volume pot after tone stack and tied the screen grids of 12au7 directly to the wiper of treble pot. It act the same

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    It is not ripple noise, is a harsh loud noise like a noise from interference

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    Please add the grid to ground resistors and post scope pictures of the noise at the plate of the third triode and amp output.

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    I disconnected 12au7 from PI and scoped it output. The issue comes from output of 12au7, meant the third triode

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    1 M grid resistors to ground did not difference. Have no ideea how to scope the noise, there are lots of components inside the envelope. Any thoughts?

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    Is the 100k resistor soldered directly to the 12AU7 socket? (It's a very large value grid stopper and wiil steel some highs).
    What happens if you directly ground the 12AU7 grid?
    Did you try a different 12AU7?
    Maybe a socket contact problem?

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    Quote Originally Posted by Helmholtz View Post
    Is the 100k resistor soldered directly to the 12AU7 socket? (It's a very large value grid stopper and wiil steel some highs).
    What happens if you directly ground the 12AU7 grid?
    Did you try a different 12AU7?
    Maybe a socket contact problem?
    100k is directly soldered to 12au7 socket. I don't know what is 12au7 input capacitance and how much treble will be rolled off with 100k grid stop. What I found is, if I put a metal sheet over the issue is gone. Not permanently. Sometimes it did , sometimes not so it is into a stability margin. I was very happy first time when I put a metal sheet over , the issue was gone and can put the treble pot at max without problem. But take it off and put it back and the issue was back. Sometimes it stops... Hmmm, is clear into a margin of stability I think.

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    Quote Originally Posted by catalin gramada View Post
    100k is directly soldered to 12au7 socket. I don't know what is 12au7 input capacitance and how much treble will be rolled off with 100k grid stop. What I found is, if I put a metal sheet over the issue is gone. Not permanently. Sometimes it did , sometimes not so it is into a stability margin. I was very happy first time when I put a metal sheet over , the issue was gone and can put the treble pot at max without problem. But take it off and put it back and the issue was back. Sometimes it stops... Hmmm, is clear into a margin of stability I think.
    Or it is actually interference from an external source like a cell phone, a switched mode power supply or a dimmer. Make sure that the metal sheet is securely grounded. Amp chassis should always be covered/closed by a grounded plane (sheet metal or foil). Also use tube shields.

    Amp oscillation/instability is typically a single frequency that should be easy to scope.

    Grid-to-plate capacitance of the 12AU7 is specified in the datasheet. Multiply with stage gain the get Miller capacitance. Miller capacitance (+ grid-to-cathode capacitance) and series resistor (grid stopper) determine low pass corner frequency. (I just realized that the 12AU7 is very low =open loop voltage gain, so the roll-off frequency is probably above audio. Forget my comment about the high value grid stopper.)

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    Strong suspect is instability/oscillation, caused by poor layout/grounding/shielding.
    The thump/pop at certain pot positions shows the start/stop of oscillation, and "hiss" means its present.
    We can not hear it directly, too high in frequency for our ears, but the indirect effect of it hitting the tubes, hard.

    Have no ideea how to scope the noise, there are lots of components inside the envelope.
    What components?
    Are you playing the guitar while you test?
    1) set controls so as to start/create the noise.
    2) set all guitar controls to 0.
    I assume you are going from guitar straight into input jack, no pedals or anything else involved.
    Put guitar on a stand and scope output.
    Take a screen picture and post it here.

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