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Thread: Oscillation problem need little help, please

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    Oscillation problem need little help, please

    Hello,
    I builded the preamp from drawing below. It works fine but...I get little low frequency osscilation just in second channel. It is very discreet but is here.It is like a low freq pulse wave. It raise in amplitude with volume and bass control. Have no instrument to scope it, did You see any reason from schematic below, please? Thanks

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    Do you think sharing the same bus bar like in drawing can be a problem,please? I can separate the preamp return and join just at the input.Do you think it worth? Thanks
    It is a SoundCity/Hiwatt layout, the PI is at opposite side of bus. Post a pic:

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Just unplug the tube from the 'good' side will do the same thing. If that is not possible what effect do the "good" controls have on the problem? If none then it rules out a cross channel issue.

    I have no idea what is going on so I'll just throw out some things to try...

    Have you tried putting another, say 47uf, cap in parallel with the 32uF just in case it's bad one.

    Is the frequency constant as you decrease the volume on the bad channel?

    Are you saying the the noise is there even with the volume almost zero?

    What is the 'bad' input connected to e.g. instrument, shorted jack etc?

    What about layout. Is there a bigger loop on the bad input when compared to the good one.

    What happens if you short the bad input to it's ground?

    Any chance this could be an HF oscillation? Try a 100pf cap from anode to grid of tube to see what the effect is.

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    Bent Member Chuck H's Avatar
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    I haven't had to deal with this problem, but...

    From what I've read here your problem is most often caused by faulty filter caps and/or grounding scheme. Meaning there is a positive feedback loop in your power supply that isn't being effectively shunted to 0V. It is most likely an interaction between the preamp and the later stages. The schematic you provided doesn't include those later stages and doesn't show the grounding scheme. Determining the circuit from the picture is sort of a pain. It may help in determining grounding scheme a little, but the HiWatt layout does hid some grounds daisy chained under the board for a neat appearance.

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    Thanks Nick. The noise is present with both input shorted. It is not audible with volume control turned down

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    Thanks Chuck H. You cannot daisy chain the ground returns when you mix channels. You can joint into a point or array along the bus. In my case there are arrayed so I.ll see what is happen if I separe the return of bad channel and I will join directly to its decoupling cap.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by catalin gramada View Post
    Thanks Nick. The noise is present with both input shorted. It is not audible with volume control turned down
    That wasn't what I asked. What I'm try to find out how much of the circuit is involved. If the noise stops BEFORE the the volume control reaches zero then both tubes are involved. If it only stop when you get to zero then only the first is involved. The question about the frequency and volume is related.

    Time you got a scope
    There's a bunch of other questions which need answers in that post too.

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    Thank You for You support gents. The problem was solved in fraction of second. Stupid begginer mistake: I turned off my digital Ersa solder station and everything goes quiet as a grave. Thank You again.False alarm, Sorry. The amp work spectacular.

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    Bent Member Chuck H's Avatar
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    You wouldn't be the first around here to have something like that happen. And neither was I

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Agreed. Hardly a rookie thing at all. But you do have to ask the question: why just CH2?

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    Quote Originally Posted by nickb View Post
    Agreed. Hardly a rookie thing at all. But you do have to ask the question: why just CH2?
    Well...I think I found the answer too. The amp was not permanent wired, just crossed some wires to get sound. Over that found in a rush forgot to put a grid stop resistor to ch2(which should not matter as time inputs was shunted to ground).I think pretty much these prone the ch 2 to get interferences. Now the amp is permanently wired and saw You post I put my solder station on directly under amp as it was and guess what: no oscillation, there are no differences
    Of course, that did not mean it not exist, till a scope investigation, but are not audible at least.
    Thanks

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    Last edited by catalin gramada; 12-02-2018 at 04:26 PM.
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    Bent Member Chuck H's Avatar
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    One of my favorite personal WTF's that I pestered members here for was a cold solder joint that would only act up when playing C# and a little at one other pitch I can't remember now. At which point it would vibrate sympathetically at frequency so that it didn't really sound like an intermittent fault, but an oscillation. We went through maybe twenty posts looking for oscillation and even speaker cone cry before I just went in and resoldered some iffy looking joints and the problem went away. Since I built that amp I was duly embarrassed and apologetic.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

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