Crossline Master Volume Mystery

Discussion in 'The Workbench' started by Marcomel79, Oct 26, 2021.

  1. Marcomel79

    Marcomel79 Well-Known Member

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    Hi everyone, i apologise since this has been discussed before but in short i built an 18W plexi clone and decided to put in a type 3 mv. Worked like a charm. Beautiful tone. Then suddenly it didnt anymore. Then only way i can describe it is like having a metal zone pedal with all the knobs on 10 on a cheap ss amp with a delay in the sound. As if the two signals from the pi have a slight delay... it really annoys me knowing that it was working perfectly before and all of a sudden it stopped. Otherwise without it works fine.
    Any wisdom is greatly appreciated
     
    Last edited: Oct 27, 2021
  2. Pete Farrington

    Pete Farrington Well-Known Member

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    It may be oscillating. How about if the tupe 3 is disconnected, does that stop the issue?

    If the margin of stability is low, then this might arise.
     
  3. Marcomel79

    Marcomel79 Well-Known Member

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    Hey Pete, when i disconnect it the amp works as it should. No problem whatsoever. What i dont understand is that it worked when i first built the amp and after a few weeks it didnt.... no clue why
     
    Last edited: Oct 27, 2021
  4. Pete Farrington

    Pete Farrington Well-Known Member

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    Might environmental conditions affect lead dress parasitic capacitance :shrug:
    If so, there’s been a change of season.
    Mains voltage variance?
     
  5. Marcomel79

    Marcomel79 Well-Known Member

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    Here in norway is 240V. I tried swapping tubes in pi position but that didnt help. I tried shielded cable grounded on bus and just two wires with no ground with same result. All the voltages are spot on as per schematic... and again, works perfectly without it..
     
  6. Pete Farrington

    Pete Farrington Well-Known Member

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    That’s a nominal level. I’m thinking of the actual voltage at the outlet, which ends up across your PT primary. And how that changes over time, hour to hour, day to day etc.
     
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  7. myersbw

    myersbw Well-Known Member

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    As a cross line, that’s usually a 1 pot configuration. Did you go with a 1M PEC pot that’ll handle 2watts or use a typical 1/2 watt pot? If the latter, open up the pot and inspect it…you may have pushed it too far. I always use a 2W PEC for that style.

    Worth peeking into. My money is on the pot failing unless you replaced it. And, yeah, oscillation is up there for a candidate, too.
     
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  8. Marcomel79

    Marcomel79 Well-Known Member

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    Im not sure how that changes... i think im better off building the attenuator thats on a thread on here...
     
  9. Marcomel79

    Marcomel79 Well-Known Member

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    I used an Alpha pot. Measured the pot yesterday and it read 1M ohm on the DMM. Didnt open it though, and yes, its rated 0,25W. I will open it today and check it. I do have a 500k pot laying around, can i just test it with that to see if the pot is the problem? This one is a cts linear pot i had bought for a tone control...i guess its also rated 0.25W..

    If oscillation was the problem, what causes it? The amp works perfectly without it, and its very quiet. I spent time doing all the wiring to make sure everything was tidy and short. Its near dead silent now...
     
    Last edited: Oct 27, 2021
  10. Pete Farrington

    Pete Farrington Well-Known Member

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    Coupling of a later stage back to an earlier stage. As might occur if a master vol, speaker jack etc was positioned in close proximity to an input jack / circuitry.
     
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  11. Marcomel79

    Marcomel79 Well-Known Member

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    The master volume is on the back panel, away from input jacks. Someone here said they use a 2W pot for this type mv, while i use 0,25W for mine. I have a 500k linear pot laying around so ill try it tonight to see if the first one went bad.
     
  12. Pete Farrington

    Pete Farrington Well-Known Member

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    I’m somewhat doubtful that the LTP anodes here can put out sufficient current to damage a decent pot. Whatever, the 10k resistors previously suggested would eliminate that possibility and also greatly improve the range of control.

    A 100pF cap fitted between anode and cathode of one of the preamp common cathode stages within the positive feedback loop would be very likely to improve the margin of stability sufficient to stop oscillation for good.
     
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  13. myersbw

    myersbw Well-Known Member

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    +1 on the cap addition! Now, I will speculate on your "somewhat doubtful" definition! LOL...define somewhat? (Yeah, I'm just razzing you because I'm jealous of the beer you have in hand.) I do think that it wouldn't be an issue right away...unless this were a junk drawer style pot that happened to be laying around. This is where a couple of good photos are worth 10,000 words. And, I'd also expect an oscillation issue to be there right off the bat, too. But, and I didn't go back and reread, I don't know if this is a build with all new parts? Some used tubes inserted? Etc, etc.

    Anyhow...swapping the pot out will be a simple enough test. Also...if this was an old pot and a strong enough DeOxit used to clean (or similar)...it can be too thick to a point that you create slight conductive paths where you don't want them if you use an excessive amount that doesn't all dissipate...like to the side of a chassis via the pot case? Just another thought. And, that's just one more reason I prefer a sealed PEC pot for this application.
     
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  14. Marcomel79

    Marcomel79 Well-Known Member

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    I do have a 100pf cap laying around. Where exactly should i fit it? B2DDB160-EB6E-4E0F-A2AB-753DE2B45780.jpeg
     
  15. Marcomel79

    Marcomel79 Well-Known Member

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    Thank you for your answer. The build was from a brand new kit purchased in june this year. Never sprayed the pot. The mv was very nice, and so easy to install. I will try the 500K pot later tonight and see if that might help although i doubt it....
     
  16. Pete Farrington

    Pete Farrington Well-Known Member

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    Do any control settings affect the nasty tone? eg does turning the treble or bright channel vol down stop it?
     
  17. Marcomel79

    Marcomel79 Well-Known Member

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    Well i dont get the crap sound when having the bright channel turned down, when its just clean. I just tested the 500k pot with same result, so i can rule out a faulty pot.

    If i strum a chord when i have the master volume real low and the bright channel into overdrive (after 1 on the dial), i hear the sound getting lower and the higher, in waves.... with the metal zone-SS sound...with the delay with waves in lower and higher volume. Not a lot of difference in volume, but noticeable
     
    Last edited: Oct 27, 2021
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  18. Pete Farrington

    Pete Farrington Well-Known Member

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    Ok, try 100pF across pins 1 to 3 on V2.
     
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  19. Marcomel79

    Marcomel79 Well-Known Member

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    is this to potentially stop oscillation as i use the mv?

    Scrap that, i reread your earlier post!
     
  20. Pete Farrington

    Pete Farrington Well-Known Member

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    Do you ever get the bad sound when using the normal channel, ie with the bright channel set low or off?
     

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