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  1. Replies
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    OK, this is what I did: 54426 At first...

    OK, this is what I did:

    54426

    At first glance The amp did not shows any sign of oscillations even pushed hard. Still have to adjust some voltages. I will do some tests tomorrow and post the...
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    Okay, if I put a cf.in front of what is supposed...

    Okay, if I put a cf.in front of what is supposed to put into 12au7 grid? I want to know what impedance is supposed to drive,? It will be classic 1M or something more 100-200k around, please ? Thank...
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    Thanks That is something I can do. The only...

    Thanks That is something I can do. The only problem for now is my amp stopped to oscillate and work without issue even I pushed hard...very confused
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    There actually good material. Thanks for share

    There actually good material. Thanks for share
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    The amp without first gain stage looks very...

    The amp without first gain stage looks very stable. Is very hard for me to understand why adding one more gain stage in front it make it unstable, as time feed if from external source shows no...
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    Question please: I have to look at a point where...

    Question please:
    I have to look at a point where are created conditions for parasitic oscillations or at the point where this condition manifest the oscillation effect ? It is not clear for me where...
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    And this is with signal generator at input of...

    And this is with signal generator at input of second gain stage (2nd half of 12ax7 preamp) before volume control, with all pots dimmed.

    54419

    I still have to see what I can do with the first...
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    I physical separate the preamp ground point from...

    I physical separate the preamp ground point from the rest of the rail and also separate supply cap,instead a dual one. Now the preamp is completely decoupled from the rest and I.m hundred percent...
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    Actually 13 around, already measured. Yes, but...

    Actually 13 around, already measured. Yes, but this did not seems to do the power stage more unstable, as it is fed in from external source, no matter how hard is pushed. Those bloody 12ax7 from...
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    Thank You. There are 3 gain stages included in my...

    Thank You. There are 3 gain stages included in my power amp: 12au7 / 12at7 / EL34. The gain of 12au7 is pretty low, 12at7 give a half of its gain as it is used as common cathode PI.
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    The outputs are EL34 into 4 ohm resistor load....

    The outputs are EL34 into 4 ohm resistor load. The voltages are peak voltages
  12. Replies
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    54411 54412 These pictures are more...

    54411

    54412

    These pictures are more appropriate and shows exactly the moment when power stage starts to clip ( as far I can see on my scope) . First without nfb second with nfb applied. In...
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    The pictures did not shows the same output level....

    The pictures did not shows the same output level. For feedback loop I just followed the amount of voltage at 12au7 input when output shows it START to distort. In both case with/without nfb that is...
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    In this conditions seems to be a problem related...

    In this conditions seems to be a problem related more to 12ax7 preamp triode. Can be by grounding, by the fact is shared the same double cap with 12au7 stage, maybe high voltage or layout position,...
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    With signal generator connected in front of tone...

    With signal generator connected in front of tone controls ( both treble and bass at max)

    54406

    and clipping

    54407

    12db nfb
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    Amp output and input of 12au7 with signal from...

    Amp output and input of 12au7 with signal from generator

    54404

    and clipping

    54405

    12db nfb
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    And a pic with the cab built for it: 3.5cf sealed...

    And a pic with the cab built for it: 3.5cf sealed cab with Eminence 158cb

    54400

    Found 10-12 db nfb to balance very well with tight and low end enough for 4 strings.
  18. Replies
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    That,s happen when try to do some in a rush, I,m...

    That,s happen when try to do some in a rush, I,m very sorry. Sent the additional Infos in attach

    54396

    54397

    It is mess due to multiple desperate soldering interventions, but You can get an...
  19. Replies
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    Some mentions are necessary as follow: 1.I...

    Some mentions are necessary as follow:
    1.I already buit an amp exactly same which not perform any issues and work flawless...who knows why...?
    2.tried 1M grid leaks did not changed nothing
    3.The...
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    Schematic and grounding layout,please 54394 ...

    Schematic and grounding layout,please

    54394

    54395
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    So the problem is related by the tone control...

    So the problem is related by the tone control and input of12au7 , that is correct ?
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    I tied also DC coupled the 12au7 to 12at7 PI and...

    I tied also DC coupled the 12au7 to 12at7 PI and is the same. Please let me know what did from posted picture a bad design related to my actual issues? I.m pretty interested to know what kind of...
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    Hey JMF , it is boomy, actually found 9-12 db...

    Hey JMF , it is boomy, actually found 9-12 db tame it enough and let me keep enough low end. This happen with cabinets I have. I made a sealed one with 158cb from eminence and found also to be the...
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    I tied 23pF ( 2x47 series) from grid of 12au7 to...

    I tied 23pF ( 2x47 series) from grid of 12au7 to ground. It osscilate on very different waveforms , depends by treble pot position
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    Thank You Helm I still have half of 12au7...

    Thank You Helm
    I still have half of 12au7 unused. I can made a cf to put in front of, between tone control and 12au7 gain stage. Do You think it solve my problem, please ? Thanks.
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    I used 20db nfb just for test the power stage...

    I used 20db nfb just for test the power stage with signal generator to 12au7 input and as I said was not issues at all. Used in conjunction with preamp stage found 12db to be desirable for my bass...
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    Thank You, Without nfb the amp is stable, meant...

    Thank You,
    Without nfb the amp is stable, meant have not the issues mentioned before.
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    Thank You. I tried to isolate the problem and...

    Thank You.
    I tried to isolate the problem and disconnected first tube, including tone controls. Put the signal from generator to 12au7 input and it works without problem , even with huge 20db amount...
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