• Sudden change in PAS sound (recent rebuild) - oldsmobuick 17:12:49 03/18/15 (1)
  In Reply to: RE: Sudden change in PAS sound (recent rebuild) posted by 63551 on March 17, 2015 at 12:51:48

  Thanks for the input everyone. I will retrace all my alterations and solder joints and test the passive components as well as the voltages. Also got some new tubes in the mail today.
  

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  • Sudden change in PAS sound (recent rebuild) - oldsmobuick 19:00:28 03/18/15 (0)
    In Reply to: RE: Sudden change in PAS sound (recent rebuild) posted by nick@perse.us on March 18, 2015 at 17:12:49

    Well, I think I figured it out and yes it was my doing as suggested. It was... drum roll... interconnects! I swapped out my interconnects the other day between the pre and the SS power amp. I went from 80pf .5 meter cable to 1 meter 90pf, but i think the main thing was going from 80pf 1m from turntable to PAS, I had subbed in a 1.5m 310pf (!) cable.
    
    I've never paid attention to cable capacitance before but I will be now! I thought the cable was better because it was longer and more convenient. Searching around the forum here clued me in to testing the cables. Thanks again!
    

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• Sudden change in PAS sound (recent rebuild) - oldsmobuick 13:10:13 03/17/15 (1)
  In Reply to: RE: Sudden change in PAS sound (recent rebuild) posted by petercapo on March 17, 2015 at 12:51:48

  
  
  

  
  Hi Peter, this is indeed with both high level and phono inputs. The boards are modified with bass/treble/loudness/scratch/tape all deleted. Also the 510k and 63k (i think that's right) resistors were removed/jumpered for use with my 100k sensitivity Adcom GFA535.
  
  And all that has been great for the past week since those major mods. It's only in the past day that this has started.
  
  One other thing: all my 12AX7s are now microphonic! They weren't before, and I even tried another set that I know to be good and those are picking up microphonics as well.
  
  
  

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  • RE: Sudden change in PAS sound (recent rebuild) - petercapo 14:24:44 03/17/15 (0)
    In Reply to: RE: Sudden change in PAS sound (recent rebuild) posted by oldsmobuick on March 17, 2015 at 13:10:13

    I am strongly tempted to say that the kind of changes you made provide easy opportunities for errors that might cause the problem you are now experiencing. Of course, this is not to suggest you shouldn't have done it - I did something very similar with mine.

    But, you are saying that everything was fine until just the past day, in which case my thought is that some part(s) may be operating outside of its normal operating parameters and may be failing(?), though I do not have a great deal of confidence in this possibility.

    If you do not have a copy already, I suggest downloading the original Dynaco manual and checking the internal voltages against the chart therein. This is best done with a variac so you can set the AC line to get 335VAC to ground at pins 1 and 6 respectively of the 12X4.

    For reference, here's a schematic showing a way to remove the tone controls. This circuit is doing the same thing that the PAS X-mod units did with the bass and treble pots centered. This should work fine with an amp having a Z-in of 100K or higher. I don't recall the implications (or lack thereof) of removing the 62K and 510K in this case.

    
    
    

    
    

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    Edits: 03/17/15   03/17/15   03/17/15   03/18/15   03/18/15

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