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Tube microphonics ... strange symptoms ... hum disappears if I switch sides.

post #1 of 9
7/15/11 at 9:37am
Thread Starter 

   I just received a set of Sylvania 12ax7WA tubes which exhibit a low level hum and when tapped with a pencil I can hear a sound in my headphones ... I presume this means the tubes are microphonic ... is this true?

 

1. Can they damage my amp? (Mingda MC 84-C07)

 

2. Should they be sent back?

 

       Thank You in advance .... bigsmile_face.gif

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post #2 of 9
7/15/11 at 11:59am

How long have you let them burn in?

 

 

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post #3 of 9
7/15/11 at 12:21pm

ALL tubes are microphonic.  The question is simply how much.  If you tap your AMP and you can hear that through headphones, then it's too much IMO.  But you should NEVER tap an operating tube with ANYTHING.

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post #4 of 9
7/15/11 at 2:16pm
Thread Starter 

  Thank you both for your answers ...  Actually its not the monophonic aspect which is disconcerting its the low level hum ... the tubes think they are Glen Gould it seems ... and since they are not NOS I suppose burn in does not apply ...  and when i tap the amp i do not hear anything so at least that is one variable less to check ... none of my other tubes hummmmm so ill contact the seller and see what he says ...

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post #5 of 9
7/15/11 at 3:53pm
Thread Starter 

   If I switch the tubes ... put the rt one in the left socket ... etc ... the hum stops ... and if i reverse back them it starts again ... can anyone tell me what THIS means?  BTW I  have been offered a refund or an exchange by the seller. ...

 

                 basshead.gif

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post #6 of 9
7/16/11 at 10:59am
Thread Starter 

  The tubes are now microphonic when i tap the amp or table ... but why does the hum stop when I switch sides with the tubes?


Edited by dancingpotato - 7/16/11 at 11:07am
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post #7 of 9
7/16/11 at 12:17pm
That sounds like one of the tubes is not making good contact in one of the sockets.
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post #8 of 9
7/16/11 at 2:50pm
Thread Starter 

  How strange when i switch tubes left to right etc ... everytime in the "good" setup the amp is dead silent and the other ... the "bad" setup hums ... always ... BTW the  very nasty microphonics are gone ... but I suppose they will come back  if i use the "bad" setup ... but why would I do that?   Hmmmmmmmmmm lil devil on my shoulder is poking me. evil_smiley.gif

 

  I cleaned the contacts like a little demon and used a pin straightener ... and rolled in other tubes to check the sockets also ... I enjoy mysteries almost as much as I enjoy music ... so i am doubly blessed. The world of tubes most certainly has its own unique beauties ... both visually and sonically ...

 

  BTW the seller is sending me some replacement Sylvania 12AX7WA NOS tubes which are more valuable then the originals at no expense to myself ... not even postage ... and i can keep the haunted tubes too !  I certainly applaud his customer service ... above and beyond the norm!  L3000.gif  Bravo !


Edited by dancingpotato - 7/16/11 at 3:02pm
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post #9 of 9
7/17/11 at 7:16pm

I've encountered that phenomenon too, where switching the tubes will yield no more noise after I've had noise in 1 channel (it's a fizzing sound). 

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