From: johno@sdd.hp.com (John Ongtooguk)
Subject: Re: Tube sources
Date: 1 Sep 1992 23:00:35 GMT

In article <1992Aug31.135044.10787@ncsu.edu>, kester@mat.mte.ncsu.edu (Dan Kester) writes:
|> Having recently bought a hybrid-tube preamp (Counterpoint with 3 tubes)
|> I have started to pay attention to tubes.  What I'm wondering is how
|> different are the different tubes?  What is a good source of tubes ? 
|> The tubes in the preamps are 6DJ8 and 12AX7.  
|> 
|> Any advice on how to tell if a tube is running as well as possible?
|> 
|> Is it desirable to mechanically isolate a tube preamp

   lots of editing above. I've noticed the biggest difference in the
   phono section of the preamp but noisy/microphonic tubes aren't
   wanted anywhere. I have a simple set of requirements for tubes
   and I usually buy a batch of them cheaply and sort them out myself
   using a 'scope and a wooden q-tip. If you buy selected tubes only
   buy the best ones as otherwise you'll get the rejects from the
   other grades....I returned a bunch for this reason. I recall paying
   about $8 to $10 for Amperex 6DJ8's and some other brands that I
   don't recall. 

   Since the phono section preamp seems to be the most senstive to
   tube problems I do my sorting there. After powering up the gear 
   and letting it warm up for awhile I check for noise, running the
   signal thru a solid state receiver to my 'scope, and then I check
   for microphonics by tapping the tube...ideally it should show a
   spike and quickly dampen. Some mil-spec JAN 6DJ8's that tried
   would ring forever; thru the speaker they sounded like chimes.
   Not the most rigorous way to sort but it seems to work. 

   I like to use four large Sorbothane feet beneath my preamp and
   I've built some little 1/2 in square stacks of the stuff to
   dampen the board at several places. It produced a marked
   difference although some people would say that there isn't
   enough 'room effect' or ambience, due to microphonics. The
   imaging is very good and the ambience comes thru when it's
   in the music.
 
   Have fun and experiment...but be careful as there can be a lot 
   of current running around tubes. 

   John Ongtooguk (johno@sdd.hp.com)