7fbcf80b   
   From: apogosso@tpg.com.au   
      
   "Engineer"  wrote in message   
   news:9a294d45-dee5-48f0-968a-f3f6a14965dd@q26g2000vby.googlegroups.com...   
   > Hi, Vcuumlanders,   
   > Perhaps this is just a minor point, or even common knowledge, but it   
   > has merit...   
   > I had put a pre-set 1 meg log gain pot at the front end of my recently   
   > built monobloc (the Williamson near-clone that some here may   
   > recall...)  It worked, of course, but I was aware that near the mid-   
   > point setting the amp had more hum and noise than at either end (with   
   > a pre-amp connected.)  With no pre-amp connected the hum and noise   
   > just went up as the pot was tuned up.  The penny dropped!  The grid of   
   > the first audio tube was picking up "under-chassis" hum and noise when   
   > the first stage grid had a high impedance to ground, i.e. 1/2 meg, or   
   > so. I had not specifically shielded any under chassis AC wiring and   
   > there was a 60 Hz power transformer under the chassis, too, for B+   
   > boost.   
   > The solution: use a 47 K log pot for the preset input gain. Now the   
   > first grid never sees more than 23 K to ground - very quiet.  But now   
   > the amplifier now has 47K input impedance. This is OK if the pre-amp   
   > has a cathode follower output, i.e. has a Thevenin impedance of, say,   
   > 400 ohms (it has - it's a Heathkit WA-P2), or if the pre-amp is solid   
   > state - 47K is easy to drive.   
   > I promptly changed my two other monoblocs, a home-brew and a Pilotone   
   > commercial unit, to the same pre-set pot input value with some noise   
   > improvement - less than the first one, but the idea is a sound   
   > one.   
   > Cheers,   
   > Roger   
      
   Just by the way: If a pot is set to midpoint, and source output impedance is   
   low, then output impedance of the wiper is R/4, not R/2.   
      
   But... obviously the smaller the pot value -- the less hum and less grid   
   current related noise will be there anyway.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)