From: not.here@invalid.invalid   
      
   On Tue, 29 Mar 2011 23:16:39 -0500, flipper wrote:   
      
   > On Tue, 29 Mar 2011 23:10:17 -0400, Don  wrote:   
   >   
   >>I have seen two methods of using a pot in fixed bias circuits. In one   
   >>method, the wiper carries the voltage to the grid, and if the wiper were   
   >>to fail, the bias voltage goes to zero, The other method has the wiper   
   >>connected to another terminal of the pot. If the wiper fails, the bias   
   >>goes up to a high negative voltage. This last method seems best to me.   
   >>Am I missing something here? Why would the first method be used? -Don   
   >   
   > Could be that a lot of people don't consider fault conditions but the   
   > plain ole wiper tap is simple and linear.   
   >   
   > Consider that in a simple 'traditional' configuration the bias pot is   
   > last in a series of resistors beginning with a plain ole RC filter.   
   > Wiper tied to one end makes it a 'variable resistor', as opposed to a   
   > tap, so, depending on how everything else is arranged, it isn't linear   
   > and impedance changes with position, which can bias shift more than just   
   > the one being adjusted.   
      
      
   That's not likely though, is it? The grid is a highZ input, so the bias   
   chain isn't loaded all that much (ok, I'm leaving out class AB1 etc).   
   Suitable values, with the variable resistor at the earthy end of the   
   chain, should give a reasonable bias adjustment. A pot would suffer from   
   the same linearity problem anyway. Just use lower values. A bad wiper   
   connection will just bias the valve to cut-off then.   
      
   --   
   Mick                      (Working in a M$-free zone!)   
   Web: http://www.nascom.info   
   Filtering everything posted from googlegroups to kill spam.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)