XPost: rec.antiques.radio+phono   
   From: phil_a@tpg.com.au   
      
   "Richard Knoppow"   
   > "Phil Allison"   
   >> "Richard Knoppow"   
   >>   
   >>>>   
   >>>  Others have said that the caps are line noise filters.   
   >>   
   >>   
   >> ** Not me.   
   >>   
   >> The action of the cap is to reduce the 60Hz voltage "floating" on the   
   >> chassis to a low value by  SWAMPING  the effect of stray capacitance in   
   >> the AC tranny windings and wiring.   
   >>   
   >> Doing this prevents AC line noise entering audio input wiring and causing   
   >> hum modulation of AM and FM radio signals.   
   >>   
   >> The same idea was used with most US made guitar amps that had only 2 core   
   >> leads.   
   >>   
   >>   
   >   The capacitors I am speaking of were much too low in value to pass any   
   > 60hz current and were found going from each side of the line to chassis   
   > ground in many pieces of equipment. These are typically about 0.01 uf and   
   > quite high voltage. The reactance at 60 hz is about 260,000 ohms. They are   
   > to by-pass high frequency current on the supply line.   
      
      
   ** The AC line caps WE are speaking of some value between 0.01uF and   
   .05uF  -   used on ONE  side only and of tubular film/foil or paper/foil   
   type.   
      
   Their working voltage rating was from 400 to 600 volts DC.   
      
   They were routinely fitted to tube power amps, pre-amps and receivers.   
      
   The stray capacitance in a power tranny is of the order of 200 - 500 pF and   
   typically causes the chassis to float at about half the AC supply voltage if   
   there is no such cap or connection to supply ground.   
      
      
      
   ....  Phil   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)