From: apogosso@tpg.com.au   
      
   "Phil Allison"  wrote in message   
   news:97q0epFvh5U1@mid.individual.net...   
   >   
   > "Alex Pogossov"   
   >> "Phil Allison"   
   >>> "Alex Pogossov"   
   >>>   
   >>>>   
   >>>> In any case, an abrupt interruption of the plate current will cause a   
   >>>> voltage spike on the OPT primary. The stored magnetisation energy of   
   >>>> Lp*(Ia^2)/2 will have to be dissipated. Lp can be quite high in a   
   >>>> decent amp. If a load is connected, then the energy will be dissipated   
   >>>> in the load giving you a loud crack in the speaker. A residual smaller   
   >>>> energy stored in the leakage inductance Ls*(Ia^2)/2 will most likely be   
   >>>> safely dissipated in a snubber of say 2200pF+5K usually connected in   
   >>>> parallel to the OPT primary.   
   >>>>   
   >>>> But what is the load is not connected? Then the huge magnetisation   
   >>>> energy   
   >>>   
   >>>   
   >>> ** Why huge ???   
   >>>   
   >>> If Lp is say 100H and  Ia = 0.1A  then how many Joules is that ?   
   >>   
   >> Apparently 0.5J.   
   >   
   >   
   > ** IOW   -  fuck all.   
   >   
   > Even the smallest Varistor you can buy will absorb it with ease.   
   >   
   > Like this one for $ 1.58 + gst   
   >   
   > http://au.element14.com/epcos/b72205s0301k101/varistor-9-6j-30   
   vac/dp/1004399?Ntt=100+4399   
   >   
   Agree, it is no problem to clamp the surges, but the point is: why I have   
   not seen varistors across the OPT primaries in various schematics? Is this   
   sort of protection not needed? Or is the assumption that no one will be   
   changing tubes on a working amp? Or occasional arcing and sparking is not a   
   problem?   
      
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