From: apogosso@tpg.com.au   
      
   If you are experimenting on a running tube amp, you might be at some stage   
   pulling a hot output tube from its socket while the amp is running.   
   Alternatively you might accidentally send a high negative pilse to a grid of   
   the tube while working on the circuit.   
      
   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   
   will either cause:   
   - arcing in the tube;   
   - breakdown in the OPT winding insulation;   
   - breakdown of the subber capacitor (say 2200pF);   
   - arcing elsewhere in the wiring.or in the tube socket.   
      
   Would it be a good idea to placa a varistor rated slightly above the +B   
   across the primary? Or a spark gap of some sort, or gas discharge surge   
   arrester tube (the later have miniscule capacitance and very reliable). I   
   remember in vertical deflection stages of old TVs such varistors were a   
   must, since the abrupt cutoff of the current (during flyback) in EL84 often   
   used for this purpose was the mode of operation.   
      
   However I have never seen any discussions on the varistor protection issue   
   on this site.   
   What is your opinion?   
      
   Regards,   
   Alex   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)