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Overvoltage protection
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 |
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