1dfa4d10   
   From: apogosso@tpg.com.au   
      
   "Patrick Turner"  wrote in message   
   news:64c707b7-51b0-430f-ba29-   
   =======   
   Now the same scheme would be impossible to implement on an SE amp   
   because there's only one phase of voltage and nothing much to stop Ea   
   rising enormously when no load is present and the tube cuts off when   
   in pentode or beam tetrode mode where grid current usually has no   
   limiting effect on Ea swings and voltage gain approaches gm x Ra, or   
   perhaps 130 for an EL34 or 170 for KT88.   
      
   But one could rig a voltage doubler supply to create a rail at 2 x B+,   
   and run SI diodes with their cathodes taken to tube anodes, and diode   
   anodes taken to 2 x B+, say +800V, so that when and not if Ea rises   
   that far the diodes conduct, and Ea is clipped. There will be a   
   negligible rise in the B+ rail as a a result.   
   Diodes may be also placed between tube anode and 0V to prevent Ea ever   
   going negative.   
      
   Alex:   
      
   Why such a complication with a voltage doubler? Is not a varistor a simpler   
   solution?   
   I fitted a 10mm 275Vac rated varistor across the primary, and it nicely   
   clamps peaks at about 420V, while otherwise without load the peaks easily   
   exceed 1kV if I hot-plug a 12AX7 tube. That means I can safely use 400V cap   
   in a snubber RC circuit across the primary. Do not need kV rated caps.   
      
   This varistor has only 150nA leakage at 250V, which is negligible. I was   
   surprised how small the leakage was, given that it is a crappy generic MOV   
   from Jaycar. It is too big. A smaller one (5mm or 7mm) would be enough and   
   even better because of smaller capacitance, but they are not available from   
   Jaycar.   
      
   ================   
   I find that SE amps don't tend to exhibit the excessive Ea problems as   
   much as PP pentode/tetrode amps, and despite building and selling   
   quite a few SE amps I've never seen a problem with arcing tubes or OPT   
   insulations.   
      
   Alex:   
   Perhaps it is better to try to preserve a vintage lousy rusty dusty OPT with   
   old deteriorating wire insulation and rotting layers. Arcing will gradually   
   destroy insulation. First you get short turns, then open circuit.   
      
   A radio which I mean has an external speaker 6.35mm jack. When you plug the   
   extrernal speaker, the internal one gets disconnected. This switching may be   
   intermittent. So you may be sure for some time the amp will be working with   
   no load.   
      
   ==============================   
   The other way to clamp output voltage and thus clamp the anode swing   
   voltages in a tube amp with OPT is to use two networks of 5W rated   
   Zener diodes in series with a normal 1N5404 3 amp Si diode between the   
   OPT secondary outlet and 0V so that if the Vo exceeds the rated Vo for   
   maximum rated power by say +3dB, then the zeners+diodes conduct to   
   clamp the voltage. Such clamping of outpur voltage is done routinely   
   on many SS amps where a diode such as 1N5404 is taken to each + and -   
   rail so that if the Vo ever exceeds the rail voltages then the diodes   
   conduct. Back emfs generated in speakers and inductiove loads are thus   
   prevented from flow *backwards* through output SS devices which will   
   destroy themselves instantly is the Ve-b back flow V exceeds about   
   7Vpk. The same sort of diode   
   clamping is used routinely in SS amps with mosfet OP devices to limit   
   the Vg-s, and thus not exceed a rated Vg-s which would punctuture gate   
   insulation which is very thin indeed.   
      
   Alex:   
   Zeners + diode is not a good solution. Firstly, why use 3A diode if you are   
   clamping 100...200mA at worst? Secondly, ZD combination has nonlinear   
   capacitance even before the breakdown threshold. Charging the Zeners takes   
   current spikes, and then they discharge slowly through their leakage. It is   
   like connecting an AM detector to the OPT. Also a reverse recovery of a slow   
   diode might add tiny but very high order distortion to the signal.   
      
   If one insists on using a ZD solution, then I would advise to reverse bias   
   the Zener chain to slight conductance and connect a cap (say 0.01uF) across   
   them. Then the diode will never be forward conducting in normal operation.   
   The diode will still present as a nonlinear capacitance like a varicap, but   
   it will be negligible. Important there will be no intermittent conducting   
   and reverse recovering. To provide this bias you really need a low current   
   voltage doubler.   
      
   =========================   
   Voltage clamping is thus very easy for anyone to achieve.   
      
   Alex:   
   What can be easier than one varistor?   
      
   Patrick Turner.   
   >   
   > --   
   > Regards,   
   >   
   > John Byrns   
   >   
   > Surf my web pages at, http://fmamradios.com/   
      
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