From: a@b.com   
      
   flipper  wrote in   
   news:9c14n5d0a4b2c143jka5855mces64ihogd@4ax.com:   
      
   > I'm wondering if anyone has investigated surge through the output   
   > tubes during turn on when using a delayed B+ timer.   
   >   
   > For background, I haven't used delayed B+ because I don't think it   
   > makes much, if any, difference but I thought I might do one just for   
   > chuckles.   
   >   
   > As for the amplifier, I thought I'd do a fixed bias triode mode   
   > 6GK6(6BQ5) PP output with cathode followers driving the grids. The   
   > preamp would be 'fully differential'. I.E. a pair of 6DT8 (12AT7)   
   > triodes on top a current source in both stages leading up to the   
   > cathode followers. So far so good.   
   >   
   > I'm using a pair of OPTs from a Harmon Kardon 6BQ5 amp.   
   >   
   > Everything, so far, is through Spice as I haven't built it yet but   
   > here's the issue. When B+ is snapped on with warm tubes there is a   
   > huge surge though the output tubes upwards to 450mA.   
   >   
   > It's easy enough to see. As B+ ramps up (through PS filters) on the   
   > preamp tubes it propagates though the coupling caps as a 'signal', as   
   > large as 35V, driving the output tubes.   
   >   
   > Discussion?   
      
   I use freds with large B+ caps instead of tube rectifiers. I'm not sure of   
   cathode stripping, but as a precaution I have a switch between ground and   
   the bias supply pot. With the switch open, there is full negative bias   
   voltage on the output grids. With the switch closed, the bias voltage   
   gradually rises, so there is no output tube surge or noise, because of the   
   high impedence of the bias supply circuit. There is no need to switch the   
   B+. I found this technique in a schematic for a tube radio transmitter in   
   an old AARL issue.   
      
   Cathode followers driving the output tubes can also act as a slow start up   
   circuit. For example, the Acrosound 209 uses cathode followers driving the   
   6146s directly. The cathode resistors are connected to the bias supply, so   
   at startup, the output tube grids see full negative bias voltage until the   
   6SN7s warm up. The Allen Type 75, which has a solid state rectifiers, also   
   has this design.   
      
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