From: apogosso@tpg.com.au   
      
   "flipper"  wrote in message   
   news:lnqbi6lb9quunqa28q318l084os4p8014c@4ax.com...   
   > On Thu, 6 Jan 2011 20:19:52 -0800, "Alex"  wrote:   
   >   
   >>   
   >    
   >   
   > Okay, I have another challenge to ponder.   
   >   
   > Let's say I have three 6.3V @300mA heaters instead of three 12.6V   
   > @150mA. I pick that because it's the same power.   
   >   
   > I do have a center tap but that would put all heater power on one   
   > winding. Not good.   
   >   
   > Series R across both windings just doubles the power, half to heat, so   
   > both windings are loaded. That's worse.   
   >   
   > Ok, so I use the full winding (both halves) with a series C reactance   
   > giving 300mA through the three heaters.   
   >   
   > Winding power (delivered) is equal to heater power due to the   
   > current/voltage phase shift.   
   >   
   > So far so good.   
   >   
   > However, we still have 300mA in the winding of a 350mA transformer.   
   > How does that affect the VA available for the DC section, or does it?   
   > Could I still get the same amount of DC power as with the straight   
   > resistive 150mA heaters?   
      
   a) The best is to equally load all the windings with active load. Therefore   
   your original setup with three 12V heaters in series is the best. 10W   
   transformer is OK, including DC supply rectifier.   
      
   b) Using three 6V tubes with double current means that you load to the full   
   or overload one winding, while the other is wasted. Strictly speaking now   
   you need a tranny rated at 11VA 36V for the heaters, since half of the   
   copper is wasted. Also you still need 5W for DC. Total 15W rated tranny.   
      
   c) Using 36V winding with capacitive reactive ballast is the worst and   
   honestly ridiculous solution. Now 300mA current will be running through two   
   sections of copper, dissipating twice the heat in the copper. "Useful"   
   output is still the same -- 5.4W. You still need 15W rated tranny, but it   
   will run hotter than in case b). Besides, you are facing problems of making   
   a nonpolarised precision and stable cap, which may not be close to any   
   standard value... Why create these problems? We are not in the communist   
   USSR. Actually it is easier to use a ballast resistor -- the same for the   
   tranny, but the resistor will dissipate 5.4W.   
      
   I think you better try to obtain 12.6V versions of the tubes, as per   
   original plan.   
      
   Regards,   
   Alex   
      
   --- SoupGate-Win32 v1.05   
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