From: zeetso@gmail.com   
      
   hi there!   
      
   first to state, i got no real expirience with vacuum tubes, but i understand   
   basic electric/electronic principles, and have some soldering skills :)   
      
   that said, i have a question regarding voltage and current for 12AU7 (ECC82)   
   heater in cheap and simple headphone amp. tube is in input stage, driving an   
   op amp.   
   now, design says that amp needs 12V DC power supply. there is an LM317   
   current regulator in series with heaters, and it has 5ohm resistor giving   
   max 250mA load (calculated value).   
   with 12V DC supply voltage there is only ~10,5V between pins 4 and 5   
   (heaters in series).   
      
   what is confusing me is the fact that every 12AU7 datasheet says it needs   
   12,6V for heaters (in series).   
      
   is this value absolutely necessary, or it might vary?   
   how is that affecting tube performance and sound?   
      
   i connected the amp to regulated power supply and noticed that it sounds   
   better with higher than 12V supply voltage.   
      
   to get 12,6V for heaters i need ~15V DC power supply, so i guess that would   
   be best option.   
      
   what says you?   
      
   p.s. original schematic is in pdf file here:   
      
   http://www.fredsamplifiers.com.au/documents/12AU7_Valve_KIT.pdf   
      
   p.p.s. sorry for gramatics, english is not my native language...   
      
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