From: apogosso@tpg.com.au   
      
   OK,   
   Thanks all. So, general consensus is that most likely there is a strict rule   
   as to which heater pin heats which triode in 12A*7 tubes.   
      
   "flipper"  wrote in message   
   news:5t38q5doktvtuii5en172ndnjbchkoq8hf@4ax.com...   
   > On Sat, 20 Mar 2010 10:42:46 +1100, "Phil Allison"    
   > wrote:   
   >   
   >>   
   >>"Alex"   
   >>>   
   >>> If I need only one triode of 12AX7, and thus apply 6.3Vac on pin 4 with   
   >>> pins 5 and 9 grounded, can I be assured that for all brands and makes of   
   >>> 12AX7s the *same* triode of the two will always be heated?   
   >>   
   >>   
   >>** Think that is a safe bet.   
   >>   
   >>The leads from pins 4 and 5 would have to cross over inside the valve for   
   >>it   
   >>to be otherwise.   
   >   
   > I pulled two from my amp, looked, and as for 'crossing' leads, the   
   > internal layout defies all logic. One of them seems almost rational,   
   > with the triode structures oriented parallel to the associated pins,   
   > while, with the other one, they seem to have gone out of their way to   
   > make it as convoluted as possible. Neither triode is physically   
   > arranged to be near the associated pins so you can't suppose that a   
   > pin will be 'near' where it is to go or that things don't 'cross'.   
   >   
   > However, on the second one they ran pin 4 all across the bottom of the   
   > base to get the heater on the far side, which looks to me like they   
   > were making sure to grab the 'right' heater end for the triode on pins   
   > 1,2 and 3 so, on that basis, I'd say it's a fairly good bet.   
   >   
   >   
   >>.....  Phil   
   >>   
   >>   
   >>   
      
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