Posted by Damir [ 213.202.69.162 ] on January 30, 2005 at 02:46:36:

In Reply to: and now to include "air resistance" posted by Tnuctipun on January 27, 2005 at 20:14:27:

Wonderful world of tubes .
Formula has it`s limitations, it is based on the analysis of the typical DHT output stage, where close to the max. power is the goal, and is assumed that current swing through the load is maximum, or from zero to the value 2*Ia dc. Formula gives such Ra with Ua/Ia proportion as described, and is not effective in the area close to the Ug=0 line - resultant load line is too steep...
For example, for AD1 tube (close to 2A3), OP 250V/-45V/60mA and rp~670 Ohms, mu~4 , Ra is:
Ra=(Ugk*mu)/Ia - rp = (45*4)/0,06 - 670 = 2330 Ohms
Just like reccomended Ra=2k3 in manuals. But, if you want the "extreme" OP, say 100V/-5V/90mA, then Ia can`t "swing" from 0-2Ia dc value and formula can`t give the "right" result, you must "compensate" it with real current swing...

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