[Amps] Pspice models for power grid tubes

[Xmitters@aol.com]

In a message dated 1/26/06 10:13:31 AM Central Standard Time, r at somis.org 
writes:

<< The curves for 500v may be converted to other voltages by applying the 
 3/2 power law.
  >>
The 3/2 power law is only applicable over a fairly narrow range. This 
approach is especially problematic when the control grid voltage is positive at the 
published screen voltage and it goes negative at the desired screen voltage. 
You also run into problems when calculating where the zero plate current line is 
going to be with the desired screen voltage compared to the published screen 
voltage. This comes about when you ratio out the desired versus published 
screen voltage. To scale the grid voltage axis, you then multiply the published 
control grid voltages by this scale factor. Well, this approach does not work 
when you have to relocate the zero control grid voltage point at the new screen 
voltage. Zero times anything is zero.

I verified this by examining several tubes that have curves for more than one 
screen voltage. I chose an observation point (plate voltage and plate 
current) on one curve, then noted the grid voltage for that point. I found the same 
observation point on the other set of curves. I then applied the 3/2 power law. 
In some cases it was close, in other cases it was off by a mile. Especially 
in the case where the grid voltage for that observation point stats out mapped 
with a positive grid voltage, and then goes negative at the higher screen 
voltage.

For relatively small deviations in screen voltage from published to desired, 
the 3/2 law works quite well.


Jeff Glass, BSEE CSRE
Northern Illinois University
WB9ETG Since 1970