Derive Triode Curves from Pentode Curves?

[dsavitsk]

Is it possible to derive at least a reasonable approximation of how a pentode will behave triode strapped from the pentode curves? Being exact is not necessary, but just getting a sense of ra, gm, and mu at some point would be great. Any tricks to doing this?

 

[rdf]

Pentode data sheets normally contain multiple plate curves representing different screen voltages. Graph the operating points where Vp = Vg2.

 

[ilimzn]

Exactly what rdf said - unfortunately, if you are lucky, you will have about 3 points per grid voltage line (most pentode datasheets have a set of curves for two or three screen grid voltages) which can give you a crude idea of what the triode curves will be (most screen grid voltages will be outside the values you'll want to use on the trioded 'plate' and if there are no screen current graphs, you are out of luck - remember that trioded 'plate' current equals pentode plate + screen current)...

 

[EC8010]

Good enough for jazz...

Most pentode data sheets give a value for the amplification factor from g1 to g2. If you were to pick a true triode with similar anode dissipation and cathode current ratings and a similar mu to your triode-strapped pentode, I doubt if you would be very far wrong.