Datasheets: RC coupled amp VS. typical characteristics

[engels]

A couple of times I've seen datasheets where cathode resistor in "typical operations" was significantly smaller than in "RC coupled amp table". I understand you can bias it any way you like but still cannot understand what's the story.

examples:


5670 - double triode.
see page 3: cass A1 amplifier, cathode resistor = 240 ohm.
in class A RC coupled amplifier table below the lowest cathode resistor is 900 ohm.

5719 - submin triode
page 1: characteristicsn- plate 100V, cathode resistor = 1.5K.
page 6: RC coupled amp - plate 100V, cathode res. = 2.7K and higher.

5718 - submin triode
characteristics: cathode 150 ohm - plate 100V
RC amp - cathode 1K and higher (under 100V)

I'm sure I am missing some point. Why is that difference?

[Miles Prower]

Originally posted by engels
I'm sure I am missing some point. Why is that difference?

What you see under the heading of "Characteristics and Typical Operation" are nominal values used to determine the parameters listed: plate resistance, transconductance, amplification factor, &c. These are nominal figures used as "figure of merit" as an aid to selecting a type for an application. It gives you a rough idea as to what the particular VT is capable of. Of course, in an actual circuit, the likely Q-point will be vastly different. VTs aren't like transistors with consistant device parameters. Plate resistance, transconductance, amplification factor can wander all over the place. I also determine these from the characteristic.

Lots of VT specs also suggest typical circuits. I ignore these completely, and design based on loadlines. Most of those sample circuits are based on 5.0% THD, which is pretty awful. Nor is it always possible to use the specified Vpp. I can do better by picking my own Q-points and distortion figures from the plate characteristics. Consider it to be nothing more than a cheat for lazy designers.