Datasheets: RC coupled amp VS. typical characteristics
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 18th November 2007, 08:19 PM #1 diyAudio Member     Join Date: Jan 2007 Location: Jerusalem, Israel Datasheets: RC coupled amp VS. typical characteristics 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?
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Re: Datasheets: RC coupled amp VS. typical characteristics

Quote:
 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.
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