Do you know how (or where i could find some tips) for pentode wired as triode output impedance calculations?

You may get a rough estimate by looking at the graphs.

http://www.r-type.org/pdfs/ef80.pdfOn page B, you can see, for example at Vgk=-2V, the 170V line shows Ia=10mA, and page C shows Ig2=2.6mA for a total of 12.6mA. The 150V line shows Ia=7mA and Ig2=2mA, for a total of 9mA.

So you get 12.6-9=3.6mA for 20V variation. You may conclude that the

**internal** resistance - also known as plate resistance - is about 5.5 kohms.

As Ian mentioned, this is not the

**output** impedance, which depends on the actual circuit.

You can see that the internal resistance is dependant on the operating point; this evaluation must be done after defining the operating point corresponding to your needs.

The problem with thismethod is that detailed information such as this particular one is not often available.

You can't measure directly the internal resistance of a tube, you can only measure the output impedance of a stage and derive the value using the relevant formula. Indeed, the best is to use a common-cathode or common-grid stage and measure the output impedance at the plate, because the calculation is easier.