The AM Forum

I found this on the web somewhere the curves for triode connected 813 tube.

Please help me out here,  Are these curves for class C operation. If I'm reading the chart correctly, it looks like an 813 tube would require -170 volts at around 1325 vdc to keep the tube within specifications or to operate class C.  I know people use these in class B service with zero bias. I know these questions will make it look as if I don't know what I'm talking about. You would be correct. Let me know what I'm missing please?    ??? ???

http://www.pmillett.com/file_downloads/813%20curves.gif (http://www.pmillett.com/file_downloads/813%20curves.gif)

I found this data at this site. http://www.pmillett.com/tube_data.htm (http://www.pmillett.com/tube_data.htm)

Lots of good old stuff here.

Those look like Plate current vs plate voltage. The blue lines are the actual current and different grid voltages. The left most line is zero bias and you work your way into Class C as you go right.  Notice how the efficiency increases as you move right - 100 watts output at much lower plate current. But look at the wide range of linearity on the zero bias line at the far left. This is where you would want to be for an audio amp/modulator or an RF linear amp.

The curves show up either too large or too small on my peecee here.

If it looks fishy compared to what you already expect maybe it is for the low Mu connection instead of the hi Mu connection. Plate and screen connected together instead of all grids tied together. You CAN do either with an 813.

Plate and screen tied together limits maximum plate voltage to the maximum screen voltage rating.  Not a problem with small receiving tubes but would be a severe limitation with transmitting tetrodes and pentodes.