Clueless Idiots and High Voltage Vacuum Tubes Really Do Mix!

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Sidebar : Graphing Resistances

May 22nd, 2011 · No Comments · Amp Repair, calculations, Datasheets, Schematics

I want to talk about the power section of my Medalist in detail (and some other things), but before I do that I’m going to give myself a refresher course on tube data sheets and graphing resistances.

Recall, if you will, that V = I * R.  An equation in three variables.  If any one of them is held fixed, then you have a simple linear equation.  For example if we held R constant at say 4600 Ω, then we would have V = 4600 * I.    For giggles, I will choose to make I a function of V,  so I(V) = V / 4600. I choose a value for V and the equation tells me I. Lets look at a picture. Here we see resistances graphed for 500, 1000 and 5000Ω. Just like any other graph, you pick a value on the x axis, go straight up for to which ever resistance line you like, and then look across to read off the current through that resistance for the given voltage.

Simple graph of Various Resistances

Simple graph of Various Resistances

That’s all well and good. The key point is that resistance is the slope of the line. The only thing is, in general, we graph voltage drops across resistors.

Circuit that needs graphing

Circuit that needs graphing

In the example above, the bottom part of the circuit is likely connected to the plate of a tube or something and we frequently need to plot how the current changes the voltage across the resistor (thus telling us the actual plate voltage).

In order to graph a linear equation, we need two points. Let us calculate two simple points using v = i*R. We know R. We pick i = 0. No current flowing. With no current flowing, there is no drop in voltage so the voltage at the bottom of our little circuit is the same as that as the supply (200V). We can graph V=200, i=0 and that is one point. For the second point, we choose V=0, which means that all of the voltage has dropped through the resistor. i = V/R = 200/100k = 2mA. We can now draw a line between those two points and read any values we need from there.

Graphing Loads

Graphing Loads

Notice I also converted to milliamps.   By now it is surely obvious why I did all this, I want to start reading data sheets, at night.. to my kids.


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