Plotting Tube Loadlines with SPICE

Here is a quick example of how I use SPICE to experiment with loadlines for vacuum tube circuits. By simply running the command spice3 Go-se.sp I get this nice graph on my screen:

The red lines are the plate curves as found in datasheets. The reason they look like a cross between pentode and triode curves is because they are! In this example I'm operating the tube (a 6V6GT beam tetrode) in ultra-linear mode. The quiescent screen voltage is the same as the plate voltage. But when the plate voltage swings to either side of the operating point, the screen voltage swings in the same direction by a factor of 40%. In real circuits this is done by connecting the screen to a tap on the output transformer primary. The red lines correspond to different control grid voltages in steps of 2 volts, from 0V at the top line down to -42V at the bottom. I haven't figured out a way to make SPICE label the lines.

The slanting blue line is the loadline. In this case, I have chosen a load of 5000 ohms.

The horizontal green line shows where the quiescent operating point of the tube is -- namely, at the point where the green and blue lines cross. As you can see, I have chosen an operating point at Va = 315V and Vg = -21V. Following the green line over to the axis shows that this gives us a quiescent plate current of about 41 mA.

Finally, the curved orange line is the plate dissipation rating of the tube -- 14 watts for the 6V6GT.

When I run this, SPICE also outputs some useful information that I would otherwise have to calculate by hand. Here is the output:

    Program: Spice, version: 3f5, patchlevel: 2
    Date built: Wed Dec 29 10:24:57 PST 2004

    Type "help" for more information, "quit" to leave.


    Circuit: *


    Circuit: 6V6 single-ended characteristics

    Values for one tube:
    va = 3.150000e+02
    vg = -2.10000e+01
    ia = 4.074158e-02
    is = 3.529980e-03
    ik = 4.427156e-02
    rk = 4.743452e+02
 

SPICE has calculated the quiescent plate, screen grid, and cathode currents for me. It has also calculated the cathode resistor I'd need (474 ohms) to cathode-bias the tube.

Trying different operating points or loadlines just takes a few keystrokes.

Here are the source files for this simulation. The 6V6GT tube model is Copyright © 1997-2000 Duncan Amplification. I obtained it from Duncan's Amp Pages and then modified it so it would work with SPICE3 instead of PSPICE.

Please note that these files are for SPICE3. They'll need to be changed slightly if you want to use PSPICE.

SPICE can also be used to draw composite push-pull curves as described at Steve's Tube Pages. Here's an example, using the same operating point as above but in a class A push-pull configuration:

Here are the source files for the composite curves:


Copyright © 2005 John D. Polstra. All rights reserved.