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- last updated 26/08/06 21:34:52


Note that these instructions only work for PSpice up to version 8.x, they will not work with Orcad/PSpice 9.x!!!

OK, so you are using PSpice from Orcad, and you want to use some vacuum tube models. Unfortunately, there are no vacuum tube symbols in the library (well, not in the demo version - don't know about the real version!!).

Here is s step by step guide to how to make up a generic triode model. Tetrodes, Pentodes, and Rectifiers follow a similar procedure.

1 Fire up the schematics program, and from the blank screen, select File|Edit Library. This will take you to the component library editor which looks almost like the schematic editor.

Select File|Open, and double click on the library you wish to add your new vacuum tube to. Suggest "eval.slb" if you are using the evaluation version.

2 PSpice componentOn the toolbar, you will see buttons to draw arcs, boxes, lines etc. You will also see one with the graphic O-- on it. This is the pin tool - more of that later.

Spend some time playing around with the arcs boxes and so on, until you can draw a triode. Here's one I prepared earlier! It looks cheap and tacky, but that does not matter. You can go back at a later stage and make it look nice - we just want to get things up and running for now...

3 PSpice componentHaving got our basic triode, we need to add some pins to it. Remember the pin tool from earlier on? Click on the pin tool and move the mouse to where the pin should go.

Tip: Pressing Ctrl+R rotates the pin. Drop them onto the triode, you will get the rather messy looking result on the right.

OK so far, now we need to tidy up some of those messy pins.

4 From the menu bar, select Part|Pin List. Change the pin name from "pin 1" to "a" - don't use the quotes - I'm only using them here so you know what to type in. Remove the tick from Display Name. In the pin attributes box, change the "pin=" attribute from "1" to "a".

PSpice componentIn the top right of the dialog box, you will now have a, pin2 and pin3. Click on pin2. Repeat the preceeding steps but use g instead of a.

Lastly, change pin3 to k, remembering to take the check mark of the Display Name box each time. Press the OK button to come out of the pin list editor. Your symbol should now look like the one on the right.


5 From the menu bar, select Part|Attributes. In the name box, type model. Tab to the Value box, and type 12AX7A. Click on Save Attr to save this attribute with the part.

Continue this, until you have the following four attributes:

model 12AX7A
template X^@REFDES %a %g %k @MODEL
part 12AX7

Then click on OK. Worthy of some further explanation is the template line. The X allows a subcircuit to be included, the @REFDES macro expands to the reference designator at run time - e.g. V7. The %a %g %k entry corresponds to the pin names we made up a little earlier. The @MODEL line expands to the model entry at run time - in this case, 12AX7A.

6 From the menu bar, select Graphics|BBox. This allows a bounding box to be drawn round the component. Draw a bounding box which is large enough to fit the triode into.

Select all the pins lines and arcs that make up the triode, and move it into the bounding box.

From the menu bar, select Graphics|Origin. Set the origin to the centre of the triode.

Select File|Save. You will be prompted for the name of the part, type in 12AX7 and click OK.

Next to the File menu, you will see a symbol. Click on it, and select close from the menu. You will now be back in the schematic editor.

So now you have a vacuum tube symbol. How do you use it? Simple - it now appears in the parts browser with all the other parts and you can select it like any normal part.

Note that you will have to include the subcircuit you require (in this example 12AX7A). From the menu bar, select Analysis|Library and Include files. Click on browse to find the 12AX7A.INC file which you can download from these pages. Click on add include, then click on OK. The 12AX7A subcircuit will be loaded up when you run your simulation.

To use other triode types, highlight the triode in the schematic editor. From the menu bar, select Edit|Model. In the dialog box, click the reference button, and type in a different model name.

Should you want to improve the look of your triode symbol at any time, highlight it in the schematic editor, and select Edit|Symbol to take you back to the symbol editor.

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