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LTSpice and vacuum tube models - last updated 11/11/07 17:12:08



Here's a step by step for using the generic triode model with LTSpice.  These instructions don't follow every possibility with every model, but if you run through this one you will get the general idea.

Any of the pictures down the left hand side can be clicked on for either a bigger picture, or a link to a web page.

Download LTSpice/SwitcherCAD III from the Linear Technology website.
Download the generic triode model (click link on the left), you want the PSPICE one as LTSpice understands them just fine.

Save it to the folder where you will be putting your schematics.

If you want to use the heater model, a symbol will need to be made up for it - this is easy with LTSpice.

File|Open, change the drop-down box to Symbols (*.asy) and navigate to the folder lib\sym\Misc.  Open up triode.asy

Use File|Save As to change the filename to triodeh.asy ("h" as been added to signify heater).  Press p to place the first heater pin, call it H1 and then press p again to add the second heater pin called H2.  Save it.
You should now have exercised your artistic skills to come up with something looking like the picture on the left.

To save time, you can download a ready made triodeh.asy if you wish saving it to the LTSpice folder lib\sym\Misc so the software can find it.  If the file comes back as a load of text, best to use right-click and Save As on the link.

Draw up your schematic, the help file is pretty good and got me up and running and about 5 minutes.

Note that AC heater is used and should be 8.91V not 6.3V as sine waves on SPICE are measured at peaks.  8.91 = 6.3 x sqrt(2).

From the menu, Simulate|Edit Simulation Cmd, and make sure that Start external DC supply voltages at 0V and also Skip initial operating point solution are checked.  Also pick a time for the simulation, in this case 15 seconds.

This is critical and applies to any simulator, not just LTSpice. If you leave them open the simulation engine will try and iterate to a solution before the heaters have warmed up.

Press S to invoke a new SPICE directive, and type .inc or whatever name your triode models are in.
Simulation|Run completes the picture.  Click on the pic on the left and see how the plate voltage sits at 250V then comes down as the tube warms up.

You can download this schematic by clicking here.  If a load of text comes up, use right-click and Save As.  Make sure that is in the same folder.

On the left, another simple schematic which feeds 200Hz to the tube and you can see the output "come alive" as the tube warms up.

You can download this schematic by clicking here.  Again, be sure that is in the same folder.  Start it running and make a nice hot cup of tea - even on my 1200MHz machine it takes a little while...

Here's another to try out, a small EL84/6BQ5 amp with a pair of ECC83/12AX7 stages and negative feedback.

Download the zip file and extract the files to where you normally store your LTSpice schematics.

Another example with a pair of EL34/6CA7 being driven by the ECC81/12AT7 as a preamp and phase splitter in guitar amp style.

Click here to download the zip file.

Any queries, comments, suggestions, complaints, please stick 'em on the forum!

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