LTSice is a powerful simulation tool that is provided free by Linear Technology Corp.   It comes with a complete library of passive and common analog solid-state components but if you want to use it to simulate vacuum tubes, it doesen't work as-is.    Even though triode and pentode symbols can be found in the "Misc" folder, they are just schematic symbols and no actual model data exists.

However it is straightforward to add tubes to the ltspice library choices.  Just be aware that this information is provided with no warranty implied or otherwise.

LTSpice is a 32 bit program and will be installed in Program Files (X86).  Use Windows File Explorer to locate the LTC folder.  Then find the \sym and \sub folders underneath it - these are where two files that are avaialble at the Attachements link below need to be added.

#1   Add symbols file.

The path to where the tube symbols file needs to be is \LTC\LTspiceIV\lib\sym   Download and unzip the tube_sym zip file to this folder.

#2  Add models file.

The path to where tube symbols file needs to be is \LTC\LTspiceIV\lib\sub.    Download and unzip the tube_lib zip file to this folder.

Start LTSpice and under the component folder (the "AND gate: icon on the menu) you should now see a folder called 'Tubes' listed.    Open this folder and you will find diode, triode, tetrode, and pentode folders. Click on the folder of your choice and you will get a number of choices. Quite a few triode models and fewer pentodes, tetrodes and diodes have been created.   Some errors may exist but most work ok.   More models can be added if desired.

It may be necessary to go under the Tools menu and click "Sync Release" to force LTSpice to update if the Tubes folder is not visible.