6021 - SQ Tube
The 6021 is of interest due to its ruggedness, small size, and generally being a fantastic example of a tube designed and manufactured towards the end of the 'golden age' of tubes.
I developed the model for use in the design of a low power guitar amplifier. The datasheet for the 6021 is available from tubedata.org
When the following circuit is simulated in LTSpice, it produces the graph below.