The usual way to deal with such data is one-hot-encoding, since the number of ASCII chars is small.

So are you suggesting one hot encoding to represent each ascii character that is present in the total 20K dataset? e.g. if only ABCdef123 is used in the entire set, then:
A = 1 0 0 0 0 0 0 0 0
B = 0 1 0 0 0 0 0 0 0
C = 0 0 1 0 0 0 0 0 0
etc?
Thanks for your feedback.