Limiter

Here's the limiter circuit (one of two separate channels is shown):

It's based on a circuit from 1959 [1]; the glass 6AL5s being replaced by the also glass 1N4148s:

Trimmer C1 is retained and proves useful in setting the left (or right) part of the transfer characteristic flat. 

[1] 'Function Generation With Operational Amplifiers', H. Koerner and G. A. Korn, Electronics, November 6, 1959, pp. 66-70.