This tone control network was submitted to the patent office in April 1937 but it is still in widespread use today. I’ll redraw the circuit and you will probably recognize it…
Look familiar? This is the classic Big Muff tone control. I left out a few of the numbers from the patent drawing that are not relevant, but you should be able to match the components in this drawing to the one at the top.
The specific values will be different for use with a tube circuit but the principle is the same.
Just as interesting is the following patent submitted in 1939:
It appears that the tone control circuit in this 1939 patent submitted by Fyler is identical to Ballard’s patent that was filed two years prior. If you are interested in the full text of these patents, you can download the complete pdf files here: