The EQ is done with passive RC filters. The topology looks like
,
where each stage performs the transfer function
,
that is mathematically described as
,
giving time constants of
.
So, you just solve these functions for the desired turnover points and viola! You're done. The hard part is figuring out what comprises Ra and Rb. For the CORNET2, Rb is simply R205. Ra is more complicated...
For determining Ra, you calculate the ac small signal plate impedance of V200, which depends on R203. Put that in parallel with R202, add R204 to the result, then put that in parallel with R207. Everything is carefully balanced as compromises between grid current, parasitic capacitance, bandwidth, plate current, available supply voltage, noise, linearity of tube operation, and deviation from nominal response caused by rolling tubes. This is my result, my compromise, my tuning. You can do it differently. In fact, most other designers do, basically by making R204 really high in value, thus dominating the response. I don't like that approach. I did exactly the opposite.
jh
