C13 (470pf) and R32 (1M) form the classic Fender high pass filter that I mentioned above. In simplistic terms, it has a cutoff frequency of 339Hz so it cuts frequencies below that and passes frequencies above that. You don't want those lower frequencies because they just muddle-up the sound of the reverb. The gain of U2A (the driver opamp) is one plus the ratio of R33 (4.7K) in parallel with the input coil to R34 (47) in series with C14 (22uf). For the frequencies that we are using, the impedance of C14 is small enough to ignore. So we can calculate the impedance of the input coil in parallel with 4.7K then divide that by 47 and add 1 to find the voltage gain. (The inductance of the input coil can be calculated from the impedance at 1KHz and is 95.5mH, needed for the calculations) For simplicity, we will ignore the 4.7K for now below 2KHz because the impedance of the coil is small compared to it. So at 500Hz, the impedance of the coil is 300 ohms and the gain is (300 /47) + 1 = 7.4 . At 1000Hz the coil is 600 ohms and the gain is (600/47) + 1 = 13.8 . The impedance of the coil doubled from 500Hz to 1000Hz but the gain almost doubled, also. So the current in both cases is very similar. As the frequencies get higher, the significance of the 4.7K increases and the gain does not increase as much per change in frequency. Eventually it will max out at a voltage gain of 101. Not only do you not want lower frequencies, but you also don't want higher frequencies, so this works out well. Also, the opamp would reach its limit if gain kept increasing in a linear fashion. The 4.7K resistor also prevents the gain from going to infinity if the tank is disconnected. The driver is a constant-current device for the frequencies of interest.