The HSG mod has a slightly different approach to the real valve gain setting and positioning of the voicing RC network attenuator compared to the original Soldano design. Though not the final design this is a dialogue covering the thoughts I had in approaching the modification. I will concentrate here primarily on the input stage and what I attempted to do to make more sense of the setting. All values are from SPICE simulation using Microcap 9 evaluation edition from Spectrum software. This is a free download and contains the 12AX7 / ECC83 models as standard from Duncan Munroe.
The standard stage is set up pretty much from the off as a high gain amplifier. The valve is operated at a high gain and then droped with a passive resistor network with 1Meg as the gain pot This generates a lot of resistor noise and susceptibility to interference being a high impedance point then amplified by the rest of the preamp. The gain of the input valve is a combination of the anode resistor (220K) and the load resistor (1Meg pot + series resistor) over the cathode resistance.
For the Soldano design below, with a 100mV input at the point marked VG1 the voltage at the V1A anode is 6.26V pk-pk or a gain of 62.37. However at point A at the input of the second stage and after the 1Meg series resistor the overall gain achieved is now 27.6 with the gain pot set to 100%. The quandary is that whilst 62.7 is pushing the valve fairly hard the overall gain achieved is modest, and I still have an issue with all that impedance and resistor noise on the input of the second stage.
All this doesn’t come without cost, the AC frequency response curves for both circuits are quite different. However the low frequency is recoverable by increasing C27 from 1uF to 4.7uF. The final curves are shown below. The red is the standard circuit with 100% gain at point A, the blue is modified including C27 = 4.7uF.
The Soldano circuit also has a low frequency response that flattens and extends way below bottom E and into hand-on-string and bump noise and I didn’t see that helping either. The modified circuit has a much wider frequency plot, so I can easily tweak the circuit to lose or re-voice this. The original circuit is pushed so especially at the high frequency end it is what it is.
I worry that I have missed something here as I seem to find 1 meg-ohm as the default go-to value for the gain pot, and to me that doesn’t make sense.
I should note that the dynamic range of the modified circuit is reduced because of the bias being unmoved but it will still take a 2.5V pk-pk signal before clipping the first stage which should be enough. If your humbucker is hotter than this then you are probably after clipping anyway. The important thing is that the modification can be tried without removing parts from the PCB and just placing parts or wires in parallel with the existing circuit. By reducing the gain of the valve I also hoped to remove some of the valve to valve dependency that other uses have reported. In the original design the high frequency end is dominated by the valve and not by circuit values.