Okay, so I wasn't planning on finalizing all of the DarkVoice modification plans today, but that's what happened, all of parts are ordered.
So here is what we are doing specifically...
1)
Maida regulated power supply. The entire stock CRCRC-CRC filter is being excised and replaced with a 330uF reservoir cap and a Maida regulator. This regulator, again the hard work of
@A2029 of 1101 Audio, has far better power supply rejection capabilities than even the biggest, clunkiest passive supplies with giant caps and chokes. It is compact and incredibly performative.
Here is the first CRCRC removed. This is where the Maida regulator will go.
It just so happens I have a regulator ready to go from another project, just have to change one or two components to adapt for this circuit. Had to get creative on how it is going to be mounted, but came up with a solution that I think will work with the existing mounting hardware, no drilling necessary. Going to attempt to get away with not using a heatsink on the TO-247 FET. Going to strap a thermocouple to it and see how hot it gets, we'll see how it goes, having to squeeze a heat sink on would make things very interesting...hoping to avoid it.
2)
Gyrator load on 6SN7. The PCBs and parts are ordered, just a matter of waiting to get them in house for assembly. As I said, these will go in the position of the last CRC of the power supply, again using the existing mounting hardware, the PCB mounting holes will align.
The bias point of the 6SN7 will be altered slightly, so the necessary cathode resistor has been ordered as well.
3)
Output tube cathode resistor swap. Based on simulations I have run and the new bias point of the 6SN7, in addition to the new higher available B+ supply, the bias of the output tube will also be slightly altered to maintain the 80mA plate current of the original design - keeping the 1K cathode resistors would push this to 100mA, which would be fine from a plate dissipation standpoint, but felt better not pushing the tubes harder than originally intended, especially since they are becoming quite rare (pushing 100mA through a 1K 25W resistor would get very hot too). This necessitates increasing the value of the cathode resistor slightly to 1.2K. Just for the halibut, will change them to Vishay non-inductive wirewound resistors, which fit in the same position as the originals.
4)
Output capacitance. The DarkVoice as built has only 30uF of output capacitance, which leads to major LF rolloff with lower impedance headphones, pretty much anything under 300ohms. Have to increase it, which is challenging as the available space is very limited. I was hoping to find something that can fit on the original PCB, and I did, but it is a tight squeeze, some 33uF 250VDC TDK film caps.
Now these are thicker than the originals at 22mm, so I am not super confident that I will be able to squeeze three of them in, which would be ideal. Even if it is just two, 66uF is better than 30uF and will improve the LF performance with headphones of moderate impedance.
5)
Alps pot. That is all.
Edit: adding another item...
6)
Heater balance. The heaters in the stock DarkVoice do not appear to be center tapped. In addition, the leads are not a twisted pair. These two oversights are very likely the cause of the widely reported "hum" issues in this amplifier. Making both of these chances should significantly reduce the audible hum.