I've gotten pretty far with my apex 460 mods, the mics are sounding great. I thought I was done until I encountered a post from Open Plan Recording where they figured out how to modify the Apex psu to provide the fixed -1.1V to the grid, like the original C12. In the sound clips below you can hear the difference compared to a diode biasing method, that I got from Fox Audio research, that I'm currently using.
https://soundcloud.com/open-planrecording-studio/sets/c12-diode-vs-psu-1-bias
The problem is I don't know where to begin on modifying the power supply. The old C12 schematics are basically illegible to me, hard to read, some numbers are smudged, pF and mF are written the same, lines zig zagging everywhere. I know JJ Audio has figured this out also for their "Dutch."
I guess the Apex 460 psu is pretty similar to the C12 in the B+ supply in that there's a lot of series resistors with filter caps between them, but the values and number of parts is different, also there's a single diode(?) in the C12 and four in the 460 in the rectifier. Also the Apex uses zener diodes to ground and these are not in the C12. And the cathode in the C12 is connected to ground directly, instead of by resistors and caps as in the original Apex 460 (or two 1N4148 in series in my case). Do you think I could just rebuild the B+ section of the PSU only following the C12 schematic if I could actually read it? And the attached grid bias section.
I don't know if anyone hear can offer advice? Like what is that 300K 1/2 W resistor in the C12 schematic going to pin 4 of the pattern selector to nowhere?
I suspect this is a valuable mod, for getting closer to the C12 sound. I do know my tube microphone cables have an extra unused conductor I can employ for carrying this voltage if I can get at how to do it.
https://soundcloud.com/open-planrecording-studio/sets/c12-diode-vs-psu-1-bias
The problem is I don't know where to begin on modifying the power supply. The old C12 schematics are basically illegible to me, hard to read, some numbers are smudged, pF and mF are written the same, lines zig zagging everywhere. I know JJ Audio has figured this out also for their "Dutch."
I guess the Apex 460 psu is pretty similar to the C12 in the B+ supply in that there's a lot of series resistors with filter caps between them, but the values and number of parts is different, also there's a single diode(?) in the C12 and four in the 460 in the rectifier. Also the Apex uses zener diodes to ground and these are not in the C12. And the cathode in the C12 is connected to ground directly, instead of by resistors and caps as in the original Apex 460 (or two 1N4148 in series in my case). Do you think I could just rebuild the B+ section of the PSU only following the C12 schematic if I could actually read it? And the attached grid bias section.
I don't know if anyone hear can offer advice? Like what is that 300K 1/2 W resistor in the C12 schematic going to pin 4 of the pattern selector to nowhere?
I suspect this is a valuable mod, for getting closer to the C12 sound. I do know my tube microphone cables have an extra unused conductor I can employ for carrying this voltage if I can get at how to do it.