When I mod these mics I upgrade all of the capacitors. Start with the 1000pf and change that to a polystyrene. The capacitor between R7 & R9 I change to a 1uf Polypropylene. The 2 red/brown film caps I change out with Wima or Philips film caps. The 47uf by R3 I use a Nichicon Muse or other high grade capacitor. All of the other electrolytic capacitors I use Panasonic, Nichicon, Elna, or Rubycon.
I also do some proprietary tuning to the circuit to "tame" the high end.
Have u heard of these mods? They're actually for the GT-55 (same mic?) and I found em on a diff forum:
1) Remove the resonator disc (carefully).
2) Install a trimpot between the resistor and source voltage on the FET to balance the microphone pair with each other. I found a 10K trimmer bringing the drain voltage to around 11 volts sounded best to me.
Also, how would u guys say that the mod affects the sound? Any mics to compare it to after?
How much someone wanna charge to do the mods if I send u my ST55? I dont have too much time to get into this and I feel weird visiting my local electronics guy since I wouldn't date his Japanese immigrant friend lol.