Not to start yet ANOTHER thread about this, but for people interested in Gus' MXL 603/990/991 mod here is all you need to know, with pics. I am just trying to make it easier and more clear.

Special thanks to Gus for coming up with this one (sorry Gus) and Flatpicker too for answering my dozens of simple questions.

I did the 990s last week and enjoyed the improvement, but the improvement to the 603s is more profound. These are the best deal around in a matched pair, and if you don't already have some you need to pick up a pair. $200 u.s. for a matched pair with shock mounts and case!

Change ceramic caps C3 and C4 from .22uf ceramic to .22uf Metal Film Panasonic caps from Digikey

Change the capsule/Gate cap C13 (on back of PC board) from a .001uf (1000pf) ceramic to a 1000pf Xicon Styroflex polystyrene cap from Mouser

After those 3 simple cap changes the already decent mic becomes wonderful. I will be using thes mics more and more.

I did not find the PC board to be fragile at all (at least compared to the MC012) I recommend a decent iron with a clean tinned tip, and desoldering braid for removing the old parts. Apply the braid, then the iron to the braid, and when the braid soaks up the solder (turns silver) the lead is clean. It helps to use the very tip of the desoldering braid for more effective heat transfer, and for an easier time clip the used tip of the braid after cleaning each trace. Then just use the hot iron to poke the leads through, leaving holes for the new components.

This mod is very easy and should take about 10-15 minutes.

Here are some pics of the mic innards before and after. If you like the mod report back here with your findings. For the price everyone should have a pair of these. 



General:
Replace the input cap off the capsule with a MIT .001 uf PPMFX. Replace the jfet with a J305 from Siliconix. Replace the electrolytics with Panasonic FM. Bypass the signal electrolytics with a Wima .01 uf MKP-2.

Its not often I make a discovery of some importance by myself, but I had to post this for anyone interested in doing the Royer mod for an MXL 2001, V67 or ADK 51.

I got my modded V67s back from Eddie King and one of them made terrible popping sounds when it warmed up. I knew immediately it was the tube. I had taken a chance on eBay and bought some JAN 6205 tubes, which are similar to the 5840 from the Royer article. They are ruggedized, but have a lower max voltage. Most of the specs are similar. Its not a big deal as both tubes can run easily at the 100v plate voltage the Royer mod runs on.

I replaced the 5840 with the 6205 and noticed an immediate improvement in sound. The lows are tighter, the highs are better and the sound is more open. Not to mention it seems they have about 10% more gain. I know its not me being optomistic as I A/B'd the new tube with the other 2 mics I have with the 5840.

The 6205 is generally cheaper than the 5840, too. By almost half at some places.