There are things that you can do to make your JT-30 look better and possibly sound better. Don't attempt these unless you can afford to screw up. Don't mod your best mic. Get a cruddy one from eBay or order a new one from Coast2Coast. There's no sense in modifying a beautiful old mic, but there are millions of unremarkable JT-30's from the 50's and 60's to experiment on.
Some of the harder mods require soldering. Some are so easy as to be obvious. I'll start with the easiest and work up to the most difficult.
your mic a bath. Clean it up good and put some wax on it to buff it up
nicely. The mic gets funky and sticky from the sweat on your hands and from
being spit at. Unscrew the grill and take it apart. Separate the element from
the grill and wash the grill in warm soapy water. DO NOT GET THE ELEMENT WET!!!
Wipe down the shell with a damp cloth or a paper towel wetted down with the
blue window washing stuff. While it's apart see 2 and 3 below. Put it all together
and polish up the finish.
You might also want to use some rubbing compound to get rid of the scratches.
2. Remove the cardboard baffle. Some mics come with a cardboard baffle to protect the element. They muffle your sound. You might consider trading the protection that they offer with more volume. Take of the grill and separate it from the element. If you have a baffle it will be between the element and the silk.
3. Change the silk. The JT-30 comes with a black piece of cloth between the element and the grill called a silk. Take out the old silk - they are usually disgustingly dirty. Replace it with a piece of red or blue cloth, preferably nylon. I have a box of nylon remnants that my wife bought from a parachute factory. I have a choice of wild colors for my silks. It can be any piece of bright colored cloth as long as it's not too heavy. Use the old silk as a template for cutting the new silk. You may have to put a small fold in the fabric to get it to conform to the concave shape of the grill. Dampen the silk and put it in the back of the grill. Let it dry in place before assembling the mic. If you have trouble getting the silk to stick, put a little rub-on glue stick stuff on the inside of the grill to make it tacky. The color peeks through the grill and looks very cool.
4. Repaint the shell. Take off the grill and tape up the element and the connector and the volume control, if you have one. You may want to unsolder everything and take it apart completely. Sand and brush the mic to get rid of any scratches and dings. If you strip the paint off the mic, it can be polished to a dark metallic glow, but this will dull up as the metal oxidizes. Paint it with several layers of spray paint. Go to an auto parts or hobby store and get something cool like "Candy Apple Red". Home depot carries hammer-tone paints in gray, red, green, gold and black. I have the red hammer-tone on one of my mics and it's a rich deep color. On second thought, though, I might have preferred a metallic color. You may want to finish up with a clear lacquer spray to deepen the finish. Let it harden and then wax it to give it good sheen and protect it from your sweaty hands.
5. Color the grill. After you have painted the mic, you might want to tint the chrome grill. You can do this with a clear lacquer mixed with a clear color tint. Auto parts stores might sell this kind of paint. Another source of inspiration is nail polish. A clear nail polish with a tint or an opalescent pigment might look good; nail polish colors tend towards pinks and reds. (If you are a guy, you may wish to bring your girl along to the drugstore, as this is definitely not a "manly" purchase.)
6. Add decals. Go to a hobby shop where they sell the stuff to customize hot wheels cars and model airplanes. They usually sell a decal sheet of flames for less than $10. Look at the flames decals here: http://www.slixx.com/other.htm. Spray a light clear coat lacquer over the decal to protect it.
7. Replace the Element. There are many different element options that you can use on a JT-30. The original crystal style (MC-151) can be replaced with a ceramic element (MC-127). If you can find a Shure magnetic element (controlled reluctance) you may wish to try that, giving you the sound of green bullet in the lighter JT-30 body. The Shure element requires that you create a way to mount it in the JT-30. I did it by stuffing the shell with enough foam to hold it in place, but I am sure that you can think of better way.
Try using cheap or odd microphone elements such as the ones at: http://www.mouser.com/catalog/cat_606/335.pdf , http://www.allelectronics.com/cgi-bin/category.cgi?category=390&type=store or http://www.surplussales.com/Microphones-Audio/MicroAudio-3.html.
8. Replace the connector. If you have an XLR type connector you might want to replace it. Even if you have the M-1 standard mic connector, you might want to go for a direct ¼" phone jack. You can buy the Switchcraft thick panel jack (www.mouser.com part 502-151 or Neutrik 568-NYS220). The jack requires 15/32 inch mounting hole.
Remove the old connector. Some unscrew, some have a set screw and the XLR ones are epoxied in place. (You have to manhandle them out.) Drill out the old connector hole with a 15/32-inch drill bit and put in the thick panel jack. Don't use the stand mount hole; this is obvious place for a volume control. WARNING: drilling out the mic takes away all collectible value from it, so don't do it on old vintage mics.
If you have access to the right size tap (they don't sell it at Home Depot), you may want to match the threads on the jack and tap it into a 7/16 inch hole so you don't have to use the nut to tighten down the jack.
If you know a machinist or someone with a drill press, this whole process is much easier.
Solder 3-inch wire leads to the connector before you install it. Screw in the jack and twist down the nut on the inside of the mic. Solder the leads to the element (or volume control) and you'll never have to use an adapter again. The mic looks much more streamlined with a direct jack. If you use a guitar cord with a right angle connector, the mic becomes much easier to hold.
9. Add a volume control. You can easily put a volume control into a mic that doesn't have one. See the Rod Piazza Circuit for guidance. I use a 1-meg pot (www.mouser.com part Xicon 31CX-601 13 mm Carbon). This is a linear taper, and I am not totally thrilled with it, but it is tiny, it has a 4mm shaft and it fits in a small hole. You have to buy the matching knobs at mouser that have go on the 4mm shaft (45KN05). It's also great for Shure bullets. If you can find a 5-meg pot that is as small as the Xicon, let me know. There's not much difference between a 1-meg and a 5-meg, but .
The pot is small and the connectors require you to use a magnifying glass to solder it in place. I solder leads to the pot before I install it. Buy several pots, because it's easy to break off a lead while soldering.
Drill out a 3/8" hole in the place where the mounting bracket goes. There should already be a small hole there where the grounding screw is placed. I use a reamer and just widen the existing hole until the pot fits (that's why I'm not sure that 3/8" is the right size hole for the pot.) Nip off the mounting tab sticking out from the pot with a pair of wire cutters.
Put the pot in place and use a pair of needle nose pliers to tighten down the mounting nut. If you can find a lock washer that fits, use it. Volume pots tend to come loose. You tend to fiddle with them and they loosen up. A drop of super glue might help.
Solder things up according to the diagram, but test it before you screw the mic back together. You will probably have to unsolder it and put it back together because you didn't set it up so that clockwise makes it louder. No matter how careful I am, I screw this part up.
Put the knob on so that you can tighten the set screw easily. If you have a small hole in the side of the mounting area then you can fit a small screwdriver through that so you can tighten it, otherwise, you have to angle the screwdriver down to get at the set screw.
10. Add the volume control at the "little-endian" side of the JT-30. (Little-Endian - from Gulliver's Travels. In Lilliputia there was controversy as whether to break the egg on the big side or the little side, so the little end of the JT-30 is the tip end at the back.) This places the volume control at the end of the mic so that you can turn it easily, but it is not in the way of your hands when you play. I've seen Kim Wilson and Arthur "Fresh-Air" Moore with mics modified in this way. I've never done this, so I don't know how thick the shell is and if a pot with a ¼" shaft will work.