Volume Control Mods
Rod Piazza "Hot Rod" Circuit
I sent Rod my first JT-30 and $60. He added the volume control, a 1/2 inch microphone scew-on adapter and he changed the crystal. He put a fairly thick piece of cardboard in front of the element and a small foam rubber dot between the crystal and the cardboard to keep it from buzzing. No one knows exactly what the capacitor does. It is too small to do much more than smooth out the range of the volume control making it more linear.
When adding a new crystal make sure that the connector that is directly attached to the casing of the crystal goes to the gounded connector attached to the shell of the mic. The insulated connector goes to the "Tip" wire which is the center wire of the coax.
When I add a volume control, I use the cap (why? It might help and it doesn't hurt). I don't use the cardboard, but I cut a new piece of nylon for the mesh (red nylon looks cool).
The 150 is the voltage on the Cap, not the value. It is small, about 1/4 inch across. I use .001 mfd.
I am informed that the capacitor in the Astatic/Blues Blaster/Piazza/etc. mic is there to compensate for the frequencies that are rolled off by the volume pot as it's turned down. The volume pot (a/k/a variable resistor) cuts the high frequencies as it's turned down (as resistance is added). The capacitor allows more highs through as the resiatance increases, so that in theory the mic will have the same frequencies response no matter where the volume control is set.
Kevin's Crystal Balls are the same circuit without the capacitor.
Hohner's Blues Blaster and stock JT-30VC have various values for the pot, but all are much less the the 5Meg shown here. Some older Blues Blasters have a small value cap. Tube amplifiers behave best with very large impedances so it is best to make the pot value very large. There is very little difference in the feel of Linear taper vrs. Audio taper and I can't get audio taper 5m pots anyway.