(Warning:Keith is not a real expert, he's just the guy with the website and too much time on his hands, so don't take anything he says as gospel.)
>Hi Keith, I have a JT30VC built by "Mr. Downchild" in Cleveland area, which
>is the same as the "hot rod" Piazza circuit you have on your web site. I
>want to reduce the brightness and feedback tendencies but keep the big fat
>bottom. Do you know if changing the capacitor will accomplish this? If so,
>what value would I start with? Have you ever heard of putting an adjustable
>tone control inside the mike (capacitor and variable resistor). Thanks for
>any advice you can give.
Replacing the cap in the Piazza circuit with a larger value will make it more like a tone control and roll off the highs, but it seems to make more sense to do this at the amp. I think the mic should send out it's signal as unaltered as possible and you should use the amp tone controls to fine tune it.
As far as the size of the cap, 30 years ago I was an electrical engineering major and I could have done the math in my head, but my mind has since been pickled. I would start small with a .001 mfd (or less) and work my way up to .05 mfd. I think that .1 would be very large and you'd loose all frequencies through the cap. You've got to keep the value fairly low. I have some discussion pages off usenet - the micK pages - on my site. Look for tone control circuits there to get an idea of the cap sizes that are used. Some pages have guitar circuits so you might also search for a stratocaster or telecaster volume and tone circuits to see how guitars accomplish this sort of thing.
This being said... I have often thought of installing one of those double pots in a mic and have a tone control in addition to the volume control. You reduce feedback by fiddling with the tone and I can see where you might want to brighten things up in one part of a song and darken them down in another.
Kevin's harps used to sell a small box with two pots. One was a simple RC tone circuit and the other was a volume control. He charged $35 for this thing and I got him to take it back (after I cracked it open and peeked inside) He sold it as a feedback eliminator, but it was no more useful than the amp knobs.
An interesting kit would be to take a small can or container and put two 1 Meg wheel pots in it with a 1/4 inch female plug at each end. You ought to be able to come up with something from Home Depot plumbing department (see my home depot mic for inspiration). An on/off switch would also help for when you set the mic down to go get a beer.
Switchcraft use to make a similar device for providing volume controls to mics. They sell now for up to $200 each.