I use a small (1.75 hp, 4 amp) shop vac to circulate air for drying. Works great, moves the air, dries it quickly, but... It howls like a banshee! I need a reostat or something so that I can slow the motor down. I could probbaly run it at 1/2 speed and still get plenty of air movement.

I tried Home Depot, Lowe's, Radio Shack, a mom & pop shop, and Ebay. No reostats to handle 4 amps. The only one HD and Lowe's had were for ceiling fans and they are only rated for 1.5 amps. Any ideas?

You could look around at surplus places for a used one. A rheostat that size will likely cost more then your vacuum is worth and it may kill the motor in no time because of the reduced voltage it will have to work harder.
I would instead make a muffler for the vacuum.
HERE is some discusion on building a muffler...
Good luck,
Jon

Using a shop vac to move a bit of air around is a big waste of energy and money. Why not just use a small blower fan? Even a hair dryer with the heat off would be better.

One thing is for sure, Jon Heron has it correct. You will toast that shop vac in no time if you reduce the voltage to it.


Just my 2 cents.

Why the heck would you not just get a cheap fan?