So recently we've been having problems with some of our fresnels (Altman 8" Fresnels.) The lamps get about 50-100 hours (?) on them and then they stop working. The filaments are fine, and all we have to do is use some fine sand paper on the contacts of the lamp and they start working again. It happened to one of my lamps tonight and it looks like arcing. Where's a good place to buy new bases for Altman Fresnels?