This vintage Bassman came here a few weeks ago for an overhaul and the first thing I noticed was that it still had the nonadjustable bias with selenium rectifier setup of yore. After a general refurbishing, I ordered some small trimmer pots from Mouser. Do not order fake Bourns pots on ebay folks. I took a small pair of pieces of vulcanized fiberboard and mounted the components that duplicated a later adjustable Bassman bias setup, and dropped it into the slot where the selenium rectifier used to live.
There are a couple reasons for doing this. Selenium is nasty, toxic stuff for starters. The selenium rectifiers are pretty unreliable as diodes and they don't have nearly the capacity that modern day silicon diodes have. Plus, you get to fine tune the bias level and get all that early fifties goodness that Leo built for you.