Mcintosh C22 schematic
 

McIntosh C22 preamp schematic. Rectifiers at bottom right SR1, SR2, SR3 are
selenium. They can be replaced with 1N4007 diodes for SR1 and SR2 and a 2
to 4 amp 50 volt or greater diode bridge for SR3.

Mcintosh C22 schematic
 

It was suggested to add a 10-20 ohm resistor is series with the diode. This
will give a resulting behavior very close to the selenium rectifier. Here
use 20 ohms with the 1n4007 diodes and 10 ohms, (or maybe 5 ohms), at least
10 watt or greater with the diode bridge. The filaments will draw about
600ma at 18.9 volts from the supply. You can try several resistor values to
get about 18.9 volts (12.6 + 6.3) at the filament center taps of V3 and V6.