[Chas]

In radios that have fewer tubes and a combination of separate filament rheostats substitutes for '99's will work successfully as the gain can be reduced for each stage. The Catacomb controls the filament voltage of but one of the tubes as a volume control the other tubes operate at the 3.3 volts, 0.060ma each with a bias of 4-1/2 and 9 volts. A further restriction is interconnecting wiring is confined to inside the catacomb. Unless there are other internal issues in the catacomb requiring a melt out, changes to the circuitry internally are not possible.