[Chas]

RA-DA initially used 2 x uv-201 and one uv-200 detector, one amp filament tubes.

There was a modification to the rheostats to use 2 x uv-201a and still use the uv-200. What the set requires depends on the resistance of the filament rheostats.

Later ux-201a's and ux-200a would work, 1/4 amp filaments. If the intent is to operate then the rheostats and the tubes chosen are important. For the one amp tubes a low impedance filament power source is required.

The '201a may be substituted for the gas detector 200a, moving the tuning circuit return from A+ to A- may be required if satisfactory sensitivity and regeneration control cannot be obtained.

The one amp filament tubes are tungsten, very bright when in operation very little if any getter deposits. The 1/4 amp tubes are thoriated tungsten, will operate at a medium orange color, thorium is diffused through the filament but the active layer is on the outside. The 1/4 tubes can be rejuvenated if the thorium layer is depleted.

There is far fewer one amp tubes to be found, therefore they are generally more costly...

YMMV

Chas