The 5C/450A is a wonderful valve and majestic in appearance. This is a true pentode transmitting valve with the suppressor grid brought out to one of the top caps so that it can be used to modulate the transmission. Essentially the 5C/450A is a 'big brother to the well-known type P15 and covers a similar frequency range but at much higher power.
The ratings given are for class C operation. The g1 current is given as 19 mA and the suppressor grid would operate at about 110 Volts.
The three grids can just be seen with the correct lighting. And two are visible in the photograph. The Suppressor grid is connected to the second top cap. All grids are wound with substantial wire and the outer grid is supported on the box frame.
The valve is directly heated and the filament is tensioned by the springs.
The bell jar envelope is 100 mm in diameter and, excluding the 4 special base pins, is 290 mm tall.
Reference: Data-sheet. Type 5C/450A was first introduced in 1947.