The SP61 was a commercial television RF pentode that became adopted by the military during WWII. We have several variants in the collection, but this is the original livery of the manufacturers and is covered by an external screening metalisation. For mixer use a separate low noise oscillator was designed this was the P61 also known as the VR66.
With the grid at the top cap, this is the normal pattern for RF and IF amplifier valves of the period. Providing high gain in tuned anode circuit applications.
This valve type was used in air-borne radar particularly GEE.
The SP41 and SP61 differ only in heater voltage rating. They resulted from Mazda's strong design initiative during the later 1930s.
The demand for efficient video pentodes for TV came a year or two earlier in Britain than elsewhere and the SP41 was Mazda's solution. Shorter leads to shorter pins, and compactness in general, are essentials where high gain-bandwidth product is a requirement and it is easy to see why Mazda's SP41 was streets ahead of (say) Mullard's SP4. Of course, the later EF50 overleapt the SP41 but the latter and its 6.3V lookalike filled vital needs in military radar and high-frequency communications during the late 1930s and early 1940s, until UK (and later US) production of EF50s was sufficient to meet demands. Moreover, one reason for the huge demand for EF50s was that they had short lives and needed frequent replacement. By contrast, the SP41/61 were virtually indestructible.
The reverse is plain zinc paint.
The classic envelope is 32 mm in diameter and, excluding the MO (Mazda Octal) base pins, is 84 mm tall.
References: Datasheet, 1043 & 1040. Type SP61 was first introduced in 1941.