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The 1936 AC/SP1 is an amplifier pentode. As the designation suggests it was for mains driven sets and had the 4.0 Volt 1 Amp heater of the time. As DC mains was also common in the 1930s an other version was available for that market, it had the Type designation of SP2220.

This is a relatively uncommon valve used in some up-market Murphy radio receivers of the late 1930s (eg. the A40 series) as the control elements in either AFC (automatic frequency control) or amplified AVC (automatic volume control) stages.

It may also have had other professional uses and is related to special long-life pentodes developed for use in undersea telephone repeater applications.

Strangely enough, brand new, boxed AC/SP1's (and also AC/SP3's) were at one time often to be seen gathering dust on the top shelves of radio service shops, perhaps because there was no call for them.

The family similarity to the contemporary ACS2/Pen is obvious. The bulb shape, the style of base cap, and the greyness of the metallised coating typify Mazda-made valves of the mid 1930s.

The balloon envelope is 42 mm in diameter and, excluding the B7 base pins, is 113 mm tall.

References: Data-sheet & ¹⁰⁴³. Type AC/SP1 was first introduced in 1936. See also 1936 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.