XFY10

The Hivac XFY10 is a directly heated subminiature pentode with five wire leads and an oval-section glass envelope. Our sources associate it with hearing aids: specified audio output is just 3 mW, a scale suited to small sensitive earpieces. This is an interesting output-valve feature, but does not establish an absolute lowest-power record. The photographed specimen is described as about 9 × 5 mm in section and 38 mm long excluding leads; these are specimen measurements, not manufacturer drawing limits.

Radio Valve Data, compiled by the Wireless World staff and published by Iliffe & Sons in its sixth edition in 1958, lists XFY10 among then-current Hivac types. Page 29 specifies a 1.25 V, 25 mA filament, 22.5 V anode and screen, −1.25 V grid bias, 0.5 mA anode current and 0.2 mA screen current. Transconductance is 0.35 mA/V and the stated output load is 50 kΩ, with 0.003 W, or 3 mW output. The chapter concerns single-valve Class A audio output; these figures describe that regime, not independent absolute maxima.

Connection designation B5A, diagram 1 in the same reference, places the five leads in the order anode, screen grid, positive filament, control grid, negative filament joined to suppressor grid. The filament therefore serves as cathode, as confirmed by the asterisk beside its voltage in the book’s legend. The reference’s 1.25 V agrees with the other incoming heating data; a different descriptive profile gives 1.4 V, which should not automatically replace the verified table figure. Likewise, 1958 establishes the type’s inclusion in that edition, not its introduction year.