12FX8

The 12FX8 combines a medium-mu triode and heptode converter in a miniature Noval bulb. Tung-Sol documents superheterodyne receivers where the triode amplifies RF and the heptode converts frequency; anodes, screens and heater obtain their supplies directly from the automotive battery. RCA expressly specifies automobile radios operating from six-cell storage batteries. The converter's oscillator function belongs to the heptode: it must not automatically be assigned to the triode merely because both share an envelope.

The heater requires 12.6 V, approximately 0.27 A. The sheets accommodate 10–15.9 V; RCA recommends 11–14 V for longest life. The triode point uses anode 12.6 V and grid about −0.8 V developed across 2.2 MΩ, giving anode current 1.3 mA, transconductance 1.4 mA/V and amplification factor 10. RCA gives anode resistance about 7,150 Ω. At approximately −3.2 V grid voltage, anode current falls to 10 µA; this near-cutoff condition is distinct from the preceding point.

The self-excited converter point uses anode and screen grids 2 and 4 at 12.6 V. Grid 3 receives the signal and has average bias about −0.5 V across 2.2 MΩ; grid 1 belongs to the oscillator and uses 33 kΩ. RCA specifies 1.6 V RMS oscillator voltage on grid 1. Anode current is 290 µA, grids-2-and-4 current 1.25 mA, anode resistance about 0.5 MΩ, conversion transconductance 300 µA/V. At approximately −3 V on grid 3, conversion transconductance drops to about 10 µA/V.

Non-oscillating oscillator tests are a separate condition: grids 1 and 3 are zero, while grids 2 and 4 are joined to the anode at 12.6 V. Between grid 1 and that electrode group the sheet gives transconductance 3.6 mA/V, amplification factor 9 and cathode current 4.4 mA. Tung-Sol also plots conversion gain against oscillator feedback tap and grid-3 bias. The 200 kΩ-load graph specifies resistors 33 kΩ and 2.2 MΩ and supplies 12.6 V: its curve depends on those conditions, not the tube designation alone.

Design-center limits are anode 16 V for both sections, heptode screens 16 V, and peak heater-cathode voltage 16 V in either polarity. Heptode grid 3 permits negative bias magnitude up to 16 V but no positive bias: its positive maximum is zero. Maximum grid-3 circuit resistance and triode-grid circuit resistance are 10 MΩ. Without external shielding, maximum capacitances are heptode grid-3/anode 0.28 pF, grid-3/grid-1 0.12 pF and triode-grid/heptode-grid-3 0.01 pF.

E9-1 basing is 9KV, bottom view: 1 heptode grids 2 and 4, 2 heptode grid 1, 3 heptode anode, 4 heater, 5 heater and triode cathode, 6 triode grid, 7 heptode grid 5 with cathode and internal shield, 8 triode anode, 9 heptode grid 3. The triode cathode's internal connection to a heater pin must be retained. Any mounting position is allowed. Both manufacturers give maxima of diameter 7/8 inch, seated height 2 3/16 inches and overall length 2 7/16 inches: 22.225, 55.5625 and 61.9125 mm. Overall length includes pins; seated height includes the bulb tip.

Documentation consulted: Tung-Sol, 12FX8 Triode-Heptode, Tentative Data, August 1, 1960, plates 5955 and 5956, four pages; RCA, 12FX8 Medium-Mu Triode—Pentagrid Converter, March 1961, three pages.