12FQ8

The 12FQ8 is a high-mu twin triode with an unusual construction: each section has one grid and two anodes brought to separate pins, while the cathode is common. RCA presents it for frequency dividers and complex-wave generators in electronic musical instruments. Our descriptive sources recall tone-generator use in electronic organs, where separate anodes reduced the number of tubes needed. It is therefore not an ordinary twin triode with only two anode outputs.

Indirect heating requires 12.6 V at 150 mA AC or DC. The miniature glass envelope is T6½ with a nine-contact noval base, 9KT. Heater pins are 4 and 5 and the common cathode is 9. Section 1 has grid 7 and anodes B and A on 6 and 8 respectively; section 2 has grid 2 and anodes B and A on 1 and 3. Two anodes per section and a common cathode make this specific diagram essential rather than inferring connections from the noval shape alone.

RCA, General Electric and Tung-Sol agree on a characteristic point at 250 V anode and −1.5 V grid: amplification factor 95, about 76000 Ω internal resistance, 1250 µmho transconductance, or 1.25 mA/V, and 1.5 mA per anode. The measurement condition matters: one anode is used and the other anode of that same section is grounded. These values do not automatically describe both anodes in parallel; General Electric supplies a separate curve family for that connection.

The sheets give design-maximum limits of 330 V for each anode, 0 V maximum positive DC grid voltage and 0.5 W dissipation for each individual anode. Total heater-to-cathode voltage, DC plus peak, is limited to 200 V in either polarity; with heater positive relative to cathode, the DC component must not exceed 100 V. General Electric also specifies allowable heater voltage from 11.3 to 13.9 V, while RCA expresses tolerance as 12.6 V ±10%.

Small internal capacitances also explain the construction. RCA and Tung-Sol list about 0.9 pF grid to either anode and 0.7 pF between anodes of the same section, without external shield. General Electric instead gives 1.2 pF for the latter coupling: that difference belongs to the maker and should not become an artificial average. Curves and capacitances remind us that both anodes belong to one section controlled by one grid; they do not form four completely independent triodes.