12B7

The 12B7 is a variable-slope pentode intended for high-frequency amplification in radio receivers. The Raytheon document of June 6, 1939 already presents it as a remote-cutoff amplifier: making the control grid more negative progressively reduces its ability to amplify. This characteristic provides the principle of gain control useful in radio stages, without requiring the valve always to operate close to an abrupt cutoff.

Construction uses a T9 glass bulb and an eight-pin locking Loctal base, with no top terminal. The indirectly heated cathode has a heater requiring 12.6 V and 0.15 A, on AC or DC. Pins 1 and 8 connect to the heater, 2 to the anode, 3 to the screen grid, 4 to the suppressor grid, 5 to the internal shield, 6 to the control grid and 7 to the cathode. Suppressor and internal shield therefore have connections separate from the cathode.

Both sheets give typical operating points with 100 or 250 V on the anode, maintaining 100 V on the screen, −3 V on the control grid and the suppressor at cathode potential. At 100 V, anode current is 8.9 mA, screen current 2.6 mA, transconductance 1.9 mS and internal resistance approximately 0.25 MΩ. At 250 V, these values become 9.2 mA, 2.4 mA, 2 mS and 0.8 MΩ respectively. A grid voltage of approximately −35 V reduces transconductance to 10 µS: this is a gain-control reference, distinct from claiming that current is completely zero.

The stated maxima are 250 V on the anode and 100 V on the screen grid. Control-grid-to-anode capacitance is limited to 0.005 pF; input and output capacitances are approximately 5.5 and 7 pF. Tung-Sol specifies these capacitance data with an external shield connected to the cathode. The available tables assign neither an audio output power nor numerical dissipation values: they chiefly describe the electrical behaviour of a radio valve with adjustable gain, with attention to connections and measurement conditions.