7L7

The 7L7 is a sharp-cutoff, high-transconductance loctal pentode. Tung-Sol explicitly specifies an IF amplifier or limiter in FM equipment. It belongs to the receiver’s signal-processing chain, with defined cutoff and curves showing current and slope versus bias. Limiting refers to the source’s FM circuit context, not an audio power-output stage.

Heater 6.3 V, 0.3 A AC or DC; RCA separately notes nominal 7 V, 0.32 A. The T-9 glass envelope has an eight-contact lock-in base, 8V diagram, any mounting position. Pins: heater 1 and 8, anode 2, screen 3, third grid 4, internal shield 5, control grid 6, cathode 7. Published operating points connect third grid and internal shield to cathode at the socket.

RCA and Tung-Sol give two class A1 points. Anode and screen 100 V, control −1 V, cathode resistor 125 Ω: anode 5.5 mA, screen 2.4 mA, internal resistance about 0.1 MΩ, transconductance 3000 µmho or 3 mA/V. Anode 250 V, screen still 100 V, control −1.5 V, cathode resistor 250 Ω: anode 4.5 mA, screen 1.5 mA, about 1 MΩ and 3100 µmho. These are different conditions; reading only anode voltage misses the bias change.

RCA specifies bias near −6 V for cathode-current cutoff; Tung-Sol gives the same voltage at 10 µA residual anode current. It describes cutoff, not ordinary amplification. Tung-Sol curves separately show anode and screen currents and, for 100 or 250 V anode, transconductance and internal resistance versus control voltage. They complement the tables without implying constant slope throughout the characteristic.

Main maxima: anode 300 V, screen 125 V, screen supply 300 V, dissipation anode 4 W, screen 0.4 W. RCA design-center also specifies 90 V peak heater-cathode either polarity and maximum positive control bias 0 V. The 4 W is anode dissipation, not audio output power; 300 V supply is not direct permissible screen voltage. With specified shielding: input 8 pF, output 6.5 pF, control-anode maximum 0.01 pF. RCA connects external shield to cathode and base shell. RCA 15 October 1947 and Tung-Sol 2 May 1949 date the editions, not independently the tube’s introduction.