7B4

The 7B4 is a high-amplification-factor loctal triode for audio voltage amplification. Tung-Sol explicitly specifies a high-gain resistance-coupled audio amplifier. Sylvania compares characteristics and applications with the 6F5G and also lists a 6SF5GT reference; RCA refers to the 6SF5 for limits and electrical conditions. These comparisons concern characteristics and must be distinguished from mechanical compatibility: the 7B4 retains its own loctal socket.

The T-9 glass envelope uses the eight-contact 5AC base diagram. Heater pins 1 and 8, anode 2, grid 6, cathode 7; pins 3, 4 and 5 are unconnected. All terminals are at the base without a top cap and any mounting position is allowed. Operating heater data are 6.3 V and 0.3 A AC or DC; nominal 7 V and 0.32 A are also listed. Sylvania associates the locking construction with compactness and shielding.

Sylvania and Tung-Sol agree on typical class A1 points. At 100 V anode and −1 V grid, anode current is 0.4 mA, internal resistance about 85 kΩ and transconductance 1150 µmho, or 1.15 mA/V. At 250 V anode and −2 V grid the values are 0.9 mA, about 66 kΩ and 1500 µmho, or 1.5 mA/V. Amplification factor µ is 100 in both cases. This characterizes the triode; actual stage gain also depends on load and coupling, so it does not automatically mean a hundredfold circuit gain.

The stated anode maximum is 300 V; Sylvania also specifies 90 V heater-to-cathode. Tung-Sol advises keeping that potential difference as low as possible when heater and cathode are not directly connected. With an external M8-308 shield connected to cathode, Tung-Sol gives 1.6 pF grid-to-anode, 3.6 pF input and 3.4 pF output capacitance. These values belong to the stated measurement arrangement and provide shielding context.

A Sylvania note explains the historical values: nominal 7 V heater corresponds to a 130 V line and is also used as the car-radio nominal. For household receivers, maxima are interpreted as design-center values for a 117 V line; automotive service uses 90% of the listed values with 6.6 V at the battery terminals. This explains how supply context affects interpretation without confusing nominal ratings and operating points.