3BZ6

The 3BZ6 is a miniature semiremote-cutoff pentode primarily intended for television intermediate-frequency amplifiers. Grid bias varies its gain, making it suitable for stages driven by automatic gain control. GE and Tung-Sol emphasize high transconductance and low interelectrode capacitances. They identify it with the 6BZ6 except for heater ratings: 3BZ6 requires 3.15 V at 0.6 A for 600 mA series heater strings, with controlled warm-up of about 11 seconds. It must not receive the 6BZ6’s 6.3 V.

Semiremote cutoff is illustrated by GE ET-T971A, October 1956. At anode and screen 125 V with a 56 Ω cathode resistor, typical transconductance is 8000 µmho, or 8 mA/V, anode current 14 mA, screen current 3.6 mA and anode resistance about 0.26 MΩ. At the same voltages, grid −4.5 V gives 700 µmho; roughly −19 V gives 50 µmho. This is an extended control characteristic with specified conditions, not a universal circuit gain. Tung-Sol’s different typical point is anode 200 V, screen 150 V, cathode resistor 180 Ω, anode current 11 mA, screen 2.6 mA, transconductance 6100 µmho and anode resistance about 0.6 MΩ.

Ratings remain manufacturer-specific. GE gives design-maximum anode and screen-supply voltages 330 V, anode dissipation 2.3 W and screen 0.55 W. Tung-Sol September 1959 instead gives 300 V anode and screen supply, anode dissipation 2.5 W, screen 0.5 W. In both, actual screen voltage must follow the dissipation derating graph: maximum supply voltage does not permit full screen power at every voltage. RCA RC-30 confirms the family’s use in gain-controlled video IF stages of color and black-and-white receivers. Adjacent types on shared pages remain distinct.

Shielding matters when reading capacitances. GE compares maximum grid-anode capacitance 0.015 pF with an external RETMA 316 shield connected to pin 2 and 0.025 pF without it; input stays 7 pF and output increases from 2 to 3 pF. Tung-Sol gives slightly different values, not interchangeable with GE’s set. Tung-Sol limits grid-circuit resistance to 0.25 MΩ with fixed bias and 1 MΩ with self-bias. RCA also distinguishes heater-cathode peak and average voltages: its November 1970 sheet gives peaks +200/−300 V and averages +100/−200 V, heater relative to cathode.

The miniature seven-pin base is B7G, bottom-view 7CM: 1 control grid, 2 cathode, 3 and 4 heater, 5 anode, 6 screen, 7 internal shield and suppressor. The application table requires pin 7 connected to cathode at the socket. Any mounting position. GE and Tung-Sol maxima: overall length 2 1/8 inches, 53.975 mm; seated height 1 7/8 inches, 47.625 mm; diameter 3/4 inch, 19.05 mm. GE separately gives base seat to bulb top excluding tip 1 1/2 ±3/32 inches, 38.1 ±2.38125 mm. Documentation: GE ET-T971A, October 1956, five pages; Tung-Sol 3BZ6, five pages with September 1959 table; RCA sheets August 1960 and November 1970; RCA Receiving Tube Manual RC-30, 6BZ6/3BZ6 section, pages 216–217.