6T4

The 6T4 is a low-mu miniature UHF triode designed by Sylvania as a tuner oscillator. The June 1955 Engineering Data Service specifically cites television tuners and documents an example at 950 MHz. Its cathode is indirectly heated, requiring 6.3 V, 225 mA. The small T-5½ bulb and seven-pin base have two plate and two grid connections belonging to one triode system.

The characteristic point uses plate 80 V and cathode resistor 150 Ω: plate current 18 mA, transconductance 7 mA/V, amplification factor 13 and internal resistance 1,860 Ω. The 950 MHz oscillator example retains 80 V and 18 mA but specifies self-bias −4 V, grid resistor 10 kΩ and approximate grid current 400 µA. The groups retain their own conditions. Grid voltage −15 V in the characteristic table applies to just 50 µA plate current, not the 18 mA operating current. No oscillator output-power figure is supplied for attribution to the tube.

Design-centre maximums are plate 200 V, plate dissipation 3.5 W, grid current 8 mA and cathode current 30 mA. Heater-cathode total DC and peak is 50 V; with heater positive, the DC component is limited to 25 V. Capacitances depend on configuration: the 1955 sheet gives input/output 3.3/1.8 pF with external shield 316, 2.9/0.25 pF without. Direct grid-plate is 1.7 pF in both. For specified-electrode-only measurements the external shield, if used, is grounded. The other Sylvania sheet gives unshielded input 2.6 pF and shielded/unshielded output 2.0/0.4 pF: separate document values, not numbers to average. The 1955 sheet also gives grounded-grid input/output 5.7/3.4 pF shielded, 5.5/1.8 pF unshielded.

Seven-pin miniature B7G, 7DK bottom view: 1 and 7 plate, 2 and 6 grid, 3 and 4 heater, 5 cathode. Do not confuse it with other B7G triodes having different connections. The June 1955 drawing gives maximum diameter 3/4 inch, seated height 1 1/2 inches and overall 1 3/4 inches: 19.05, 38.1 and 44.45 mm. Bulb height excluding tip is separately 1 3/16 inches, not overall length. Any mounting position. Plate and transfer curves show changes in current and transconductance with bias. Documentation: Sylvania 6T4 Engineering Data Service, June 1955, four pages; Sylvania Type 6T4 UHF Triode, two-page data and curves; Precision 660 test table, 6T4 row.