6FQ7

The 6FQ7 is a miniature twin triode with a medium amplification factor. General Electric, RCA and Tung-Sol data sheets specify it chiefly for vertical- and horizontal-deflection oscillators in television receivers. Tung-Sol also lists phase inverters, multivibrators, sync separators and amplifiers, and resistance-coupled audio amplifiers. Thus the same tube can serve different functions depending on the circuit, and no single operating point suits all of them.

The heater is rated at 6.3 V and 0.6 A; the documentation also covers series heater strings and gives an average warm-up time of 11 seconds under the test defined in the sheet. On the nine-pin noval base, viewed from below, pins 4 and 5 are the heater and pin 9 is not connected. Section 2 uses pins 1, 2 and 3 for plate, grid and cathode; section 1 uses pins 6, 7 and 8 respectively.

As a Class A1 amplifier, the GE sheet gives two typical conditions for each section. At 90 V on the plate and 0 V on the grid, it specifies 10 mA, 3 mA/V transconductance and approximately 6.7 kΩ plate resistance. At 250 V on the plate and −8 V on the grid, it gives 9 mA, 2.6 mA/V and about 7.7 kΩ. The amplification factor is 20 in both cases. These characteristics apply to those test points, not to maximum ratings.

For Class A1 amplification, the GE limits are 330 V plate voltage, 22 mA cathode current and 4 W dissipation on either plate. With both sections operating, however, maximum combined dissipation is 5.7 W: two 4 W allowances cannot be added. Maximum grid-circuit resistance with fixed bias is 1 MΩ. The sheets also distinguish heater-to-cathode voltages: 200 V total, but no more than 100 V for the DC component when the heater is positive relative to the cathode.

For deflection-oscillator service, the GE sheet gives separate pulse limits: for the vertical oscillator, 440 V negative grid pulse and 77 mA peak cathode current; for the horizontal oscillator, 660 V and 330 mA. These figures apply to pulsed television service under the stated timing conditions, not to continuous bias. The RCA manual groups the 6FQ7 with the 6CG7; before substituting a related type, the heater, socket and actual operating conditions of the equipment still need checking.