6HM5

The 6HM5 is a miniature high-mu triode designed for the grounded-cathode RF stage of VHF television tuners. General Electric, RCA and Westinghouse agree on this use; the RCA manual specifies both black-and-white and colour reception. Grounded-cathode operation amplifies the signal ahead of later tuner stages, although actual performance depends on the circuit and its alignment. RCA also presents the data under the combined heading 6HM5/6HA5, while Westinghouse notes a difference in bulb length from the HA5 series: the socket, available space and heater supply must still be checked before substitution.

The heater operates at 6.3 V AC or DC: GE states 0.185 A, RCA 0.180 A. The base is seven-pin miniature E7-1, with the grid on pin 1, cathode on pins 2 and 7, heater on pins 3 and 4, plate on pin 5 and internal shield on pin 6. The shield and the low grid-to-plate capacitance, specified by GE as 0.34 pF maximum with an external shield connected to the cathode, matter in a VHF stage. Westinghouse includes the 2HM5, 3HM5 and 4HM5 on the same sheet, but their heaters differ: membership in the same family is insufficient for direct substitution for a 6HM5.

The GE sheet gives two characteristic points: at 120 V plate and 0 V grid, 15 mA and 18 mA/V transconductance; at 135 V plate and −1 V grid, 12.5 mA and 14.5 mA/V. At the 135 V, −1 V point, the RCA sheet instead states 11.5 mA, an amplification factor of 72 and plate resistance of about 5 kΩ, with the same 14.5 mA/V transconductance. These are typical figures under the stated manufacturer conditions, not limits. GE also reports a 4.2 dB noise figure measured at 200 MHz in a noise-matched, optimized and neutralized grounded-cathode stage; it must not be assigned to every tuner.

Published maxima also differ: using design-maximum ratings, GE specifies 200 V plate voltage, 2.2 W dissipation and 20 mA continuous cathode current; RCA specifies 220 V, 2.6 W and 22 mA respectively. RCA also gives 600 V as the plate-supply limit: this is not the permitted plate voltage, which remains limited to 220 V on its sheet. During restoration, identify the documentation for the actual valve, check the shield connection and measure the real voltages before comparing them with the limits. Sources: General Electric, RCA and Westinghouse 6HM5 sheets; RCA Receiving Tube Manual, 6HM5/6HA5 entry.