6AM8

The 6AM8 is a miniature valve combining a diode and a pentode in one envelope. Sylvania designed it for video detection and the final intermediate-frequency stage of television receivers: the pentode provides amplification and the diode detection. The pentode has a sharp-cutoff characteristic. The data sheet compares it with the 6CB6 and the diode with one section of the 6AL5, but these similarities do not establish that the valves are interchangeable in a circuit.

The 6AM8 heater requires 6.3 V and 450 mA. The 5AM8 variant instead requires 4.7 V and 600 mA; the 6AM8A retains the 6AM8's 6.3 V and 450 mA. Sylvania specifies controlled warm-up for the 5AM8 and 6AM8A, with 11 seconds measured by the test defined in the data sheet, for use in series heater strings. It does not give that time for the 6AM8: similarity between the electronic sections does not justify automatically transferring heater characteristics from one variant to another.

Sylvania describes the base as a nine-pin E9-1 'small button' with 9CY pinout; the pinout code is not the name of the socket. Viewed from below, the pentode section uses pin 1 for its cathode, 2 for its control grid, 3 for its screen grid, 6 for its plate and 9 for its suppressor grid; pins 4 and 5 feed the heater, while the diode has its cathode on 7 and plate on 8. For that construction, the Sylvania drawing of August 1957 gives a maximum diameter of 7/8 inch, maximum overall length of 2 3/16 inches including pins, maximum height of 1 15/16 inches from the seating plane to the glass tip, and maximum height of 1 21/32 inches from the seating plane to the envelope shoulder. These are dimensions of the documented outline, not measurements of every specimen.

In the August 1957 sheet, a pentode test point at 200 V plate voltage and 150 V screen-grid voltage, with a 120 Ω cathode resistor, gives 11.5 mA plate current, 2.7 mA screen current and 7 mA/V transconductance. These are characteristics under those conditions, not maxima. The same sheet sets design-center limits of 300 V and 2.8 W for the plate, 0.5 W screen-grid dissipation and 5 mA continuous diode current. The diode's 50 mA figure appears only in a test condition: it does not replace the 5 mA continuous limit.

Another Sylvania sheet gives a different test point, at 125 V for both plate and screen grid with 56 Ω at the cathode, and higher plate limits of 330 V and 3.2 W. The values from the two sheets should not be combined without identifying the applicable documentation. The 1957 sheet also shows the importance of shielding in separating the two sections: the stated capacitance between the diode plate and pentode plate is 0.035 pF with Sylvania shield No. 315 and 0.10 pF without shielding. These are stated measurement conditions, useful when interpreting a valve intended for video and intermediate-frequency stages.