E81L

The E81L is an output pentode in the Philips SQ series, intended for telephone equipment. The December 1968 documentation also gives the designation 6686 and describes a Noval base with gold-plated pins. The available June 1957 sheets and later revisions describe a valve intended for prolonged service, with characteristics and tolerances specified for equipment design. Indirect heating requires 6.3 V and 375 mA; AC or DC supply, in series or parallel, is permitted. The 1968 edition warns that heater voltages outside 6.0–6.6 V shorten life.

As a class A output valve, with anode and screen at 210 V, suppressor at zero, a 120 Ω cathode resistor and a 15 kΩ load, Philips specifies 1 W at 5% total distortion. Under these conditions currents are 20 mA at the anode and 5.3 mA at the screen, nominal transconductance is 11 mA/V and internal resistance 0.3 MΩ. Amplifier operation is also documented: still at 210 V, with a 180 Ω cathode resistor and a 20 kΩ load, the currents become 15 and 4 mA. The booklets include anode curves, triode-connected characteristics and graphs relating power, distortion and load, helping select an operating point while keeping the different connections distinct.

A distinctive feature is the description of life testing. In the 1968 booklet, production samples are tested for 10,000 hours with anode and screen at 210 V, suppressor at zero and a 120 Ω cathode resistor. The documentation distinguishes initial values from end-of-life limits, including 13.5 mA anode current, 3.1 mA screen current and 7.8 mA/V transconductance, alongside a reverse grid-current limit. This is a test specification, not a certain prediction of the remaining life of a valve stored for decades. Mechanical tests are also described: 500 g shocks, five times in four positions, and vibration at 50 Hz and 2.5 g for 32 hours in each of three positions. Philips states that these are test conditions, not normal operating conditions.

Nominal capacitances include 6.5 pF at the anode and less than 0.02 pF between anode and control grid. Input capacitance is specified as 11.5 pF in the 1957 sheet and 11.2 pF in the 1960 and 1968 editions, with ±0.8 pF tolerance; its increase with cathode current is also shown. The 0.2 mV grid-referred hum limit is accompanied in the 1968 booklet by specific conditions: a 0.5 MΩ grid resistor, earthed heater centre and bypassed cathode resistor. These details make the value useful without presenting it as independent of the circuit.

Operating limits for anode and screen are 210 V, distinct from the 550 V specified at zero current; maximum dissipations are 4.5 W at the anode and 1.2 W at the screen, with 30 mA cathode current. Maximum grid resistance is 0.5 MΩ with automatic bias and 0.25 MΩ with fixed bias; 120 V heater-to-cathode and 170 °C bulb temperature are also specified. Maximum diameter is 22 mm, height 61 mm excluding pins and 67 mm overall. Connections are: 1 and 6 internal shield, 2 control grid, 3 cathode, 4 and 5 heater, 7 anode, 8 screen grid and 9 suppressor. The preserved photographs show specimens with different brands and visible structures; alone they do not establish electrical equivalence or differences in sound.