EC80

The EC80 is a radio-frequency triode designed for grounded-grid UHF amplifiers up to 500 MHz and for mixer circuits. Philips documentation describes communications, radio links and measuring equipment, as well as wideband amplifiers and intermediate-frequency stages following a crystal mixer. Its specialization appears in the connection: the signal enters the cathode and the grid is common to input and output circuits. The grid thus helps screen the cathode from the anode; Philips also links the favorable noise behavior to the absence of the partition noise inherent in a pentode.

The base is nine-pin Noval. To reduce common-lead inductance and instability, the grid connects to four contacts, all to be grounded. Anode is pin 9, cathode 3 and heater 4 and 5. The socket matters too: the application note recommends type 5908/46 for decimetric-frequency requirements. Mounting is possible in any position, with support recommended under shock or when not upright. Heating is indirect, 6.3 V parallel, AC or DC. Current is 0.48 A in the 1955 Philips sheets, application note and July 1956 La Radiotechnique sheet; the 1959 and December 1968 Philips sheets instead give 0.43 A. These figures belong to different editions and should retain their context.

The datasheet characteristic point uses 250 V anode, −1.5 V grid and 15 mA anode current, with transconductance 12 mA/V and amplification factor 80. The application note uses a 100 Ω cathode resistor for automatic bias and recommends this arrangement for stability. At 300 MHz with 4.5 MHz bandwidth, approximately 15 dB power gain and 8 dB noise figure are given. The manufacturer stresses that at decimetric frequencies power gain and bandwidth must be considered together: the 15 dB is not a circuit-independent voltage gain.

Capacitances illustrate the construction: cold anode–cathode capacitance is at most 0.075 pF, or 0.08 pF including the heater with the cathode. The two tabulated input configurations, with different heater grouping, give 5.1 and 9.3 pF; output is 3.4 pF. Limits include 300 V operating anode voltage, 4 W anode dissipation, 15 mA cathode current, 100 V cathode to heater and 0.3 MΩ in the grid circuit. The 550 V cold anode allowance is distinct from the operating limit. The December 1968 Philips sheet also identifies EC80 as 6Q4, in the S.Q. series, with gold-plated pins and a 500-hour life test on production samples: a test procedure, not a promise that individual life is limited to 500 hours.

A particularly interesting Philips example is the radio section of a 300–400 MHz receiver. It contains two grounded-grid EC80s, an amplifier and a mixer, and an EC81 local oscillator. The actual tuned circuits are coaxial lines with movable short-circuiting plungers, rather than the ordinary coil-and-capacitor circuits shown in the simplified diagram. Two knobs move the plungers through rack-and-pinion mechanisms. This example specifies 5 MHz bandwidth, approximately 12 dB RF-stage gain and approximately 8 dB noise figure.

In that receiver mixer, both the amplified aerial signal and oscillator signal reach the cathode of the second EC80. The EC81 operates as a Colpitts oscillator using its interelectrode capacitances; the documentation also describes interstage matching through the coupling capacitor and the next tube’s input capacitance. Even aerial lead length contributes to input matching. These details explain the EC80’s nature: a small radio tube whose UHF performance depends on the whole assembly, connections and tuned lines as well as the triode’s static values.