UCH81

The UCH81 contains a triode and a heptode in a miniature noval tube for AM and FM radio receivers. In frequency conversion for AM reception, the triode generates the local oscillation and the heptode mixes this signal with the received signal. Philips also specifies the heptode for use as a radio-frequency or intermediate-frequency amplifier. Tesla documentation describes other arrangements: in FM reception the triode can act as a self-oscillating mixer, while the heptode becomes an additional intermediate-frequency stage; it also lists the heptode as a VHF preamplifier and the triode as an audio-frequency amplifier. These are documented circuit options, not functions performed simultaneously in every receiver.

The two sections share a cathode, a detail that matters when interpreting the connections between oscillator and mixer. The datasheets give capacitances between electrodes and between the two sections: in the heptode, for example, Philips specifies no more than 0.006 pF between the plate and first grid. For mixer operation Philips presents operating points with 100, 170 and 200 V supplies, each with different currents and conversion transconductance. Conversion transconductance must therefore be read together with the circuit and voltage that produce it, rather than confused with the heptode's characteristic transconductance measured separately.

The UCH81 is indirectly heated for a 100 mA series heater chain; its heater voltage is 19 V. The technical sheet comparing ECH81 and UCH81 identifies the former as a 6.3 V, 300 mA version with a different heater. For continuous operation, Philips sets the maximum plate voltage of both sections at 250 V; maximum plate dissipation is 1.8 W for the heptode and 0.8 W for the triode. The heptode's screen grids 2 and 4 have an ordinary limit of 125 V, while the 250 V value in the sheet applies to the specific condition of plate current below 1 mA. This distinction prevents a value intended for a special case from being used as a general limit.