UF89

The UF89 is a variable-transconductance pentode for radio- and intermediate-frequency amplification. Philips documentation describes an indirectly heated valve on a nine-pin Noval base. Its distinguishing feature is the way transconductance falls as the control grid becomes more negative: the circuit can thus reduce gain as the received signal grows stronger. This suits stages governed by automatic gain control, which must handle signals of widely differing strength without always applying maximum amplification.

The heater requires 12.6 V at 100 mA; the sheets provide for its use in series heater chains supplied by AC or DC. In the connection diagram the suppressor grid has a separate pin, while two contacts are assigned to the internal shield. These details matter during restoration: before fitting a UF89 in an existing stage, both heater-chain current and the actual base connections must be checked, rather than inferred merely from its membership of the pentode family.

A Philips example makes the change in transconductance concrete. With a 170 V supply, a 15 kΩ resistor feeding the screen grid and a 130 Ω cathode resistor, a control-grid bias of −1.95 V corresponds to about 11 mA anode current and 3.8 mA/V transconductance. When the bias is taken to −20 V, the stated transconductance falls to 0.11 mA/V. These figures belong to that circuit and its conditions; they are not measurements that can be assigned indiscriminately to every receiver. The published curves also show changes in current and internal resistance throughout this adjustment.

The audio examples show a less obvious use of this pentode: in the Philips table, with a 170 V supply, 220 kΩ anode load, 620 kΩ screen resistor, 1.8 kΩ cathode resistor and 1 MΩ grid resistors for the UF89 and the following stage, gain is 95 and specified distortion is 0.4% at 5 V RMS output. This is the result of that resistance-capacitance circuit, distinct from valve transconductance. Philips limits include 250 V operating anode voltage, 2.25 W anode dissipation, 0.45 W screen dissipation, 16.5 mA cathode current and 150 V heater-to-cathode voltage. A construction detail described for Mullard specimens is an anode made of two small plates joined at their ends; the three grids can be distinguished through the mesh shield.

Philips specifies no more than 0.002 pF capacitance between control grid and anode, a relevant figure for high-frequency use. The sheets also give audio-frequency amplifier examples, so the principal purpose does not exhaust the documented circuits. A Philips note warns that grid current may occur at one of its typical operating points and recommends at least −1.5 V bias if that current is unacceptable. It is a reminder to read the conditions attached to each figure, especially when reconstructing a circuit. The examined material includes Philips sheets dated 1954–1955 and later sheets, as well as a Mazda-Belvu sheet; their dates identify technical editions, not by themselves a first production year. Documentary sources: Philips UF89 sheets and characteristic curves, and the Mazda-Belvu UF89 sheet.