PC900

The PC900 is a miniature triode for radio-frequency amplification in the VHF television band. The Philips sheet specifies an RF stage in TV receivers; Mazda-Belvu also describes it as a VHF amplifier. It has a seven-contact base and an indirectly heated cathode with the heater intended for a series supply: current is 300 mA and nominal voltage 3.9 V. That rating should be checked in the heater chain when working on a receiver.

Philips separates quick-reference data from characteristics measured at specific points. One typical condition uses 135 V on the plate, 0 V on the shield and −1 V on the grid, giving 11.5 mA plate current, 14.5 mA/V transconductance and an amplification factor of 76. The same sheet also lists 20 mA/V and a factor of 84 in a different operating arrangement: those latter figures do not belong to the condition just described. Capacitances are specified with an external shield connected to the cathode, so comparisons also need to preserve that measurement condition.

Philips design-centre limits include 200 V plate voltage in operation, 2.2 W plate dissipation and 20 mA cathode current. The sheet separately gives 550 V for plate voltage at zero current: it is not an ordinary loaded supply point. Maximum grid-circuit resistance is 1 MΩ, increased to 3 MΩ in automatic gain control (AGC) circuits. Keeping these limits apart from the characteristic points prevents a maximum value from being read as a recommended operating condition.

Our sources also describe a construction detail: in the specimens examined, the plate has two active faces on either side of the cathode and grid, with a small surface close to the grid and a larger metal area to dissipate heat. Plates guiding electrons into two beams are also described. This illustrates how an RF tube can combine a compact active region with an area useful for heat dissipation. These observations concern the specimens examined and do not certify every commercial variant.