ACPT8

The Marconi ACPT8 is a transmitting pentode for high-frequency power amplifiers and oscillators. The January 1959 datasheet explicitly identifies it as a pentode; this is the reference for resolving conflicting secondary classifications. Its thoriated-tungsten filament requires 11 V and 16 A; the copper anode has a radiator. These are features of a transmitter power valve, rather than an ordinary receiver output valve.

Cooling determines part of its operating capability. Permissible anode dissipation is 750 W, rising to 1.1 kW with forced air under the stated conditions: 10 cubic feet per minute at a pressure equivalent to a 3-inch head of water. These figures concern heat dissipated by the anode, not useful RF output. Marconi describes alignment of the screen and control grids as contributing to high overall efficiency.

The quoted maximum ratings apply at wavelengths of 15 metres or longer. At 13, 11.5, 10.5 and 9.5 metres, maximum anode voltage and permissible input must be reduced respectively to 85, 70, 60 and 50% of the quoted figures. Marconi nevertheless specifies useful output down to 9 metres. The full maximum-rating table cannot therefore be applied indiscriminately at every frequency: wavelength restrictions and electrical derating must be read together.

For unmodulated class-C telegraphy with one valve, maximum ratings are anode 4,500 V and 600 mA, screen 800 V and 100 W dissipation, anode dissipation 750 W. One typical example uses anode 4,500 V and 550 mA, screen 800 V, suppressor +80 V, control grid −250 V and anode load 3,600 Ω: output is 1,800 W, anode dissipation 700 W and drive 40 W. These are associated conditions of one example, not maxima to combine freely.

The documentation distinguishes grid, anode and suppressor modulation, as well as class-B telephony. For 100% anode modulation, one example specifies 3,500 V and 300 mA, screen 500 V, anode load 5,500 Ω and carrier output 700 W; maximum anode dissipation for this mode is 500 W. The relatively low suppressor voltage needed to cut off anode current explains Marconi’s emphasis on suppressor modulation. Tables warn that some drive figures vary widely and distinguish carrier conditions from modulation crest: these are not interchangeable mean powers.

Curves supplement the tables with anode characteristics, screen/control-grid currents, suppressor-voltage behaviour, and output and efficiency versus wavelength. Approximate transconductance is 6.5 mA/V at the stated anode 4,500 V, screen 800 V and dissipation 750 W; the sheet warns against attempting static measurement of marked values. Capacitances are anode-control grid 0.1 pF, input 65 pF, output 26 pF. Maximum drawing dimensions are 7.874 inches wide and 18.312 inches overall, about 200.0 and 465.1 mm; stated weight is 10.5 lb, about 4.8 kg. The connection view identifies two filament terminals, screen, control grid and locating pin, with a separate anode connection on the radiator assembly. No pin numbering is invented. Documentation: Marconi’s Wireless Telegraph Company Limited, Pentode Type ACPT8, HF Power Amplifier and Oscillator, January 1959, twelve-page booklet, data and curves on pages 1–9.