CV688

CV688 is the British service designation associated with the 2C43 transmitting triode, of planar lighthouse construction. Our cross-reference sources explicitly point to the RCA and General Electric 2C43 sheets. These describe the family and its very-high-frequency uses, with each data edition kept distinct. The indirectly heated cathode’s heater nominally requires 6.3 V at 0.9 A. Documented applications include Class C RF amplifiers and oscillators, including pulsed operation; this is not an ordinary audio preamplifier triode.

Its most interesting feature is the connection to the circuit. The anode and grid have external disc contacts concentric with the cathode RF contact. General Electric describes an incorporated disc capacitor of about 100 pF between cathode and external cathode RF connection, providing a low-impedance radio-frequency path. The envelope facilitates fitting into coaxial circuits. The base, described by RCA as a small H-wafer octal with six pins, therefore does not carry all tube connections: heater on 2 and 7, cathode on 3, 5 and 8, while pin 1 must not be used. RCA’s 1945 sheet requires mechanical support through the metal shell rather than the base or other terminals.

The sheets also show why dates and rating systems must be retained. RCA’s tentative data of 15 November 1945 specify design-center maxima of 450 V anode voltage, 36 mA and 10 W anode dissipation for Class C telegraphy. RCA’s May 1962 edition and General Electric ET-T1463A of December 1961 instead give absolute maxima of 500 V, 40 mA and 12 W for RF service up to 1500 MHz. At the characteristic point of 250 V anode voltage and a 100 Ω cathode resistor, these later documents give amplification factor 50, transconductance 8.1 mA/V and anode current 21 mA. These are figures for the respective 2C43 editions, to be checked against the particular CV688 specimen and equipment specification.

Pulsed service is a separate case: the later documents reach 3370 MHz and 3500 V positive peak anode voltage, with maximum duty factor 0.006 and pulses no longer than 10 µs. These limits do not permit a continuous 3500 V supply. One General Electric example at 3370 MHz, with 1 µs pulses, 1000 pulses per second and 3000 V peak, specifies about 1.75 kW during the pulse, not continuous average power. RCA requires at least one minute of heater operation before applying anode voltage in this service, whereas simultaneous application is allowed for Class C RF telegraphy. Such details show how performance depends on operating regime and mounting as well as designation.