Valve Millivoltmeter TF899

The Marconi Instruments Type TF899 Valve Millivoltmeter was an advanced R.F. measurement instrument introduced in the late 1940s. It represented a departure from conventional designs by employing a triode valve mounted in the probe assembly instead of the customary diode detector. This configuration offered improved linearity and reduced loading on circuits under test, allowing accurate measurement of small R.F. voltages in high-frequency circuits.

Within the main unit, the TF899 used a pair of EL91 triode valves, one acting as the active measuring element and the other serving as a balancing component within a bridge detection circuit. The instrument provided three measurement ranges covering 20 to 2,000 millivolts and was calibrated to read R.M.S. voltage values for sinewave inputs, though it responded to peak voltages.

Operational performance was specified to ±1 dB accuracy up to 50 MHz, and ±2 dB up to 100 MHz, making it suitable for laboratory testing, receiver alignment, and R.F. amplifier development.
With its triode probe design and extended frequency range, the TF899 offered a significant improvement over typical diode millivoltmeters of the time in both sensitivity and accuracy at very high frequencies.

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