IEC 60100: Interelectrode Capacitance in Tubes — The Root Cause of RF Amplifier Oscillation

Measuring Interelectrode Capacitance in Tubes — The Root Cause of RF Amplifier Oscillation

IEC 60100 specifies interelectrode capacitance measurement for electronic tubes. In RF amplifiers, the anode-grid Miller capacitance (C_ag multiplied by voltage gain) can cause self-oscillation even when the DC simulation looks perfect.

Measurement challenge: Interelectrode capacitances are typically 1–20 pF; stray capacitance from test leads alone can reach 5–10 pF. IEC 60100 specifies the substitution method — measure fixture residual capacitance without the tube first, then measure total capacitance with the tube inserted; the difference is the interelectrode capacitance. This demands 0.1 pF precision.

TN Lab — A few picofarads determine whether an RF amplifier oscillates or amplifies.