A precise SPICE model for miniature (MT) triode tubes of high-µ 12AX7 and medium-µ 12AU7 is proposed, based on the physical analysis of the measurement results. Comparing the characteristics between these tubes, the grid current at lower plate voltage and positive grid bias condition is modeled successfully with novel perveance parameters for the first time, though it was known that the perveance depends on both grid and plate bias. It is shown that the modulation factor of the space charge for the MT triodes is different from the other classic tubes. The model is implemented in LTspice to result in a good replication for a variation of three-order magnitude of grid current and cathode current.
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