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The 1A4-P is a pentode version of the 1A4, which is a tetrode (sometimes designated 1A4-T). This means that the P version has an additional grid, (suppressor grid), close to the plate to prevent secondary electron emission from the plate from landing on the screen grid. This improves the stability of the tube.
Since the secondary elections are repelled by the suppressor grid, and returned to the plate, plate current could be higher in the P version of the tube, which may account for differences in tube checker readings.
The additional grid in the P version is internally connected to the heater/cathode, so the tube base connections are the same.
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