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How does a lower current for the heater at the same voltage affect GM?

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I have many 1193/CV6/6S5S tubes. They all have a 300mA heater draw and measure from 27 to 3300 for mutual conductance. The plate current varies on the tube but that is not the question.

I have over 100 E 1148 Hytron tubes that were made in 1944 under contact to the US government for the military. They have a heater draw of 200mA. The spec sheet shows they would measure the same as a 7193 but they do not.

They sound excellent and work fine but I am trying to figure out why they by design or because of a lower heater current, measure typically 2000 to 2300 in Mutual conductance/GM and a lower plate current. Gain seems to be the same but for some reason it seems that Hytron varied from the specs in a lower measuring tube. I would put the minimum at 1300 to 1400 GM vs the 1900 (on a Hickok) as the minimum.

These are all NOS in original military boxes and never used. All they 7193 types use 6.3 volts for the heater.

Any ideas?

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