OK, so maybe I don't know enough about this but with my little knowledge, I never had problems testing any of my other tubes. If I use the settings you gave me am I only suppose to use the diode check? If so, none of mine, with those settings get into the good diode area. If I try to use the normal tube test (P4) I have to set the scale to the 15000 level and then the tubes really come up then the needle drops way back as I hold down the P4 button.
The 6GM8 is a tube designed to operate with low plate voltage around 25 to 30 volts max. This is why testing is done with the P1 button as that applies low voltage to the tube under test. This is also needed to protect sensitive diode tubes. If you test with P4, this applies higher voltage then the tube was meant for and may overstress it. However, a tube that falls off under test is indicative of a weak one. I suspect that a strong 6GM8 would exceed even the 15K position of your tester.
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