Hi Steve:
In addition to the things that Jim (JKO) said and to answer your
question "is 8.5 volts OK?", (assuming your meter is correct) no 8.5 volts
is not OK. Not only will the emission be low and power out low but the
tubes will suffer from a special illness that occurs from insufficient
heater voltage. I do not know the details of this illness but Don, K4KYV
can tell you more about it.
Below is a paragraph segment from
http://www.transfixr.com/strafe_tube.htm
section A: cathodes
paragraph 3. Lifetime of cathodes:
"Operate it too cool and life may be shortened (especially in thoriated
filaments, which depend on replenishment of thorium by diffusion from within
the filament wire). A few researchers have observed that the lifetime of an
oxide-cathode tube can be greatly increased by operating its heater at 20%
below the rated voltage."
John, WA5BXO
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