The filament supply has inrush current limiting to insure maximum tube life. Ameritron AL-Variants (all contain two - 500Z graphite tubes). The center “ pin ” in the base of this tube is hollow.
In a valve the cat. All - 500Z amplifiers use a grounded grid and drive the cathodes.
I tested the filament pins (and 5) on the suspected culprit - 500Z and I AM reading continuity. When the product is of quality (i.e. - 500Z triode sold by RFParts) the amateur can. V at 11A power supply. Remove the - 500Z tube from its shipping carton.
One of the pins is separated by a large space from the other four pins. SPECIAL TUBEBASE in general Top contact with a cap.