You know, this discussion of filament voltage and tube life just prompted me to why I originally joined the AMPS group and then consequently leading up to this group. The pieces finally fell into place. About 20 years ago I purchased a well-used, (but still very operational) RCA Radiomarine SB-1K amplifier. It uses a pair of 3-400Z¡¯s at 2800VDC for a solid 1KW out. When I connected it to my electrical system, I was only getting ~~500W? out. I had 5.2 VAC at the transformer but 3.8 VAC (IIRC) at the tube socket! The short answer was the previous owner used a piece of lamp cord (gauge escapes me now but I wouldn¡¯t run a filament with it!), to the filament. Replaced it with some stouter wire and I had 5.0 on the filaments again and a solid 1K out. I scratched my bohunkous many times wondering why somebody would do that. This recent discussion gave me the answer I believe.
