[Amps] Checking Filament Voltage

[Bill Turner]

Apparently your filament transformer is separate from other
transformers (plate). 

I'd suggest running the AC line outside the amp and using a Variac. 
The drawback to taps is your line voltage can vary with the season or
other factors and you'd be stuck. 

As others have suggested, bring the filament connection out to a pair
of easily accessible banana jacks and you're all set for whatever
happens. Be sure to use an accurate meter such as a Fluke. This is not
an area to save a few bucks on a cheapie. 

Used Variacs are widely available. Go first class!

73, Bill W6WRT