Hang on I just reread the 1st post and as this is your first build,, I'll assume you don't have a lot of teck skill.
Warning!!! RE this;
"The power supply for the amp I'm trying to build features an inexpensive 7-volt Antec Toroid transformer with the 120-volt coils split, making it a small isolation transformer. The 7-volt side is used for the tube filament."
Unless I've missed something in translation,,,that tells me it is a 120/120 split *Primary* so something sounds wrong

Also the way in which the power Tx is drawn up in that schematic seems strange.
Others here may know more but meantime be careful cause if I'm right as it could kill you.

Normally when a Tx states 120/120 split primary that means it can be wired up to either 120 mains ans 240 mains. It is most likely NOT an isolation Tx

Maybe post a picture of the Tx you wish to use ideally showing the label.
Phil.