What does this protect? (heater circuit)

[g1]

What part is SIDAC TR1 protecting? And under what circumstances?
Wild guesses on my part: a) if ground reference resistors are fried and there is htr-cathode short?
b) PT failure and shorts between windings?
c) power tube cathode fuse opens combined with heater to cathode short?

Thanks for any suggestions.

[Jazz P Bass]

Crate BV120H, correct.
My guesstimate:
Seeing that the sidac conducts at 120V, they must be trying to ameliorate damage to the heater winding, should an output tube plate puke into the heater circuit.
Why a 120v rating, I do not know.

[pdf64]

Wow, what a good idea.
120V may have been chosen due to tubes heater cathode insulation rating being around 100-200V.
Pete.