cooler running power transformer with change in driver tubes ?

I early on had switched out my tube rectifier for a Weber WZ34, but most recently put in RCA 12AU7A’s (the two outside tubes on the driver board) replacing Northern Electric 12AU7’s I had for each of the 3 driver board tubes, and it seems that my power transformer (ST-120 amp) is running cooler (probably fairer to say not running quite as HOT). While I expected to mod the sound a bit, question: but these are the same family of tubes (12AU7A) I didn’t expect impact on power transformer- is there something to this ?

Thanks

You should check your bias after changing the rectifier tube.

sorry to confuse with the rectifier change - yes I did re bias after the change (this was mentioned as a fyi as I have read this also helps the power transformer run a bit cooler as drawing less power)