You could pull the chassis and measure the voltage between pins 9 and 4&5.
I'm attaching a picture from the GVT15 schematic that shows the DC filament circuit. I couldn't find a schematic for the GVT52. So things will be different on the GVT52 but they should be similar enough that you might be able to figure it out.
it seems unlikely that the rectifier would go out, but its not out of the realm of possibility.
switching the power mode may have sent a transient spike through the filaments and smoked the preamp tube filaments? hard to say.
I'm attaching a picture from the GVT15 schematic that shows the DC filament circuit. I couldn't find a schematic for the GVT52. So things will be different on the GVT52 but they should be similar enough that you might be able to figure it out.
it seems unlikely that the rectifier would go out, but its not out of the realm of possibility.
switching the power mode may have sent a transient spike through the filaments and smoked the preamp tube filaments? hard to say.