From IR application notes it looks as though the internal bootstrap diode is
actually a gated powerFET in recent manifestations of that IR215. series.
The accepted external diode for a IR2153 seems to be MUR160 so placed a
BYV96E in there and it is comfortable. Removed the droppers
Raised supply to 66 percent mains and voltages coming up to +/- 36V,
presumably between 50 and 55 V main rails with 100 percent (this amp is 250W
rated, that schematic is 500W)
Same oscillator frequency
Time has come to replace the IRF740s with pair of 20 amp /600V powerFETs