Measure with x10 probes on each half bridge. Just under dt 240 nS rise and fall times on pulse wave with a dv of 142VDC! Output seems almost too good to be true, but this is a really low frequency and nothing is being stressed as there is no load. However with the fans off on the heat-sinks after an hour running like this no noticeable heating, heat-sinks remain cold to touch. This tells me that losses in the driver circuit is very low, so I can move onto higher power tests.
Ran the full bridge at 24.6kHz to test the low frequency response, did probe the gate waveforms they look ok but did not upload a picture here. Will include gate waveforms after driver is working at higher frequencies ~290kHz. The bridge functions flawlessly at this frequency showing that the gate drive transformer has the correct number of windings and the right core material eg no droop little overshoot.
Only pulling 50mA at 12V as seen above from the power supply of the Gate Driver IC to the GDT. However no load and low frequency this low power is expected under these conditions. Low losses, this is all a good sign that things are working as expected. Variac was set to just under 50VAC.
Top-down picture of setup with a probe on the DC-blocking capacitor and one probe on the raw output leg of half the bridge. Successful test of H-Bridge at low frequency (24.6kHz) conclusion is everything is good to go for higher power tests.