Going back to your post #1 question; yes, if Vcc is changed then the gate drive voltage will change by approximately the same amount, for either IC. In the case of the IR2110, though, Vcc must stay above 10V to prevent the under-voltage lockout acting.
Looking at the schematics and scope pictures, I think the 12V has serious ripple on it. Caused by a lack of capacitors on the 12V at the IC. There needs to be a 0.01 to 0.1uF cap at the IC and about 100uF high current cap some where near the IC.
This looks like a RF project, where two feet of wire going to a supply will act like an inductor. Some wire and a 0.1uF cap can resonate at the transmitting frequency and cause strange problems.
Hi Ron, the schematics are not 100%, ages ago I added 0.1uF ceramics directly at Vcc pins, and a 1uF tantalum and a 100uF electrolytic on the 12V supply right near the chips. But would this ripple, if indeed that's what it is, come and go as Vcc was raised and lowered? Surely it would be present all the time? Many thanks for the replies.
Hi Ron, the schematics are not 100%, ages ago I added 0.1uF ceramics directly at Vcc pins, and a 1uF tantalum and a 100uF electrolytic on the 12V supply right near the chips. But would this ripple, if indeed that's what it is, come and go as Vcc was raised and lowered? Surely it would be present all the time? Many thanks for the replies.