Thanks Alan for the direct e-mails.
I use a driver transformer wound on a chunky ferrite tube with 5 wire
ribbon cable, one primary and 4 secondaries, instead of the H bridge
driver chip. Seems to work OK although Bob ZL2CA's comments are worth
looking into regarding the not-so-good waveform shape but it is running
extremely cool at 500/600 watts.
Jim, I'm going to try the series resonant tank again without the
secondary 'guard' circuit to see what it buys me.
I'll let you know when the first set of STW20NB50 FETs blows!
Christer tells me that Johan SM6LKM is also building a bridge PA - be
interested to know how he gets on.
73, Tom G3OLB.