Before I Drive Myself Nuts!!

[pwdrive]

Hi Rob, the heaters should work with a scale lamp o/c, that's the purpose of R37 (30R)

With the set unplugged from the mains switch set on an do an R check from the neutral pin on the plug to the chassis, should be a max. of 30R, if that is ok then do an R check from the live pin on the mains plug to the voltage tapper on the dropper, that should read 0R (s/c).
If both of those are ok then work your way down through the dropper and the valve chain, the readings should show up the problem.

Lawrence.