[Arran]

There is nothing wrong with metal tube, at least in the front end of a set, though I don't really care for rectifiers or power output tubes in metal unless they were stock. But it seems like Philco had a temper tantrum against anything but ST style tubes in 1938, they went out of their way to make sure you could not use anything else without butchering the shield base, even GT style tubes won't fit. I don't think that metal tubes were any cheaper then glass when they were new, but during WW2 and Korea there were some tubes in some styles that you could not get at certain times.
Regards
Arran