Thank Ian for the info on HA filters. I think that explains a lot. I just tried some temp measurements in my working head which has the 300w bulbs. I got one reading with the IR thermometer at over 400F - that was likely aimed directly at the lamp. With a Type K probe near or touching the metal frame the temp rose to over 350F within a minute (focus). It would seem that we are operating at or over the spec for glass. Your comment about the glass splitting and staying in place I think is very valid. The frame only holds the glass on two sides so depending on the break the glass might sit there for sometime, maybe only falling out when moved or turned on the side. ( Both Jeff and I noticed it was broken when it rattling around after shipping or moving. )
Since most or all HA glass is tempered it creates another problem. All I read indicates that the glass must be cut to size before it is tempered, so unless someone has a glass tempering oven at home, do-it-yourself is ruled out.
I think this gives us few options:
1) Find a source of the 15x30x2 glass from old stock
2) Find someone to make the required glass (with a min order less than 1000sq meters!)
3) Rig another piece of glass to the metal frame
4) find and buy another head.
5) operate without the HA glass and hope the Dichro filter holds up.
I got lucky and did #4 although I'd like to fix up the broken head I have.
Number 1 is looking very remote right now, after all it's been long time since these were made (10 years ?).
3,5 might be doable but results will vary - as they say.
the search continues,