[1100 man]

Good evening,
Well, despite my best efforts, the EHT obstinately remains at 10KV. I tried a new U26 rectifier, 30P4 LOP valve (and a PL36 & 30P19 as well).
All the other components were new anyway. The picture size & linearity are good and in fact, the picture is not too bad, it's just the EHT should be 16KV.
My conclusion, therefore, is that it's the LOPT.

We have recently been discussing the issue of the wax or pitch coatings reducing the 'Q' of the transformer and therefore the efficiency. Any major failings, such as shorted turns would have a more dramatic effect.
This LOPT has the primary windings on one 'limb' and the EHT winding on the other. The primary is wax impregnated but the outer layer is just stiff paper. The EHT winding has a thick layer of wax all over.

By way of experiment, I measured the boost and EHT volts and the temperature over 1 hour.
At the start, boost was 525V and EHT was 10.5KV
After 1 hr boost was 485V and EHT was 9KV
Temperature of the inner part of the primary near the core was 50C and the EHT winding was 40C. Although I would not be unduly worried about these temperatures, the wax was starting to bubble on the primary and was soft on the EHT winding. Also a good discharge was obtained to an insulated screwdriver held onto the wax so it's not doing a great job of insulating!
In the past, I have mounted small cooling fans near the LOPT to keep them running cool which works well so long as the EHT starts off OK.

So there are two courses of action open to me. Either fan cool it and live with low EHT, or experiment with removing the wax & re- coating it in the hope that things will improve.
Obviously, the second option is much more interesting, even though it might kill it completely!
So that's what I'm going to do, inspired by Argus's de- waxing methods and general insight.
Earlier today, I dropped a small piece of the wax coating into some White spirit to see how good a solvent it was. It's very slow, but it does soften it and with some agitation will dissolve it. I might consider placing the wax covered winding in a small oven at 60- 70C and letting the thick wax drip off before using the white spirit.
White spirit is pretty tame and hopefully won't affect the other component parts of the winding.
It then leaves the question of what to re- coat the windings in?

The LOPT from the GEC 2000, which is of later construction, uses a sort of plastic encapsulation over the EHT winding rather than wax. Despite the cracks in it, it gives full EHT which doesn't change over time.
So we will have to wait and see what this experiment will prove: it may all end in tears!!
All the best
Nick