...

...

...

I had considered using a fan, but the HV cage is completely sealed. I'd have to modify it to get any air circulation.
But don't you think the cage is absorbing heat from the rest of the chassis, in addition to what's inside it? A fan is a simple, non-invasive solution that just might do the trick. Extra air could help the rest of a hot-running chassis, as well.

I don't know how this cage is constructed, but perhaps you could loosen the screws and maybe put in a thin shim to hold the cage open a crack to let in a little more air.

If fan noise bugs you, one trick is to connect two of the same type fan in series. Two slow fans move air more quietly than one fast one. A series resistor could also slow down a single fan, although I imagine it would put out a little heat on its own.

Phil "just improvising here" Nelson

...

...

I added a 22 ohm resistor in series with the power supply filter choke which brought the 390v supply down by about 10v to around 385-390v.

Before doing this, why not lowering the screen grid voltage of the horizontal output tube? If you could scan the schematics for the line output unit, it would help us here.

185 mA seems to be OK.

Have you checked the wave form of the signal at the grid of the line output tube?

Eckhard