Filament switching off in Tubelab SE

[Nikon1975]

I have a strange problem with the filament regulator. From time to time it switches off.

At the beginning I thought it was a thermal problem, but now I have a big heatsink, that does not become hot at all, and also the chip is cold.
After the filament goes off, if I switch off and on the amp, then it runs for hours.

I have installed shotky diodes for all the four diodes, and the voltage on the transformer is 6.4 V. I tried to download the data-sheet of the regulator, but there is a broken link on the Sharp website.

Did you see a similar behavior ?
I understood that the regulator does not need to be electrically insulated from the sink, the diodes are. Am I right ?

Thanks,

Davide

[rknize]

One of the two chips needs to be isolated, yes. Are you powering 300Bs? I wouldn't expect the heatsink to be cold, regardless of how big it is. Is does sound like it is shutting down still. Are you sure there is good thermal conduction between the IC and the heatsink?

The only other shutdown scenario I can think of is a short. Could the tube filament be shorting in the tube or perhaps a short around the socket?