[Mondial]

As Greg said, one of the output tubes is gassy or has grid emission. It goes into a thermal runaway condition where as it gets hotter, the plate current increases which makes it hotter.....

The Russian tubes are also probably run over their ratings, as the 7189 has a higher plate voltage rating than the EL84

The problem is compounded by the fact that there is a common cathode bias supply for all four tubes. If one tube draws excessive current, the negative bias only increases by only 1/4 the amount as if there were individual bias resistors.