Amplifier problem

Please help:

I have a problem with my amplifier - one resistor (R47) got loose, the technician soldered it, all tubes are working, I can hear the music, but in a 3-5 minutes there is a buzzing (or, rather, scratching) sound from the right speaker, after which I switch the amp off.

Before the damage, the amp cooled down rather quickly (after 5-10 minutes all KT88 tubes were cool), and now only one output tube gets coolo quickly, while others remain hot for approx. 20 - 30 minutes (like in Cary SLI-80). All fuses are OK. The amp is Unison Research Sinfonia.

I tried other quads, but in approx. 3-5 minutes of operation one of them became hot red in the plate. The technicial could not identify the problem. I reported the situation to the manufacturer, but so far did not receive a reply.

I will appreciate all recommendations.