Marshall tubed amp real red glowing outputs

[evimarn]

Hi guys got this tubed amp a Marshall DSL100 which is causing one side of the output valves to become really red glowing but all components check good valves have been replaced but still same prob. it starts by having its bias increas a lot and then a loud hum and tubes start to glow real red.
Any idea what to cause such a prob.Thanks

[micko]

have you checked the output transformer? or the capacitors? or maybe it's a bad earth on the input - which can cause supersonic feedback (at a high frequency above the audio spectrum).