Carvin X-60E problem - Loud distortion howling noise

[janupa]

I have a Carvin X-60E tube amp that makes an awful distorted loud howling noise. It plays fine for maybe 30 seconds then the noise starts abruptly and is exceptionally louder than the volume setting that's set for the input.

In my naiveity I replaced the tubes thinking they were causing the problem but that didn't change a thing.

Any advice or suggestions?

Thanks in advance.

[Jazz P Bass]

Do any contros affect the howl.
This may help in diagnosing where the noise is originating.
Is there an FX loop?