Firstly I would think you'd need to switch the amp to standby or to 0 volume before switching between Triode & UL modes, though you would need to consult the owner's manual. Switching modes with the volume level up could cause a loud poping sound.

Other possible causes could be cables not securely connected, speaker cables connected in the wrong polarity, or a ground loop issue. Re: Ground loops, this thread on WBF will give you a good overview - https://www.whatsbestforum.com/threads/ground-loops-101.19891/

I'd troubleshoot those above steps, and if all else fails, the amp might need to be brought back to the store or an authorized service centre for service.

You should have it OFF or in standby to make a change.

Bodhi wrote:

Firstly I would think you'd need to switch the amp to standby or to 0 volume before switching between Triode & UL modes, though you would need to consult the owner's manual. Switching modes with the volume level up could cause a loud poping sound.

Other possible causes could be cables not securely connected, speaker cables connected in the wrong polarity, or a ground loop issue. Re: Ground loops, this thread on WBF will give you a good overview - https://www.whatsbestforum.com/threads/ground-loops-101.19891/

I'd troubleshoot those above steps, and if all else fails, the amp might need to be brought back to the store or an authorized service centre for service.

commsysman wrote:

You should have it OFF or in standby to make a change.

For changing tubes or removing/connecting cables, yes, but I wasn't 100% sure re: switching between modes of tube operation. Thanks for chiming in.