Amp sound occasional momentary cut out

Lights stay on so it is not the power cord or fuse. It would only be a power supply tube or tubes none of the others. I assume it does it with all your sources? Say turntable and CD player? A protection circuit could do that via a relay but you would probably hear a click when it happens. I have never seen a protection circuit in a tube unit. They are usually pretty hard to hurt. 

bremner, assuming it occurs with all sources it would have to be a power cord or power supply issue as you are losing both channels. If all the lights go out it is the power cord or it's connections or a power fuse if the lights stay on it may be an internal fuse or a tube if it has a VT rectifier in it but tubes usually do not act this way it is acting more like an intermittent connection. Start wiggling! 

Mike