In amps with a effects loop or with power amp in, always check the IN socket. Squirt in some switch cleaner and put a jack in and out a few times (can do with power off. I Have a had a few amps over the years where the signal from the pre to power amp is broken by a dodgy connection. I have had a few amps over the years with this problem
I'm assuming that no sound applies to both the clean and ultra modes. You didn't say which tubes you replaced but I'm guessing the input tube was not replaced. Did you keep the old tubes?
When "no sound comes out", do you hear any kind of a low level hum on the speaker output?
I think the front end tube is open or the switch logic switch is defective.
Hope this helps.
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Sounds like a bad power supply filter cap...the buzz you're hearing is the 60 cycles hum that filter caps reduce or (hopefully) eliminate altogether. It may also be related to the tubes but finding the bad one will require some work. Any friends have this same amp? If so, try swapping one tube at a time and see if you get lucky. If this amp is new take it back ASAP as it is a manufacturing defect.
The amp is doing what is called "motorboating". This is usually caused when the voltages to sensitive preamp stages are inadequately filtered or regulated and the sudden load of the output stages cause a dip in the voltages sent to the preamps which in turn vary the input to the output stages starting the cycle over. This will require electronic troubleshooting and if your not skilled in that, then it is NOT a DIY fix. What you are seeing is NOT normal, there is a defect.
Take it for repair immediately as there is something that is thermally sensitive and it may cause worse damage with continued use. The problem is LIKELY a broken solder joint. Depending on where it is, it could cause damage to the power amp portion. It OFTEN is at one of the leads to a volume control and those USUALLY don't cause damage, but one can't be sure till it is repaired. These broken joints oxidize and then eventually warm and punch through and the device starts working. You MIGHT want to try wiggling the controls to see if one brings the unit on immediately showing itself as the culprit.
Buy ONE new 12AX7 and try it in location V1 and if it doesn't help, try it in V2. This tube has TWO heaters in it and if one is intermittent that could cause your problem. The vibration MIGHT be "fixing" a briken heater after some time.
There are MANY components that could cause this, so you might have to take it in for repair if it isn't a simple tube problem. Since it works on the clean channel, V3 should be OK.
Try checking to make sure that all the tubes are properly seated in the tube sockets and that one wasn't broken at the base. Considering that you just bought it you should be able to take it back and have it serviced for free.