I'm at band practice right now and I was playing through my blackstar ht 40 tube amp and all the sudden an obnoxiously loud hum and whine started squeeling at full volume through my amp. We tried several cables and everything. I found part of the problem. When I unplug the cable from the input, the hum stop, when I plug in a chord (I tried several) it starts humming super loud. I turned the amp off, unscrewed the back and looked at the tubes. One of the power tubes is glowing orange, the other one isn't glowing at all, it doesn't even look like it's working. Do you think I just blew a tube, or a tube died? I bought it scratch n' dent from musicians friend about a year ago and use it pretty consistently. If a fuze blew, wouldn't the amp just not turn on all together? Please help!
I'm not super familiar with these amps but if you unplug the guitar and the hum stops then I'd be leaning more toward the guitar/cable/insert jack on amp. Tubes glow orange but you would have to be more specific. Some preamp tubes glow very faintly. If it was a fuse, there is a good chance the amp would not turn on at all. It is very difficult to diagnose stuff like this over the internet.

Try a different guitar
Take amp somewhere else and try
Determine which tubes are power tubes
Determine which tubes are preamp tubes
Note which ones are lit while playing
Take it to a tech
if one of the power tubes is glowing and the other isn't that is a problem.

is the one that is glowing, glowing normally? or is it brighter than usual?
The amp is a closed back so I'm not sure if it's glowing normally compared to what it usually is, but it doesnt look excessively bright compared to other tubes I've seen. The cable's not the problem. I tried 4 different cables, with and without guitar, as well as a couple different outlets. When I turned the amp off for a little while, then back on, the noise doesn't start right off the bat. As soon as the tubes warm up it makes the hum. You know the sound when you strum your guitar before your tubes warm and you cam hear it fade into full sound as soon as the tubes are ready? The hum fades in like that too when the tubes warm up. Another reason why I think it's the power tubes. I have just 2 power tubes and 2 pre amp tubes. The preamp tubes are concealed, the power one's aren't. One of the power tubes does not glow, the other does. Would it be wrong to assume that the one not glowing just needs to be replaced? How hard are they to change?
If only one tube is glowing, and the other isen't, that is a sign that the tube that is not glowing is prolly blown out. Typically, if your amp uses both powertubes at the same time, which, some don't, then if they both went at the same time then it'll blow a fuse, if only one went then it will still work, just not very well.

I think it's some kind of Mesa with 4 power tubes, 2 of them are associated with the clean channel, and 2 are for the gain channel. Who knows, you'd have to look it up, I don't remember where I even saw that.

Point is, power tubes are blown, time for a retube more than likely.
Quote by sanjisan43
One of the power tubes does not glow, the other does.

Not good. You can buy a pair of 'matched' power tubes and replace but I can't speak to whether or not it needs to be biased or not (I am assuming it does.) I'd take it to a tech to be sure. Otherwise, I have a biasing blog that may help you out a bit -just note I do not own a Blackstar.
Thanks guys, I have a feeling it's a power tube issue too so if we all agree on this, that's probably the route I'll go. I'll ask on the Blackstar forum too and I'll plan on calling a tech tomorrow.