Have I damaged my amp?

I have a Music Angel 300B amp which has been fine for sometime now. A friend visited to hear the new single driver speaker cabinets I've made, to compare these with my previous Kef's (the amp had been on for hours) I unplugged the Rt channel only and we had a listen for 30secs or so then I swapped back, the amp was still on. A few days later it started making loud rustling sounds 'under' the music (not volume dependent) on the right channel with some squeal when I tap the chassis, there is also quite a lot of noise when I power down from the Rt channel. I have tried swapping the valves with no change and have checked inside for loose connections 'bulging' capacitors etc and can see nothing wrong. So is this a probable cause for damage and what could it be? the problem is becoming noisier but only starts after the amp has warmed up an hour or more. My first thought is to replace the main capacitors under the transformer. I don't have a schematic for it but does anyone have any views on this.

bad power tube unfortunately...
I did try gently tapping the 300B's with a pencil and they were silent. The 6SL7's were microphonic, could it be that both of them are dodgy, they're the original Chinese ones and didn't look very new out of the box.

If it were a ss amp, I'd say, sure, you fried one channel. But not tubes. Hard to say it was a coincidence...but that's my thinking. Good luck. Without schematic you are entering deep waters indeed.
I disagree with Tarsando. Tube amps are never to be driven with no load on the outputs.
Tube amps are never to be driven with no load on the outputs.

This has been my understanding too. It's always a good policy to shut it down, give it about 30 seconds to bleed off any residual charge, and then do your changes in wiring. Never run a signal through a tube amp without a connection to a load. Opposite of what Tarsando said, I've do not associate a similar limitation with SS amps (running a signal through an SS amp with no load connected will not likely result in any risk of damage - but I would NOT do any hot-swapping regardless). Since this has been my understanding, I hope anyone with contrary information might chime in. So, with SS I'd still power down before doing any wiring changes. I would not hot-swap with a live component, SS or tube. I've seen others do it with no consequence, but I think it's a bad habit to get into.

When you say you "unplugged the Rt channel only", do you mean you unplugged the interconnect from the amp/preamp, or do you mean you pulled the speaker cable connection?

It wouldn't hurt to replace the Chinese 6SL7's with some better NOS tubes from a reliable source. That would likely improve your performance, though it may not solve the problem. Not surprising that a pencil tap yields noise though...I should think that's pretty normal for an input/driver tube. That might be a good, and inexpensive start to troubleshooting the problem.

If you swapped valves, as you indicated in your post, with no change (noise is still in the right side/amp) it would seem to eliminate a bad valve. Your description of a "rustling sound" does, however, sound like a bad tube, so I still think either testing the tubes, or swapping them with tested tubes that you know are good would be a worthwhile approach.

I don't think it's a good idea to run any amp without a load, but a tube amp with a transformer is more likely to endure it than an ss amp. That's always been my understanding of it. I know for sure that SHORTING the speaker leads blows an ss amp almost every time (unfortunately for me) but doing so with a tube amp doesn't seem to harm it. I could be wrong about the whole thing I guess....
Thank's for the replies,

I turned the volume down and then swapped the speaker, I wasn't thinking. I'm currently using another amp because it's not enjoyable waiting for the noise to start, as I said I have tried swapping the valves one by one (with the amp off and cooled down ;) and the noise stays in the right channel which is why I'm puzzled (I wouldn't have posted if it had followed a valve). At switch off it's loud more tearing than rustling.

Hmmm, if you turned the volume all the way down and pulled the speaker connection, that in and of itself would not likely damage the amp since there's no signal to the amp. Still, not a good habit to foster. Had you shorted the connections on the disconnected wire you would likely have blown a fuse (assuming they are fuse-protected). Could you have crossed the terminations and shorted the amps? That's very easy to do with a speaker wire flopping around. Regardless, short of the tubes being bad, it seems like you should take the amp to someone who has the expertise to test out individual components within the circuit. I don't think damage will always necessarily be visually evident.
Shorting is a different can of worms. The output transformer makes a tube amp see a contant load even when shorted most of the time. This is why a speaker selector uses a transformer for multible speaker connections. But no load at all is a problem with tube amps. SS amps usually have protection for a no load situation.

This was borrowed from the Enjoy Music site.
"Note: Did you know that you should never turn on an amplifier in a system that is not connected to some type of load at the speaker terminals? Some amplifiers have built in protection for just this situation and some might not. The problem is no load means the amplifier might see an infinite load and may try to drive it. Of course an infinite load requires an infinite amount of power, which translates into heat. At a bare minimum most tube amplifiers have a resistor, which will melt and an open the circuit"
I would say it's very likely you damaged the output transformer. Never ever ever run a tube amp without a speaker or load resistor connected. The speaker impedance is reflected to the tube output via the transformer. No speaker means no load which means a short across the transformer.
Did you try switching the tubes from the right to left and vice versa? Replacing a tube is a lot easier/cheaper.
Make sure you turn off the amp first and don't burn yourself either!
Yes your amp sleeps with the fishes. No load is seen by the amp as no impedance except the cable. We all no that you created an impedance mismatch, tubes love impedance the higher the strain on the tube to transmit it's energy the load has to be close to the tubes output or it create a form of fusion and melt the tubes glass which why a tube amp can't run without being coupled to a transformer. Atmasphere and those other companies that don't have autoformers are just a myth. In truth you could have a very bad explosion because the mismatch also cause the reversal of the physical vacuum the tube must have for the electron to fly from one plate to the other. Bummer
Well, thanks for the responses, I guess I should take it to a repairer. I'm Ok with a soldering iron but I would need to know where to start.

You could start by checking for any blown resistors.