"Crack" sound in left speaker tube amp

Hi everyone,
I'll try to be as precise as I can be, but I'm not a native english speaker.

First of all, here are the specs :
Power tube Amp - PrimaLuna Prologue Five
Tube Preamp - Shanling MC30
Scandyna "The Drop" Speaker + "The Ball" Subwoofer

Since a few weeks, I can hear a "craking" sound on my left speaker everytime I switch on the Power Amp. I know the problem comes from there because if I connect the speakers to the Shanling MC30 only, the problem doesn't appear.

What does it sound like ? Well, it's like if you unplug a jack from a switched on amp, you know ? This harsh sound that feels like something is tearing apart... When it happens, the speaker seems to shrink in, then out, following this noise, and the sound gets a little lower, mushy.

The noise often occurs some 5-10 minutes after the Power Amp is on, as if it came only after it had heated up. Then it happens every 5 seconds perhaps. And everytime it sounds really scary..

I've tried everything so far : I unplugged everything but it's still the same...

I don't know what to do, I didn't come accross anything like this on any forum. Is it a known issue ?

I hope someone here will have a magical answer ; I always receives great help from you guys !

All the best.
Did you switch the tubes in the left channel with the right channel tubes to eliminate the possibility of the tubes being the problem?

I think a "crack" or "clack" -type sound can also indicate an A/C 'leak' within the amp, though I'm not sure if that would present itself in only one channel.

I had a Jolida 502A years ago that developed that exact problem - that same "clack!" followed by the volume-fading-out-then-fading-back-in phenomenon. I took it back to the dealer, and, of course, they couldn't get it to reproduce the problem there in the store (even leaving it on for days at a time). I really loved the sound of that Jolida otherwise (it was my first foray into the world of tubes), but had to get rid of it when I was unable to get to the bottom of what was causing that noise.
I had a similar issue when I first got into tube amps and it ended up being a bad power tube. I got some great guidance in this forum. "Before you do anything else: Next time you hear the crackle, tap each of the tubes in that channel with the eraser end of a pencil. It the sound gets worse, or disappears- you've found your culprit. Start with the driver/phase splitter tubes, as those usually generate that type of sound when faulty." Came from Rodman99999. It wasn't the driver/phase tubes, but a KT88 power tube, but the advice lead me to a quick discovery of the problem, and of course to the store to buy more tubes! Good luck.
Agree exchange the output tubes and see if it changes channels if so it is definitely the output tubes.
I am quite sure that it is the output tubes. I had similar situation with my Mid-monos, I replaced the tubes and problem was solved.

However, I do not recommend tapping on the tubes. Instead (be sure tubes are cool and amp is switched off) swap the left channel tubes with the rigth channel tubes. Turn the amp back on, if the cracking follows you will know it is the tubes.

The problem will likely get worse if not fixed immediately, the cracking sound will start to occur at random points during listening.

Ah the joys of tube gear!
It may be that you have an arcing tube. Arcing gives off a loud "pop/crack" heard thru the speaker, usually after the first few seconds or minutes of being powered up. Turn the lights off, power up, and watch the power tubes. You will be able to clearly see the arcing in one tube (looks like a mad scientist's Tesla experiment gone bad).
I had a similar problem with my tube amp a few years ago, a really loud crack in 1 speaker a minute after starting up. It turned out to be a damaged cap, which was still working, but the top literally snapping off on startup
Wow ! I hadn't noticed there had been so many answers yet, thank you !!

I managed to isolate the problem : one of the KT88 was faulty, I switched the tubes by pair and could identify the problem right away.

Hey, hey, a bad for a good : I ordered a quad of Shuguang KT88-Z, and now I'm thankfull this factory tube passed away ! :)

Just hope it didn't hurt my speakers though...
Oh, and I have to say that tapping the head of the tube with a pencil (very gently I must add) did the trick as well !
Agree that it very well might be a tube. Swapping tubes will tell you in very quick manner.

However, if you find out that isn't the problem, another possibility is that it's actually the tube socket. I've seen this many times. Today's tube sockets often do not make the best contact with the tube pins, and in the "cold" state, the contact with the tube pin can be compromised. Within the first few minutes of operation, as the parts heat up, they expand, and then everything is fine. If you deduce this is the problem, use something like a toothpick to gently bring the contacts of the tube sockets together a bit, after that, the problem should be solved.

Another possibility, though more remote is a less than ideal solder joint, but explore the tube and tube socket scenarios first. Beyond that we get into issues that the average owner must seek outside help to resolve.