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"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.
Franz
abalem
nick_sr
263 posts
 
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!
bojack
652 posts
 
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).
david12
1,454 posts
 
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
abalem OP
10 posts
 
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...
abalem OP
10 posts
 
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 !
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