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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Crackling Amp

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K

Knoffers

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Joined 2010
#1
I was wondering if any of you knowledgable folk may be able to point me in the right direction in solving a problem with my valve amp.

The amp is a SE Kt88, around 6 months old. I use it a fair bit and when I switch it on tend to leave it on for most of the day whether I am listening to anything or not. Having been listening to some music for an hour or so I stopped the CD leaving the amp powered on, about 30 mins later there was a sudden loud crackling / static noise coming from the speakers. There was no arcing or flames from any valves and I shut the amp down in a bit of a hurry and left it alone for a few days! In an attempt to do some trouble shooting I disconnected the input and switched it back on with the volume turned right down. It warmed up ok it seemed but there was an intermittent popping type sound every 30 seconds or so. I reconnected the input and the music played fine except for this continuing intermittent pop from the speaker(s) (I couldn't determine whether it was both). I've switched the amp off now as I don't want to do any more damage. The speakers are connected correctly and the valves seem seated ok. The amp has performed flawlessly until now and I can't understand what would have caused this.

Any guidance would be much appreciated

Thanks
 
#2
Hmmm.....

It could be a number of things....

Is the crackle/pop through both speakers, and is it present with the vol turned right down?

Does the crackle get worse or is it constant over time its on?

You can try carefully tapping each valve in turn to see if there's one more sensitive/causing the noise....

Is this amp valve rectified or SS?
 
#3
I would suggest that you buy a set of replacement output tubes and try swapping them out one at at time. If this turns out not to fix the problem then it's nice to have a set of spares anyway.
This problem can be caused by a failing componet such as a resistor or capacitor in almost any part of the signal chain. You could also get a look see at the insides to check for blackened resistors, loose connections. After that, a voltmeter and a scope would be valuable tools. Did you build this amp from a kit, or buy it assembled? Do you have a schematic you can post? (this often helps generate suggestions). Finally what is the brand and model. Perhaps somone else has had a similar experience.
 
#5
Crackling can be loose or dirty tube sockets or pins. Rocking the tube in the socket will show which one and sometimes fix it temporarily. Tube pins can be cleaned with a pink pearl eraser, then a little alcohol. My lower serial number organ is plagued by poor quality tube sockets, corrected in later production.
Mild crackling can also be arcs on the circuit board or across the tube socket. A little leftover solder flux can cause an arc. I scrubbed & scrubbed with a paper towel after replacing the carbon comp resistors and paper caps in my tube amp, still had some crackles. They show up with the lights off as faint blue spots on the circuit board or at the tube socket. Newbie warning(if youare new), high voltage can kill you, don't touch anything under the chassis before the cord is unplugged and the caps are discharged to less than 25 VDC.
 
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#6
Thanks for the quick replies. The amp in question is in the link below
Separo se88i valve amplifier, KT88, integrated, single ended, valve amplifier

I'm not sure if the problem worsens with time as I didn't want to leave it switched on to risk further damage. As I said when I first noticed the problem the crackling static noise was constant and quite loud, when I was troubleshooting I only left the amp on for a couple of minutes and just got a couple of 'pops' in that time. I will try and leave it on for longer and try some of your suggestions.

The pop was definately present with the volume turned all the way down and before switching it on again I removed and re-inserted all the valves.

Thanks for taking the time to reply
 
#7
I had the same exact issue a few months ago. It turned out to be a loose ground (-) speaker connector. Tightened all the speaker jacks and all was well. The loose speaker ground was causing arcing in one of the rectifier tubes. The tubes were OK I had tested them all. Unless you have them running hot, KT88 tubes will last a long while.

Cheers,

Bob
 
#8
I have tried swapping the valves around and checking all connections but no difference. There is also quite a loud hum from the amp now from both speakers even with the volume turned right down and no line input. I have taken the back off but can't see anything obviously untoward like burned out components. I have requested the circuit diagram to be sent to me and will post further information when I have it

Thanks
 
#9
If it's coming from both speakers, the culprit is probably in the PSU. You could try the old wooden chopstick test by carefully prodding around the board to see if you can induce the behavior. The new loud hum, all things being equal, points to a failing cap or rectifier.
 
#11
pedroskova said:
If it's coming from both speakers, the culprit is probably in the PSU. You could try the old wooden chopstick test by carefully prodding around the board to see if you can induce the behavior. The new loud hum, all things being equal, points to a failing cap or rectifier.
Click to expand...

It could also be a ground issue - I'd look fast and hard at your ground wire going to your inputs or speaker posts and where it's connected to the chassis. Be careful about that with the amp powered on - chop stick approach there.

Cheers,
Bob
 
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