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Framus Cobra NOISE/CRACKLING, Advice needed

MMHC

Member
Messages
2
Hi everybody, I'm posting here because of a problem happening with my amp, a Framus Cobra 100w Head. Sincerily this problem begun not too much time after I bought it, let's say more or less one year (amp is 5 years old). I first ignored it because I always blamed the genre I was playing (fast deathcore) and the amount of gain used. Lately (during the last year) I found myself needing a less extreme sound so I was quite disappointed noticing that even with small gain amounts the problem was still there.
I searched plenty of forums/threads with similar problems, most say it's a matter of tubes, but after some diy troubleshooting I can say that I'm 99% sure I ain't got a power amp problem, but a Preamp section problem (even tho tubes aren't the cause IMO). Let me explain the problem:

A constant noise, similar (but smoother) to white noise, coming up eventually with pops, cracks and fried-bacon sounds in all of the 3 channels, Crunch and Lead channels being affected much more by it, but still present on the clean channel.
http://soundcloud.com/mmse/cobra-crackling-noise

Every channel has a gain knob and a volume ( output ) knob before entering the power amp section. Gain affects the quantity of noise, even if at 0 gain I can still notice the problem. Output knob totally kills the noise if set at 0, plus if i connect a jack to the "return fx" with the mix level set at "100%" the noise is gone too. That's why I'm saying the issue must be in the preamp. (according to schematics v4 is effects path and v5 is phase inverter, so if I'm not mistaking the thing i did with the "return" jack ensures that v5 and everything ahead is fine).
Read everywhere that 90% this is a preamp tube problem, changed them all with no luck. Tried cleaning and retensioning sockets, no luck.
This thing appears to happen very randomly in intensity, sometimes it's less significant and sometimes it seems like the amp is going to start sparkling and blowing itself up.

More weird things also happened..
Before changing tubes it happened 2 or three times to find myself stuck with no volume at all while playing, with just a 50Hz hum coming from the speakers. "restarting" the amp solved the problem. Both times the failure was anticipated by more or less 5 mins of an ugly-overcompressed-distorted sound.
Today it even did another crazy thing. I turned the amp on and the 2 overdriven channels didn't work. Swapped v2(new) with an old one solved. Tried that v2(new) in other positions and the tube just failed everywhere.. tried that v2(new) again after 8 hours and the tube is just fine (noise/cracks/pops still there).

So guys... anyone here have a clue of what's going on here??It's really making me crazy, I'm not a tech so I can't go further than swapping preamp tubes...
I need to go on tour in 4 weeks and it would be also nice to know if in your opinion it could be something easy and quick or something that could require more time...if so I need to start looking around for another amp..

Thanks in advance for your help!!:banana
 

ledzep618

Member
Messages
4,751
Crackling and popping is almost always a preamp tube, but since you swapped those out I don't know what to tell you. Forgive me for sounding obvious, but why don't you just take it to a tech? You said yourself that short of swapping tubes you can't do anything, so isn't that the only real solution? Especially if you're taking it on tour soon, don't want some real failure on the road.
 

8len8

Silver Supporting Member
Messages
13,712
I had a similar problem that was a bad solder joint by a tube socket.
 




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