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.
Cobra crackling noise by MMSE on SoundCloud - Create, record and share your sounds for free
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).
______
Brought it to a tech who does repairs for Tvs, recorders and amps, unluckily the problem persist.
He said he couldn't find the real cause of the noise, he just supposes that it may be some faulty integrated circuit on the board ( 6-pin things, maybe optocouplers?? )...
He did the bias of the power tubes, only positive thing is that now the amp sounds louder at less "knob" volume, so the noise is now a minor issue while playing.
He also did a test of the "old" preamp tubes and said they're fine and I can keep them as good spares if needed.
Tho I'm still experiencing strange things:
Turned the amp on, and it had almost no volume and no gain in all of the 3 channels. Turning the volume knob on the guitar would give me A LOT of crackling/oxid-like noise. Noise (fried like) was present and consistent. Swapped the v1 with a spare one and it got way better. (gtr volume knob ok, volume and gain fine).
Then i noticed the noise would be present also with a 100% wet on the fx loop, thus making me think about a problem on the phase inverter ( tube or anything??)... I was playing and then after almost 1 hour the amp volume dropped almost to 0, so I shut off the amp. After shutting off the power, I heard strange popping sounds from the cabinet for like 3 seconds ( discharging/faulty caps?!? - I'm sorry, I dont know what it could be, just wondering ). Turned it back on, and all I could hear from the amp was a 50hz hum. Swapped v5 with a spare one and the amp worked (for an hour so far).
All this is really frustrating, is it possible that so many new/not old tubes fail randomly like that and then work fine?! If it was every other component, would the problem get better/change just swapping tubes?
I'm really hoping the problem gets costant now, because I can't bring to a tech and stay there and play for 2 hours hoping the problem will happen. Problem is that the problem doesn't seem to get consistent in time...
And I can't rely on this amp to play live because of the shame it would cause if it shuts off during a show.
Any help would be greatly appreciated, as I'm starting to get depressed about this...thanks in advance!!!!
Cobra crackling noise by MMSE on SoundCloud - Create, record and share your sounds for free
http://www.framus.de/media/manuals/Amps/Cobra/Cobra_V5.pdf
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.
Cobra crackling noise by MMSE on SoundCloud - Create, record and share your sounds for free
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).
______
Brought it to a tech who does repairs for Tvs, recorders and amps, unluckily the problem persist.
He said he couldn't find the real cause of the noise, he just supposes that it may be some faulty integrated circuit on the board ( 6-pin things, maybe optocouplers?? )...
He did the bias of the power tubes, only positive thing is that now the amp sounds louder at less "knob" volume, so the noise is now a minor issue while playing.
He also did a test of the "old" preamp tubes and said they're fine and I can keep them as good spares if needed.
Tho I'm still experiencing strange things:
Turned the amp on, and it had almost no volume and no gain in all of the 3 channels. Turning the volume knob on the guitar would give me A LOT of crackling/oxid-like noise. Noise (fried like) was present and consistent. Swapped the v1 with a spare one and it got way better. (gtr volume knob ok, volume and gain fine).
Then i noticed the noise would be present also with a 100% wet on the fx loop, thus making me think about a problem on the phase inverter ( tube or anything??)... I was playing and then after almost 1 hour the amp volume dropped almost to 0, so I shut off the amp. After shutting off the power, I heard strange popping sounds from the cabinet for like 3 seconds ( discharging/faulty caps?!? - I'm sorry, I dont know what it could be, just wondering ). Turned it back on, and all I could hear from the amp was a 50hz hum. Swapped v5 with a spare one and the amp worked (for an hour so far).
All this is really frustrating, is it possible that so many new/not old tubes fail randomly like that and then work fine?! If it was every other component, would the problem get better/change just swapping tubes?
I'm really hoping the problem gets costant now, because I can't bring to a tech and stay there and play for 2 hours hoping the problem will happen. Problem is that the problem doesn't seem to get consistent in time...
And I can't rely on this amp to play live because of the shame it would cause if it shuts off during a show.
Any help would be greatly appreciated, as I'm starting to get depressed about this...thanks in advance!!!!
Cobra crackling noise by MMSE on SoundCloud - Create, record and share your sounds for free
http://www.framus.de/media/manuals/Amps/Cobra/Cobra_V5.pdf
The Tubes / Valves forum is for all things related to vacuum tubes with the EXCEPTION of musical instrument amps which it is clearly spelled out belong in the Instruments & Amps forum. It will be relocated there.
Hey Loudthud, I attached a link to the schem. at the bottom of the first post..
By the way, a user from another forum posted this:
" First I would rule out tube problems as you have swapped them in and out and really changing them does not make the basic problem go away. Here is what I see, it is a switching problem. If I interpret the schematics correctly the switching is done with processor IC. The symptoms you are having, loss of signal, noises, hum could well be related to the processor not getting the proper power. I wold carefully examine the behavior of the circuitry associated with IC2 on the processor schematic. It is a 5 volt regulator. If it drops out or goes high all sorts of odd things can occur. Additionally the sources of the input voltages (listed as 17 volts) might be erratic or out of spec. Personally I would attach a meter to the 5 volt output of the regulator and monitor it and see if it misbehaves. "
This made me think about one thing I forgot to mention:
when I switch from and to clean/crunch/od I hear a more or less loud "pop" coming out of the speakers as i press the button (again, the intensity vary from time to time). It's a pop similar to the one you hear when you turn on an audio device with the amps and speakers ahead already "on". Could it be a proof that the switching processor is not working properly?
By the way, a user from another forum posted this:
" First I would rule out tube problems as you have swapped them in and out and really changing them does not make the basic problem go away. Here is what I see, it is a switching problem. If I interpret the schematics correctly the switching is done with processor IC. The symptoms you are having, loss of signal, noises, hum could well be related to the processor not getting the proper power. I wold carefully examine the behavior of the circuitry associated with IC2 on the processor schematic. It is a 5 volt regulator. If it drops out or goes high all sorts of odd things can occur. Additionally the sources of the input voltages (listed as 17 volts) might be erratic or out of spec. Personally I would attach a meter to the 5 volt output of the regulator and monitor it and see if it misbehaves. "
This made me think about one thing I forgot to mention:
when I switch from and to clean/crunch/od I hear a more or less loud "pop" coming out of the speakers as i press the button (again, the intensity vary from time to time). It's a pop similar to the one you hear when you turn on an audio device with the amps and speakers ahead already "on". Could it be a proof that the switching processor is not working properly?
With problems like this, tracking down the cause is very time consuming. It could be as the person on the other forum says, or some other component. One low tech approach that sometimes works is a close visual inspection under magnification of the PCB. The PCB would have to be removed from the amp because usually the "solder side" is hidden from view when the amp is all together. Broken solder joints or a PCB fracture might be found. It does seem to be some kind of control issue having to do with the controller chip. Do the LEDs change in brightness or malfunction in other ways when the amp acts up?
V1A is common to all channels, 220K (R3) anode load for max gain AND no grid stop.
I think Wavebourn is most likely right, that input stage is oscillating.
What I would be trying is:
Identify the grid pin of V1A (most likely pin 7) by chasing down the connection to R1.
Use scalpel or stanley knife to cut the pcb track right at the grid pin.
Scrape back the solder mask on the track each side of the cut to reveal bare copper.
Solder a surface mount 33K grid stop resistor across the track cut.
You can use a ordinary resistor but when modding PCBs like this, where the component relies on the solder to the track for physical mount, the surface mount gives a more robust mod.
33K is a "nominal" value, 1/2 the "usual" grid stop value, in fact a 1K would probably be sufficient but its not a mod you want to rework too many times so that is why I suggest 33K (or at least 10K minimum).
Cheers,
Ian
I think Wavebourn is most likely right, that input stage is oscillating.
What I would be trying is:
Identify the grid pin of V1A (most likely pin 7) by chasing down the connection to R1.
Use scalpel or stanley knife to cut the pcb track right at the grid pin.
Scrape back the solder mask on the track each side of the cut to reveal bare copper.
Solder a surface mount 33K grid stop resistor across the track cut.
You can use a ordinary resistor but when modding PCBs like this, where the component relies on the solder to the track for physical mount, the surface mount gives a more robust mod.
33K is a "nominal" value, 1/2 the "usual" grid stop value, in fact a 1K would probably be sufficient but its not a mod you want to rework too many times so that is why I suggest 33K (or at least 10K minimum).
Cheers,
Ian
Gingertube, the mod you suggest is to fix what? You guess R1 is not working properly? Or is just a mod you suggest after looking at the circuitry?I don't know about grid stop, but I read that this is mainly to create a lpf to prevent rf to get into the circuit... but what I'm hearing doesn't seem radio frequencies at all... Sorry to ask that but I'm not an expert so I didn't understand what was exactly to blame...
first of all I would get all new tubes for it preamp/power and go from there even if they test out fine on a tube tester I have still seen some that make noise/faulty and if it still does its obvious signal issue/ connection etc being the amp is only 5 years old i really doubt a cap issue but its still possible that's one of those intermittent nightmares where an oscilloscope and variac would be your best friends lol
honestly myself I would use current production for testing purposes only I have had horrible and i mean really bad luck with them like JJ ecc83s, el34s dead out of the box had some ruby el34s that red-plated blew up within 2 days lol, groove tubes that were dead blew up within days honestly I have never had any current production power tube make it past 1 year the JJ6v6s made it past 1 year but started rattling/mechanical noise yet it still tested ok on my tube tester lol I know people are gonna disagree with me but save yourself some time and money and try to find some good deals on NoS glass they do sound way better and out last all this new **** big time
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