Amp technical problem

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Wireneck

Wireneck

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I recently purchased a used Framus Cobra. The price was a little on the low side but instead of getting suspicious, I let my desire for a new amp over power my senses. The amp arrived last Friday in perfect condition, original box, the foot switch had never even been unwrapped, not a scratch on the amp at all. I put in about an hour a day on it for the past week with no issues until today.
So I am playing today and the lead channel out of no where sounds like I turned the gain wide open (mega distortion on this amp). I switched to the crunch or channel 2 and I am getting the same thing no matter where the gain knob is set. To make things even more strange, channel 1 is still as clean and pristine as can be.
So my question is, could this be pre-amp tube related? The seller claimed he had just had the amp re-tubed and re-biased. I am thinking that maybe something got damaged in shipping. Any suggestions before I have to take this into a tech are greatly appreciated.

The tube map is

V1/ Ecc 83 selected / Input/Clean Eq
V2/Ecc 83 Selected / First and second distortion levels
V3/Ecc 83 Selected/ Third distortion level/ Crunch and Lead EQ
V4/ Ecc 83 standard/ Effects Path buffer
V5/ Ecc 83 standard/ Phase reverse stage
V6-V9/ El 34 selected quartet/ Power amplifier
 
Wireneck said:
I recently purchased a used Framus Cobra. The price was a little on the low side but instead of getting suspicious, I let my desire for a new amp over power my senses. The amp arrived last Friday in perfect condition, original box, the foot switch had never even been unwrapped, not a scratch on the amp at all. I put in about an hour a day on it for the past week with no issues until today.
So I am playing today and the lead channel out of no where sounds like I turned the gain wide open (mega distortion on this amp). I switched to the crunch or channel 2 and I am getting the same thing no matter where the gain knob is set. To make things even more strange, channel 1 is still as clean and pristine as can be.
So my question is, could this be pre-amp tube related? The seller claimed he had just had the amp re-tubed and re-biased. I am thinking that maybe something got damaged in shipping. Any suggestions before I have to take this into a tech are greatly appreciated.
Click to expand...
It sounds to me like the gain pot on channel 2 is shorted.
 
GGUN- a little more info, the problem seems to be intermittent. I fired up the amp today and it was doing it for the first minute or so. After that, I played for a good 30-40 minutes and it never happened again.
Would the gain pot on channel 2 also affect channel 3? Both channels have independent volume,gain, eq etc....
 
Wireneck said:
GGUN- a little more info, the problem seems to be intermittent. I fired up the amp today and it was doing it for the first minute or so. After that, I played for a good 30-40 minutes and it never happened again.
Would the gain pot on channel 2 also affect channel 3? Both channels have independent volume,gain, eq etc....
Click to expand...

In your first post you never mentioned channel 3. I'm not familiar with that particular amp, but yes, shorts can be intermittent. When this happens, is it on more than just the one channel? I got the impression that it was only on the one channel; the gain pot would only affect that channel.

This kind of thing is really hard to diagnose remotely; have you got access to a good amp tech locally? If you are not comfortable tearing into an amp, or if you have never done it, then I advise you to have it looked at. Problems like this are rarely something that you can just slap a new tube in place and it's fixed, although that's not to say that it can't be something simple and cheap. It just takes someone with experience at this kind of thing to know what to look for.
 
a lot of controlls have one side tied to ground... i suspect you've got an intermittent on one of these... perhaps a search for cold solder joints as well???
 
Have you tried taking the footswitch out of the circuit and switching manually from the amp itself? It may even be the switches going out, since they are a moving part in the circuit.
 
Thanks for all the advice guys.

GGUN- I guess my original post was not as clear as it should have been. The "lead" channel is channel 3 and the crunch channel is channel 2. I am having issues with both channels.

Dementedchord-Ill look into that.

Anfontan-Thanks for the suggestion. I took the footswitch out as soon as it happened because I had another amp that did something similar once. It turned out to be the switch.
 
I did a little more experimentation today. Tapping lightly on the first pre-amp tube with a pencil eraser induces the problem I have outlined in my earlier post. I also realized after getting the amp up to full volume last night, I was getting some weird chimey microphonic sounds on the clean channel. So I am more inclined to believe I have a bad tube but I can't be certain.
So here comes my next question. The amp currently has JJ/tesla tubes in the pre-amp section. As far as I know, Guitar center and Sam Ashe (closest stores around here) do not carry JJ's. I can order them online but I'd like to try and weed out the bad tube scenario before the weekend is up. I am mainly wanting to buy a replacement to see if it is in fact the tube causing this problem.
Can I mix a groove tube, tube in with the JJ's in the pre-amp section? Is this a bad idea? Will this affect biasing or does that only matter with power tubes?
Lastly, the manual has the preamp tubes listed as Ecc 83 but the JJ's are 12ax7's whats up with that?
Any insight is greatly appreciated.
 
Wireneck said:
I did a little more experimentation today. Tapping lightly on the first pre-amp tube with a pencil eraser induces the problem I have outlined in my earlier post. I also realized after getting the amp up to full volume last night, I was getting some weird chimey microphonic sounds on the clean channel. So I am more inclined to believe I have a bad tube but I can't be certain.
So here comes my next question. The amp currently has JJ/tesla tubes in the pre-amp section. As far as I know, Guitar center and Sam Ashe (closest stores around here) do not carry JJ's. I can order them online but I'd like to try and weed out the bad tube scenario before the weekend is up. I am mainly wanting to buy a replacement to see if it is in fact the tube causing this problem.
Can I mix a groove tube, tube in with the JJ's in the pre-amp section? Is this a bad idea? Will this affect biasing or does that only matter with power tubes?
Lastly, the manual has the preamp tubes listed as Ecc 83 but the JJ's are 12ax7's whats up with that?
Any insight is greatly appreciated.
Click to expand...

ecc83 and 12ax7 are the same thing. different countries, different names for the same tube.

you can freely swap tubes in the preamp section.
 
http://thetubestore.com/tungsol12ax7.html

I put a matched trio of these in my Quiana55 2-12 after trying GT Chinese Mullard knockoffs that were microphonic as hell. They sound great, a few pops and clicks, but the GT's were giving me stuff that sounded like a guitar cord shorted to ground.

I'm coming to the conclusion it's difficult to get pop/microphonic-free sound from a high-powered combo amp. :(
 
Just wanted to let everyone know that replacing one of the pre-amp tubes solved the problem. Thanks for all the help and info.
 
Wireneck said:
Just wanted to let everyone know that replacing one of the pre-amp tubes solved the problem. Thanks for all the help and info.
Click to expand...

Cool. I guess you fgured out that biasing only affects the power tubes.

Glad you solved your problem.
 
So are we gonna hear this bad mama?
Click to expand...

Unfortunately my mix console has had a fried transformer for a few months now. I am still trying to coral up the funds for parts and labor. That is the main reason I was trying my best to fix the amp problem on my own.
Its a pretty cool amp. I own a Marshall and a Mesa and this is a very different beast. Not totally convinced it is better yet though haha.
 
That sucks about your board's transformer. I've never heard a Framus Cobra so I thought I would try to milk a clip out of you. :p
 
http://rocksolidamps.com/index.php?contain=description&ii=64&mid=2

That site has some ok clips. A studio I was recently recording in had one and that is what kind of turned me on to them. We didn't actually end up using it for the tracking though.
Thanks for the concern about the console. Note to self....never let friends borrow your console who are going to leave it on, inside of the flight case.
 
sounds like a pretty kickass metal amp. i liked the clean. thanks for clips/link.

ouch! (on the console being in the case) :eek:
 
The clean definitely rocks. Why many of these high end amps opt to have no reverb is something I will never understand.
 
I have a Framus Cobra as well, and I'm just not sure I like it. I own a Peavey JSX, and I was planning on replacing it with the Framus, but I don't think I can do it. The Framus has a totally different sound, for sure. To my ears, it has a lot more presence to the sound, more Mesa sounding than Marshall. I'm not sure if the sound is affected by my cabs (2 ADA 2x12's with Celestion 50 watt speakers), or if that is just the sound of the amp. I unfortunately didn't have a chance to play one before purchasing it used, since I live in the sticks in PA, so I was going by the sound clips. I just can't get a sound anything like the clips. I'm going to give it a little longer, but it's going to have to go if I can't get it to sound like I want. The Peavey JSX is such a super amp, I guess it's better for my ears. More of a hot-rodded Marshall tone, which probably works much better with my cabs.
 
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