Framus Cobra - Troubleshooting...

[fallenzero]

Hey Guys, a week or so ago, I got a Framus Cobra. However, there were some issues with it.

It's actually with a local tech now...but I just wanted to discuss and idea jog while i don't have it in my possession.


First off, the amp works.

However, there is a loud buzzing that might be coming from the transformers...that is seperate from the regular hiss of a high gain amp. This buzz is louder on the clean channel, which has an incredibly noisy and poppy gain knob.

Next, The lead and crunch channels sound great. I looked up settings and how to adjust the channel and master volumes and it sounds pretty good. However, I have to turn the treble at 1, or completely off for the amp to not sound harsh. This is at all volume levels. I know that this amp is apparently finnicky with pickups and cabs, but the amount of harshess that the treble knob adds is completely abnormal. This is with the presence at any setting. Even with the presence off.

And as background info, the previous user has it tubed with all groove tubes. 12ax7c's all in the preamp, and E34L's in the power amp.

After taking it in to the shop, they could see right away that the right two power tubes were running waaaay hotter than the other two. I could see this being responsible for the noise. However, could this also contribute to why the treble on the amp sounds so ice picky and harsh? Or is that a seperate problem?

Like I said, it's already with the tech. But since I won't have it for a couple weeks, I'm just trying to make sense of what is going on with the amp.

Thank you for any input/ideas!

 

[titanamps]

the noisy pot is either dirty or just bad. typically, the buzzing noise comes from dirty power, a failing or crappy power transformer, or a bad preamp tube. it can also come from poor grounding inside the amp, or a non grounded outlet. another common source is the guitar and cables.

there are just a LOT of sources of noise in an amp. i'm not at all familiar with a cobra, so i can't really get very specific with what might be wrong. hopefully the tech can sort it all out for you, though

 

[fallenzero]

thanks for the info!

do you think that the incredibly harsh treble can be a result of something wrong in the power section? or maybe a bad preamp tube?

 

[titanamps]

i really doubt it's in the power section, but that's just a hunch. it could be a preamp tube, but i've never seen one that was bad in such a way as to cause a harsh high end.

did you notice any other controls that didn't work very well?

if it were me, the first thing i'd do is clean the pots and replace the preamp tubes. if that didn't clear up most (or all) of the noise/tone problems, then i'd start looking for failed components. there could be a shorted treble peaking cap in the preamp section....that might account for the buzzing as well. i'm really just grasping at straws here. it's impossible to troubleshoot an amp that's not on the bench, ya know?

 

[fallenzero]

thanks for your help. i definitely know it's hard to diagnose without having it in front of you.

the only controls that worked poorly were the gain knob on the clean channel, and the treble control on the crunch and lead channels. the master volume control was slightly crackly, but that was mostly fixed by turning the knob up and down.