Does this sound like an Output Transformer problem?

[mightywarlock]

You amp tech folks, please watch this and let me know if you think this might be an Output transformer problem or some other issue?
thanks!

 

[glpg80]

Has the amplifier ever been biased?

 

[mightywarlock]

I just got it...it was used. It has been at a tech, but you think it may be tube related?

I replaced all preamp tubes, and swapped 2 power tubes around, but no change.



By the way, this is a V1 version of the Cobra. Don't know if that makes any difference...

 

[jerrydyer]

no it sounds like a connection issue. when it power surges there is enough juice to get over a bad connection. cold solder OR a problem with a switching device, if it uses like a vactrol in series. got a schem?

 

[Ventura]

That's weird... The caps are bleeding off the tone when you go into standby, right? How are the fuses in this amp? I'd pull the chassis out and flip 'er over and have a look for any blackened components on the board - fried, poofed, or the like. I'd also consider throwing in a quad of tubes - whatever you have laying around - just in case this is a power to tube to power to tone sitch.

Mo

 

[jerrydyer]

if the B+ were shorting to ground a fuse would blow... power tube issue usually the same fuse blows. ... youve got a cold solder (bad connection) somewheres. could be something simple like input grid resistor at tube bad connection, check the very first few components you see by the input jack you might find something.

 

[glip22]

I would guess a connection in the preamp. I would poke around in the chassis with a chopstick to see if you can isoalte it. You may get lucky and hit a lead or component that brings your signal back up.

 

[mightywarlock]

Heres a link the the Schematics from the Framus page


http://www.framus.de/media/manuals/Amps/Cobra/Cobra_V1.pdf

 

[jerrydyer]

1 are you able to get any channel to work without probs.
2. can you plug into the effects return and make sure your power section si good. it will just be clean tone a little muffled, slightly.

I see two spots where i think the prob is. depending on what device is used to turn on lead gain and crunch gain. it shuts off the signal in two spots right there and turns on the signal in two spots. thats a lot of relying on switching devices to keep working.

I think the problem is within the switching system itself. incorrect voltages not opening and closing correctly. Does the clean channel work. if so then I would focus on the switching system right at the first gain stage output.

 

[JerryP]

Try pluggin the guitar into the effects loop return jack and see what you get.

Also try taking a line from the effects loop send of the Framus into another power amp or another amps effects loop return with your guitar plugged into the front end of the Framus. These thigns will tell you if it's in the power amp or the preamp section.
Jerry

 

[mightywarlock]

1 are you able to get any channel to work without probs.
2. can you plug into the effects return and make sure your power section si good. it will just be clean tone a little muffled, slightly.

I see two spots where i think the prob is. depending on what device is used to turn on lead gain and crunch gain. it shuts off the signal in two spots right there and turns on the signal in two spots. thats a lot of relying on switching devices to keep working.

I think the problem is within the switching system itself. incorrect voltages not opening and closing correctly. Does the clean channel work. if so then I would focus on the switching system right at the first gain stage output.

1 - Nope, no sound out of unless cranked on ch.2 or 3. clean nothing audible, only on the distorted channels, i get that distorted messed up sound.

2- I will try the Effects loop tricks tonight and report back.

 

[mightywarlock]

Try pluggin the guitar into the effects loop return jack and see what you get.

Also try taking a line from the effects loop send of the Framus into another power amp or another amps effects loop return with your guitar plugged into the front end of the Framus. These thigns will tell you if it's in the power amp or the preamp section.
Jerry

Thanks for the tips Jerry (actually, both Jerry's)

I'll give this a try later tonight when I get home from work and report back.

thanks!

 

[TrueTone500]

Try a known good complement of tubes first. If the problem persists, do an inquiry with Joe "Joey Voltage" Nazari. You can find him over at the Metro amp forum. Email your video description to him.

pm sent...

 

[mightywarlock]

Alrighty.

Did a little more troubleshooting and testing tonight with the Framus Cobra.

First, i pulled all the brand new JJ E34L's out of my Mesa Roadster, and popped them in the Framus. (the Framus came loaded with 3 Groove Tubes and 1 Sovtek EL34. To me they all look exactly the same, so I would say they are ALL Sovteks.

1st, tried plugging the guitar in the front, as usual. Same problem. Out of standby, gets loud, then quickly quiets down and makes bad noises.

Next, plugged the guitar into the Effects Return of the Cobra.
Still sounded bad.

Then I plugged the guitar back into the front of the Cobra, and took another cable from the Effects loop Send to the Effects Loop Return of my ENGL Special Edition. Set the ENGL on the clean channel and took out all the options, so it would be clean.
Shot this video...And thanks to FJA, now have it narrowed down just a little more...

Preamp section sounds fine!

So...at least we have that narrowed down...

 

[duesentrieb]

To me it sounds as if the powertubes cannot draw enough current. There are a couple of things which can cause this to happen which should be diagnosed by a tech.

 

[glip22]

Are your power tubes biasing ok? Do a pinout where you take readings on each powertube pins with a meter. Duesentrieb can give you what you should be reading. Unfortunately if your powertube voltages checkout you may have been correct at the beginning. Now that I think about it I had an amp do something very similar. I eliminated as much as I could. I swapped in another OT and problem fixed. An OT can sometimes get a minor short rather than just blowing completely. You will get output like you are and you also get a bad distorted sound like you are and the cutting out. Since the preamp checks, I would guess it is a minor shorted OT. They are usually not hard to replace. Six or seven leads. Depends where it is mounted. Getting under the boards to get at the bolts can sometimes be a challenge. I have some good tricks for that though if the board cannot be lifted. Yours probably can. Modded amps are another story. An OT should run you 160-200.

 

[JerryP]

Next thing you have to do is look at voltages on the power tube sockets. If you're not familiar with electronics take it to a tech. This stuff is not for a beginner so if you have ANY doubts keep your hands out of the amp.

Check to make sure the screen resistors are good. They are the large 1K resistors in the picture at the bottom of the first page. You should read a voltage on both sides of each resistor. You can measure the bias by measuring the voltage across the 1 ohm resistor on that same circuit board.

Jerry

 

[duesentrieb]

This stuff is not for a beginner so if you have ANY doubts keep your hands out of the amp.

--> tech.

The OT is most probably ok, the symptoms audible in the vid are caused by simple parts or circuitry faultyness in the supply, maybe those dropper diodes have gone south.

But please leave this stuff to somebody who knows what he (or she) is doing. Don't grill yourself.

 

[mightywarlock]

Alright.
I'm going to drop it off at a tech tomorrow (just need to figure out who to go to, so many to choose from here in L.A.)

I will print out this thread and at least give the tech some idea's to work off of.

Thanks guys!

(well, i might pull it out first to see if it's anything obvious, but if not, off it goes...)

 

[TrueTone500]

Someone probably ran it with no load plugged in, and trashed the OT.