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Post subject: Vibro Champ XD squealing
Posted: Tue Jan 15, 2013 7:26 pm
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Aspiring Musician
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This afternoon a friend brought over his Vibro Champ XD - purchased new about six months ago - which is squealing and making all sorts of high-pitched sounds within 15 seconds after powering up and with no guitar plugged in. The noise is continuous and not effected by changing volume, FX, or models.

My novice diagnosis was a microphonic preamp tube. I had a spare GT12AX7 so I swapped it with his VC's 12AX7. No change whatsoever. Obviously my diagnosis was incorrect. Good thing I'm not a doctor.

At this point I'm thinking that it could be the 6V6 power tube - I don't have a spare - or it could be something more serious which would (should) be covered under warranty.

The plan is to swap the power tube and if this doesn't work to bring the amp in to an authorized Fender dealer for service. Of course my absent minded friend can't find the sales receipt but I'll throw him off that bridge if and when he gets there. :twisted:

Is there anything else I should check before?

Thanks and Cheers!]
BM

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Post subject: Re: Vibro Champ XD squealing
Posted: Tue Jan 15, 2013 7:35 pm
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You could try one other thing, if swapping 6V6GT doesn't solve the issue. Try placing your finger lightly on each tube. Does the "feedback noise" change in timbre?

If so, it could be parasitic oscillations. Sometimes, a slight change in capacitance (like the one inherent in the human body), is enough to stop these bad vibes. It's one of the reasons you see small pico-farad caps in many amps.

In any event, a good tech will be needed to isolate the bad portion of the circuit causing the feedback.


Good luck with the unit. Keep us informed.


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Post subject: Re: Vibro Champ XD squealing
Posted: Tue Jan 15, 2013 7:52 pm
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Thanks for the feedback, BMW2002Ti!

For a moment there I had a nightmarish image of some alien crystaline entity growing in the amp but then I Googled "parasitic oscillations" and breathed easier. :lol:

My friend is coming over tomorrow - hopefully with the receipt - so I'll have him put his finger on the tubes while I power up the amp. :twisted:

Cheers!
BM

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Post subject: Re: Vibro Champ XD squealing
Posted: Wed Jan 16, 2013 7:32 am
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Aspiring Musician
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BMW2002Ti wrote:
... Try placing your finger lightly on each tube. Does the "feedback noise" change in timbre? If so, it could be parasitic oscillations. ... Good luck with the unit. Keep us informed.
My friend came by this morning, we did the finger on the tube and there was absolutely no change. We even tried a little light tapping on the tubes and still no change.

Anyhow he's found the original receipt so he'll be taking the amp to an authorized Fender dealer for servicing. I'll post back with any developments.

Thanks & Cheers!
BM

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Post subject: Re: Vibro Champ XD squealing
Posted: Wed Jan 16, 2013 1:05 pm
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Good luck with the new amp. Original problem prolly a QA one with the board and/or soldering. Not unheard of, these days...


:mrgreen:


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Post subject: Re: Vibro Champ XD squealing
Posted: Wed Jan 16, 2013 1:12 pm
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Bluer Monkey wrote:
Thanks for the feedback, BMW2002Ti!

For a moment there I had a nightmarish image of some alien crystaline entity growing in the amp

BM


Parasitic? Makes me think of bankers. :lol:

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Post subject: Re: Vibro Champ XD squealing
Posted: Thu Jan 17, 2013 6:41 am
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Parasitic is the best adjective for this instability. You can watch it on a scope.

Input a signal, whose frequency generates the instability. You see the tracings start to go UP and UP and UP. Without any change to the volume setting. The instability is actually feeding back to a previous stage and generating MORE and MORE gain in that frequency range.

Just like a car with poor shocks in its suspension. You hit a bump and the darn thing starts uncontrollably bouncing up-&-down.

The solution is better shocks which alter the spring's time-constant of the car. Or a more suitable relationship between the suspension's spring characterisitcs, shock, and suspension arm.

In amps... Same thing. Alter the time-constant (resistor, cap, inductance) of the portion of the board that is inducing the unstable resonance.

There, that was easy. No? :lol: :lol:


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Post subject: Re: Vibro Champ XD squealing
Posted: Thu Jan 17, 2013 7:05 am
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Aspiring Musician
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BMW2002Ti wrote:
Parasitic is the best adjective for this instability. You can watch it on a scope. ... Just like a car with poor shocks in its suspension. You hit a bump and the darn thing starts uncontrollably bouncing up-&-down. ... There, that was easy. No? :lol: :lol:
Maybe not easy, but your car shocks example is a good illustration.

While we await the diagnosis from the Fender tech, I have a question about ruling out the tubes as culprits. I swapped the 12AX7 and there was no change. Assuming I had a 6V6 power tube to swap and if there was still no change then I would have concluded that the tubes are fine. But what about both tubes failing? Theoretically it's possible, of course, but is this a realistic possibility? Would an amp tech verify all possible Good/Bad combinations when checking tubes?

For the 2-tube Vibro Champ XD there are only 4 possible states but for the 5-tube HRD, for example, there are 32 possible states. Seems it would take a long time to check out all the combinations.

Cheers!
BM

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Post subject: Re: Vibro Champ XD squealing
Posted: Thu Jan 17, 2013 9:10 am
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Bluer Monkey, in my 1000 year experience ( :lol: ) --- ok, ok, 34 years of working on tube amps --- have I ever had two sets of different makes and different lots tubes go bad immediately in any amp. Not without the amp having issues. Could be a resistor(s), a coupling cap(s), bad sockets, bad PS caps or resistors, bad diodes, rectifier, or bad PT or OPT.

Good tubes are relatively hardy and not the weak link, as a lot of salesmen would have you believe. I've had tweed Bassmans from the 1950s with the same Mullard GZ34 and TungSol 5881 in them, that came from Fender. And they still sounded good.

The key is a stable enviroment for those tubes. And GOOD tubes.

Hope all works out for you.


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Post subject: Re: Vibro Champ XD squealing
Posted: Thu Jan 17, 2013 9:50 am
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Aspiring Musician
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BMW2002Ti wrote:
Bluer Monkey, in my 1000 year experience ( :lol: ) --- ok, ok, 34 years of working on tube amps --- have I (n)ever had two sets of different makes and different lots tubes go bad immediately in any amp. ... Good tubes are relatively hardy and not the weak link, as a lot of salesmen would have you believe.
I thought that such a failure would be highly unlikely but I've made wrong assumptions before so I thought I'd better ask. :wink: Thanks for clearing that up, BMW! :!:

BTW, your 1000-year experience figure is not that much of an exaggeration. I think it would take me about 1000 years to gain the knowledge you and some of the other amp gurus on this forum have acquired in thirty. :lol:

Cheers!
BM

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Post subject: Re: Vibro Champ XD squealing
Posted: Sun Jan 20, 2013 9:33 am
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BMW2002Ti wrote:
Good luck with the new amp. Original problem prolly a QA one with the board and/or soldering. Not unheard of, these days... :mrgreen:
You're right, BMW!

Got word from my friend who was contacted by the repair shop. He told me that the tubes are fine and the problem was a "bad resistor". I asked for more details but he didn't know. Anyhow it's under warranty and the turn around was about two days so he's a happy camper. :)

Thanks & Cheers!
BM

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