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carvin XV-112 X-AMP problem



 
 
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  #1  
Old May 25th 04, 04:46 AM
timbo
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Default carvin XV-112 X-AMP problem

Hi guys,

Dont know if this is the right group, but i'll try anyway -

got a carvin X-Amp model XV-112.
this thing is a hybrid valve/op-amp type thing (3x12AX7, 4xEL34) 100w
output. whenever you turn the gain (either the clean channel or the lead
channel) up past about 5, the amp lets out a really high pitched squeal &
any signal level (like a guitar input) drops out to almost nothing, so all
you hear is the squeal. if the eq is set with the highs boosted, it's much
worse..

anyone got any ideas what might be wrong with it? tried changing valves, but
that makes no difference..

cheers,

tc.
..
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  #2  
Old May 25th 04, 07:46 AM
Phil Allison
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"timbo" >

>
> got a carvin X-Amp model XV-112.
> this thing is a hybrid valve/op-amp type thing (3x12AX7, 4xEL34) 100w
> output. whenever you turn the gain (either the clean channel or the lead
> channel) up past about 5, the amp lets out a really high pitched squeal &
> any signal level (like a guitar input) drops out to almost nothing, so all
> you hear is the squeal. if the eq is set with the highs boosted, it's much
> worse..
>
> anyone got any ideas what might be wrong with it? tried changing valves,

but
> that makes no difference..
>



** Have all the 12AX7s got their metal covers on ?



............ Phil





  #3  
Old May 25th 04, 08:17 AM
timbo
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Default

In article >, Phil Allison wrote:
>
> "timbo" >
>
>>
>> got a carvin X-Amp model XV-112.
>> this thing is a hybrid valve/op-amp type thing (3x12AX7, 4xEL34) 100w
>> output. whenever you turn the gain (either the clean channel or the lead
>> channel) up past about 5, the amp lets out a really high pitched squeal &
>> any signal level (like a guitar input) drops out to almost nothing, so all
>> you hear is the squeal. if the eq is set with the highs boosted, it's much
>> worse..
>>
>> anyone got any ideas what might be wrong with it? tried changing valves,

> but
>> that makes no difference..
>>

>
>
> ** Have all the 12AX7s got their metal covers on ?


no, they don;t have metal covers - doesn;t seem to be any provision for them
either.. would that help?

cheers,

tc.
..
  #4  
Old May 25th 04, 08:39 AM
Phil Allison
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"timbo" <
Phil Allison wrote:

> >> got a carvin X-Amp model XV-112.
> >> this thing is a hybrid valve/op-amp type thing (3x12AX7, 4xEL34) 100w
> >> output. whenever you turn the gain (either the clean channel or the

lead
> >> channel) up past about 5, the amp lets out a really high pitched squeal

&
> >> any signal level (like a guitar input) drops out to almost nothing, so

all
> >> you hear is the squeal. if the eq is set with the highs boosted, it's

much
> >> worse..
> >>
> >> anyone got any ideas what might be wrong with it? tried changing

valves,
> > but that makes no difference..
> >>

> >
> >
> > ** Have all the 12AX7s got their metal covers on ?

>
> no, they don;t have metal covers - doesn;t seem to be any provision for

them
> either.. would that help?
>


** What you are describing is high frequency oscillation due to positive
feedback which is in turn due to stray capacitance coupling the signal at
the output of the amp (likely at plate voltage levels) back into the
sensitive first input stage.

This is quite common in guitar amps due to the ridiculous amounts of gain
and high frequency boost that is available at high knob settings. I have
measured a "Marshall Master Model" as having a max sensitivity of ** 60 uV
** at the ( 1 Mohm ) input to give 100 watts output at 6 kHz. This is over
160 dB of power gain !!!

Wiring layout, grounding and shielding of the input circuitry are all
crucial if such oscillation is to be avoided.

Take your amp to a tech who specialises in tube guitar amps for the
squealing to be exorcised.




.............. Phil





  #5  
Old May 25th 04, 03:46 PM
Fred Nachbaur
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Default



timbo wrote:

> Hi guys,
>
> Dont know if this is the right group, but i'll try anyway -
>
> got a carvin X-Amp model XV-112.
> this thing is a hybrid valve/op-amp type thing (3x12AX7, 4xEL34) 100w
> output. whenever you turn the gain (either the clean channel or the lead
> channel) up past about 5, the amp lets out a really high pitched squeal &
> any signal level (like a guitar input) drops out to almost nothing, so all
> you hear is the squeal. if the eq is set with the highs boosted, it's much
> worse..
>
> anyone got any ideas what might be wrong with it? tried changing valves, but
> that makes no difference..
>
> cheers,
>
> tc.
> .


I would suspect filter capacitor problems (dried out, lost capacity).
You can temporarily bridge a known-good capacitor, say about 20 uF at
adequate voltage, across the existing ones, to see if that makes a
difference. If it does, replace them.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
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