stages gave more depth, and eleven stages even more. Beyond this point the extremely high
gain of the system became a problem, but we determined that eleven stages would be
manageable. The resulting overdrive sound has a depth and a degree of complexity never
achieved before.
With eleven tube stages operated in cascade there were quite a few variables to work with and it
took many weeks to finally arrive at the configuration that we provide in the Quad X-Amp. The
name for our new circuit was fairly obvious: the Tube Cascade™.
6.3 Tube Maintenance and Troubleshooting
The Quad X-Amp employs nine 12AX7A tubes. These tubes all perform preamplifier functions
and are operated well within the tube's maximum ratings. As a result, the tubes should have a
very long life span and no routine maintenance will normally be required. In many cases the
tubes will likely last the life of the preamp.
Like any electronic component, there will be occasional tube failures. Some of the symptoms of
a bad tube would include: crackling noises, rattle, hum, or even a dead channel. Should you
ever experience any of these problems you will need to take corrective action. You could return
the unit to Carvin for repair but in most cases, with just a little effort, you will be able to locate
the defective tube or tubes and replace them yourself. Following is a procedure for identifying a
bad tube and replacing it. 12AX7A tubes are among the most commonly used tubes for guitar
amps so you should have no trouble finding a new replacement tube. If you can't find a local
source (check the music dealers) then you can purchase replacement tubes directly from Carvin.
Equivalent tubes to the 12AX7A are: 12AX7, 7025, and ECC83 (alternate names for the same
tube). In a pinch you could use a 12AY7, 12AU7A (aka: 6189, ECC82), or 12AT7 (aka: ECC81
or 6201).
Quad X-Amp
About Tubes Section 6
6-3
How to Identify a Defective Tube
1. Turn the unit on and determine which channel is experiencing problems. If the
problem is in all channels then it is probably not a tube problem and you should call
Carvin for further instructions on having the unit repaired. If the problem is only with
some of the channels then proceed as follows.
At this point you will need to remove the top cover to get to the tubes.
CAUTION do not touch anything except the glass tube bulbs!
2. If the problem is with channels 1 or 2 or both then the suspect tube is V1. V1 is the
tube located at the far left of the unit as you look at it from the front. Try replacing this
tube with a known good (new) tube. If the problem is corrected then replace the
cover and enjoy. If not, call us for assistance.
3. If the problem is with channel 3 then check V2, V3, and V4 by listening to the amp's
output while you tap (pluck) each of these tubes with your fingernail. A bad tube will
usually reveal itself with a crackling sound when it is tapped. Replace any bad tubes.
4. If the problem is with channel 4 then check V4, V5, V6, V7, V8, and V9 as in step 3
above. Replace any bad tubes.
5. If you cannot correct the problem please call Carvin for further assistance.
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