Direct Box Distortion and Twin Trouble
I have a Bellari ADB3B direct box which Here’s to clean living – I mean signals.
has a tube. I hooked it up to the mixer at
church and the soundman said it produced
a distorted sound in the lower notes on
my five-string bass. I don’t hear the distortion in headphones or through an amp.
Could the tube be blown? How can you
tell when the fuse is blown?
To answer your questions, if
either a fuse or a tube was
blown the unit would likely not
function. However, the tube
could still be contributing to the
problem. The tube used in your
unit is a standard 12AX7/7025
tube, but there are many different 12AX7 tubes currently available and some seem to reproduce low-end better than others
in different applications. My
suggestion would be to audition as many tubes as possible
to see if you can find one that
minimizes the distortion.
My ’ 65 Fender Twin reissue is about ten
years old. The reverb on it is very weak,
even when fully cranked. I gave the amp a
good kick with the reverb at 10 and heard
a huge “sprong!” that even woke my kid
up, but when I plug in it is very subtle.
It used to sound like a canyon of reverb
– is there a problem with the tube? Also,
my vibrato is on the skids. It’s as if it has
no depth, just a slight shimmer. At some
for troubleshooting purposes. If you don’t
have a spare 12AX7, you may borrow the
12AX7 from the first position on the right
– the normal channel preamp tube.
If the tube is not the cause of the problem,
I would next suspect the reverb tank or
cables. Unplug the reverb send and return
cables from the underside of the chassis.
Then, using a multimeter, measure the resis-
tance at each plug from the center pin to the
outer shell. One plug should read approxi-
mately 1-2 ohms, with the other reading
approximately 175-200 ohms. If you are
missing the latter, the problem is either
a faulty RCA cable or an open trans-
ducer on the input of the reverb tank. By
removing the reverb tank and measur-
ing directly at its input jack, you will be
able to determine if the problem lies in
the tank itself or the connecting cables.
If this all checks fine, the cause is either
a bad reverb drive transformer or other
internal components, and is something a
good amp tech will need to verify.
The unit can be configured a
couple of different ways and you
may be using it incorrectly for
this application. According to
the manual the unit has two gain
settings, -20db and +20db. The
-20db is recommended for a line or speaker
level signal, while the +20db is recommended for guitar level. In reviewing the specs,
the -20db setting has a frequency response
of 20hz to 40khz, which is fine for guitar or
bass. The +20db setting, however, has a
stated response of 50hz to 20khz – not so
good for bass guitar, where the low E string
has a frequency of 41hz and worse for your
bass, since the low B string has a frequency
of 31hz. Sending a signal through the unit
that is out of the useable frequency range
can cause the signal to become distorted.
My suggestion would be to try running the
extension speaker output of your amp into
the unit using the -20db gain setting and see
if that cures the problem.
faster settings it doesn’t even sound on.
I had the lamp replaced three years ago,
but that didn’t seem to help.
As far as the vibrato being weak, you
mentioned having the lamp replaced.
Most of the time when servicing a
vibrato problem, a tech will replace the
entire vibrato assembly. If you literally
meant just the lamp in the assembly,
there is little chance that the problem
would be repaired since a weak photo-
resistor in the assembly is usually the
cause of the problem, not the lamp itself.
You could try replacing the fifth tube from
the right – the tremolo oscillator tube
– with a known-good 12AX7 and see if that
cures the problem, but other than that the
amp will need to go to a good tech for further troubleshooting.
From the sound of your explanation we can
assume that the reverb recovery circuit is
working fine. This leaves the reverb drive
components as the cause of the malfunction. I would start by replacing the reverb
drive tube. This is a 12AT7 tube and should
be the third tube from the right when viewing the amp from the rear. If you have an
extra 12AX7 tube handy, you may use that
Co-Founder and Senior Design Engineer
email@example.com or www.budda.com
©2007 Jeff Bober