Direct Box Distortion and Twin Trouble
February 8, 2008
I have a Bellari ADB3B direct box which 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.
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.
Here’s to clean living – I mean signals.
|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 faster settings it doesn’t even sound on. I had the lamp replaced three years ago, but that didn’t seem to help.
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 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 resistance at each plug from the center pin to the outer shell. One plug should read approximately 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 transducer on the input of the reverb tank. By removing the reverb tank and measuring 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.
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 photoresistor 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.
Co-Founder and Senior Design Engineer – Budda Amplification
©2007 Jeff Bober