Troubleshooting Help on DBX 376

W

Wit'sEnd

New member
Couldn't find this topic elsewhere, so I'm starting a new thread...

I have a DBX 376 "tube" pre... I know, from reading all the other info here, that the role of the tube is a touchy subject, but I am having a separate issue and I'm hoping someone can confirm my suspicions...

The unit will work for a period, then the output will just stop. The LEDs indicate input signal, but all of the other indicators (gain reduction, de-essing, etc.) show no activity. If I create a transient (e.g. bang on the mic) then the output signal comes back, but after a period of low or no input it "jams up" again.

I looked at the block diagram, and it appears that the tube is "in series" at the input stage. My theory is that the tube is bad, and if I replace it the problem will go away. If I am going to replace it, I have found lots of discussion about spending the $ for a tube upgrade that may make the unit more flexible.

Trying to determine if I should go that route or use it as a boat anchor...
 
if you suspect the tube, whats the tube #, you can probably pick one up cheap and substitute it and see if the problem goes away.
Did you open it up to observe it when it is not working? Is the tube lit. If not, then the heater isn't getting voltage, if when you bang it it starts to light then it is a bad connection or bad tube. Could be the connection just needs cleaning. Take the tube out and spray the pins with cleaner, or wipe with them with alcohol using a cue tip, maybe in the socket also,and put it back in the the socket and wiggle it around, do this two or three times and maybe it will clean the socket and the pins. Then see if it does it again. If the tube is lit while this trouble is happening then it is not the heater circuit. I will try and take a look at a schematic.
 
Thanks very much. The tube is lit, and I verified the connections, seating, etc. are good. The "bang" is not a physical jolt to the unit, but a transient in the circuit from "banging" on the microphone.

I am wondering if a bad tube can manifest itself as a "threshold" problem like this.

The tube is a 12AU7, pretty common, and they range from $10 thru $40 before going into the stratosphere for "boutique" stuff. Like so many others, I have not been overwhelmed by the contribution of the "tube-pre" functionality of the 376, so if I'm getting a tube I would like to maybe upgrade a bit.

Really appreciate the input. So to speak! (I NEVER use the term "feedback...")
 
Thank you! I have exercised all the switches, but did not try contact cleaner. Good call on the insert jack--I'll give that a go now. I have been all over the 'Net just looking for some sort of evidence that the specific symptoms I'm having can be from a suspect tube. It's funky, so the solid-state button monkey in me immediately went there... Knowing you had the insert jack issue is a good start.

Awesome forum, really appreciate the replies and effort.
 
Wit'sEnd said:
Thanks very much. The tube is lit, and I verified the connections, seating, etc. are good. The "bang" is not a physical jolt to the unit, but a transient in the circuit from "banging" on the microphone.

I am wondering if a bad tube can manifest itself as a "threshold" problem like this.

The tube is a 12AU7, pretty common, and they range from $10 thru $40 before going into the stratosphere for "boutique" stuff. Like so many others, I have not been overwhelmed by the contribution of the "tube-pre" functionality of the 376, so if I'm getting a tube I would like to maybe upgrade a bit.

Really appreciate the input. So to speak! (I NEVER use the term "feedback...")
Click to expand...

i would just try another 12au7 before doing anything. If it is in the tube circuit you might have the same problem after upgrading to another tube. For ten bucks you can eliminate if it is a bad tube or not. If you put the new 12au7 in and still have the same problem then it isn't the tube, but may still be in the tube circuitry. Did you check the link above I provided. I didn't look to see if it has a schematic or not.
 
Thanks all. I checked the link, but there was not anything immediately useful that I could find. The switches on the front panel are mechanical switches with contacts, which I carefully cleaned with contact cleaner. I also loaded up a TRS plug with contact cleaner and worked it in and out of the "insert" jack.

For now, I cannot get it to replicate the problem--I think we may have nailed it with maintenance on the non-tube components!

If it comes back, I'll try swapping with an inexpensive tube before doing anything else, but my guess is the problem was not tube related.
 
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