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Thread: Gibbs 4L reverb tank

  1. #1
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    Gibbs 4L reverb tank

    Working on a Shure VA300 vocal master and the reverb tank output transducer is bad. No ouput signal on scope but signal on the input. Does anyone know what the impedance is supposed to be on the Gibbs 4L tank? I am measuring about 180 ohms on input and output but don't trust the reading with the bad output. BTW, insulated output.

    I have looked but info on these old tanks seems to be all over the place.

  2. #2
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    All of the tanks that I have ever seen in any application have had the same output transducer resistance of around 180 ohms.

    Is the return circuit functional? Try reversing the leads on the tank to see what happens.

  3. #3
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    What I did was unplug the output and put my scope on the connector. Zero signal. The input shows signal bright and clear. The way the tank is mounted in the bottom of the amp, there is no way the leads could be reversed as they are not long enough to swap. I could jumper the input and output to see if signal passes. Will let you know what I find.

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    Jumpered the in and out cables and the reverb recovery circuit works as it should. Intensity, treble an bass all have an effect.

  5. #5
    Senior Member Enzo's Avatar
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    Get an RCA to 1/4 cord. Plug the RCA into the pan out jack, and plug the other end into some guitar amp. Now rub the springs. Get noise?
    Education is what you're left with after you have forgotten what you have learned.

  6. #6
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    Ok. I did as Enzo said and... I have reverb. I was able to hook up a mic into a channel on the shure, then run the output of the reverb tank to a guitar amp. Turn everything on, adjust volumes and reverb signal and my voice came out the guitar amp with reverb. So, going even further I hooked up the reverb tank from the amp to the shure and I have reverb. When I hook up the original tank to the shure and turn on reverb, I get nothing but feedback no matter the reverb level. Seems I get reverb for just a second before the feedback takes over. Maybe I have something going on in the reverb recovery section as 52 bill suspected. Possibly some out of tolerance resistors and/or caps causing too much gain on the transistors? I have already identified a bunch of 5uf caps that need to be changed but haven't got the parts yet. I guess I need to get out my DMM and ESR meter and start digging deeper.

  7. #7
    Senior Member Enzo's Avatar
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    These are simple circuits, a part would have to be WAAAAAYYYY out of tolerance to cause all this. Nothing precision here.

    You demonstrated that the reverb pan is OK< right?

    My Shure schematics are in a warehouse somewhere, a schematic would help.

    Does it still feed back if you ONLY connect the return cable? If you cover the reverb pan with a towel or put it in a box, does it affect this? (In other words is the feedback acoustic or electronic?)
    Education is what you're left with after you have forgotten what you have learned.

  8. #8
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    shure.png
    Reverb pan is ok. I will try the other tests and report back.

  9. #9
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    Feedback stops when either the input or output is pulled. I wrapped it in a towel and noticed the feedback stopped when the springs were touched by the towel. If the towel is removed from the springs it would begin to feedback again. Moving the tank around had no effect.

  10. #10
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    I've been doing some more testing on this and what I found is interesting. Turning on the master Reverb switch causes the feedback. It doesn't matter if the individual Reverb switches are in or out or if there is a mic connected or not. Pretty much, no matter what you do, if you have the input and output to the Reverb tank and you turn on the master Reverb switch, you get feedback. I can unplug the input to the Reverb tank and hear the Springs come through the speaker. I can unplug the output and see the signal coming into the Reverb tank input. If I connect them both to the tank I get nothing but a feedback distorted sine wave. I'm not sure where to go, when checking the circuits with the individual reverb cables unplugged they both seem fine. Plugging in the tank, everything Changes.

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