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Thread: **Exciting reverb noise question**

  1. #1
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    **Exciting reverb noise question**

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ID:	40001Greetings.

    Now that I have your attention I hope that you may indulge me some of your time.

    I have successfully (mostly) lifted the reverb circuit from a tube amp and built it from scratch. It sounds really good after some tweaking. However, there is some noise which I have provided a recording of, in the hope that from your vast experience you may recognize it. I have included the modified schematics for your perusal.

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    Please note that the tank is now loaded on the flip-up lid to reduce noise, and I am confident that this is not the issue. The noise increases as the guitar amp is turned up. It occurs with a lead running from the reverb to the amp, with or without a guitar input to the reverb. The caps, zeners, etc in fact all components apart from the tank are new. I get the same sound from different tanks connected.

    Here is a quick link to a recording of the sound:

    https://soundcloud.com/ieurfhehfemrlfumhref

    Thank you for any responses.
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    Last edited by minim; 07-24-2016 at 07:28 AM. Reason: Spelling

  2. #2
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    Maybe you should mention what schematic (amp model) you used for reverb driver. The changes that you implemented are at least risky. Why did you change 1n capacitor to 680n, or removed the 5.6n capacitor? They play important role in the circuit. For the start I would go back to the original circuit and modify it only if it works correctly. But then why would you like to modify correctly working circuit

    Mark

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  3. #3
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    Ok so apologies. In my haste I failed to mention that the reason I had to modify this circuit is because it is from a Crate VC50, but I am using it with a different tank- that of a JCM900. I was getting oscillations and generally weird/distorted reverb prior to making the changes. I do not like the Crate tank-the decay is way too long for me. I decided to build this for fun, and because I had the Marshall tank hanging around.

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    The questions is why did you use incorrectly modified Crate circuit while you could simply use Marshall circuit? It is possible that Marshall reverb tank (you haven't specified the symbol of the tank) has lower impedance and it requires different driving circuit. You can easily see that Marshall used two opamps to drive the tank. The circuit most probably is capable of providing more current to the tank. This may be important.
    I hope that you noticed that the input of the reverb tank is not connected to the ground.
    I think that you first should start with correctly build circuit (the one that you build is definitely incorrect) and then try to solve the problem with hum.

    Mark

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  5. #5
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    I had the Crate head sitting around. It had undergone severe board-warping/tube socket-melting trauma prior to my owning it, and also I never respected the construction of it. I missed spring reverb so I disconnected all the HT stuff, everything except the reverb circuit and basically modified it to insert my guitar into the reverb circuit alone. This incarnation always buzzed also. Yes, I could try and build the Marshall version however my feeble/fledgling electronics knowledge found the Crate version much more easy to interpret.

    It's ok. I realize that these sorts of issues are simply mundane and frustrating to actual technicians. I regret posting it now but cannot delete. I will just frick around with it further to try and solve the problem. Thanks Mark.

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  6. #6
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    I wouldn't resign. I would revert the Crate circuit to the original version, make the circuit running correctly, and then (if needed) modify the high frequency response. Actually, it's a quite easy task. The problem is that you need to do it one step a time. Otherwise, you won't be able to solve the problems you encountered.

    Mark

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