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Fix for ground loop hum in Gibson Falcon GA 19 RVT

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  • Leo_Gnardo
    replied
    Originally posted by Tom Phillips View Post
    Good job and excellent clear writ-up.
    Agreed! I've done these and similar amps like SoulFetish but ... hey dude you wrote it up, with photos, schematics & everything! Good work!

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  • Tom Phillips
    replied
    Good job and excellent clear writ-up.

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  • SoulFetish
    replied
    Originally posted by loudthud View Post
    I see the amp has no ground switch on the schematic. Has the line cord been replaced with a three wire type ? If not, try spinning the line cord plug around 180 degrees.
    It has been updated with a grounded power cord.

    So, here's what was going on.....
    The amp came in like this:

    Click image for larger version  Name:	Gibson ga 19 rvt as it came in.jpg Views:	0 Size:	716.8 KB ID:	935043

    Firstly, the 5Y3 has a design max input capacitance of 32F, so the 47F replacements for the 20F listed in the schematic was probably installed as a punt to try and solve the hum issue.
    The strange part was, if I pulled the reservoir capacitor out of circuit altogether, the hum almost disappeared completely. If I reconnected it back in circuit, the hum returned. If I pulled the second 47F cap after the 470Ω dropper, the hum died down, but not as quiet as the first. (lifting last two 10F caps made the noise worse).
    Clearly, the amp was never intended to be operated with no reservoir cap, so I was pretty certain we were dealing with a ground loop in the power supply. It turns out, that's exactly what the issue was, and here is the fix:

    Click image for larger version  Name:	Gibson ga 19 rvt Hum Ground Loop Fix.jpg Views:	0 Size:	715.4 KB ID:	935044

    The amp is now extremely quiet at idle, and the customer should be very happy with the results. The amp is in fantastic condition as well.

    Click image for larger version  Name:	Gibson Falcon GA 19 RVT.jpg Views:	3 Size:	1.40 MB ID:	935045
    Last edited by SoulFetish; 06-20-2021, 09:10 AM.

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  • loudthud
    replied
    I see the amp has no ground switch on the schematic. Has the line cord been replaced with a three wire type ? If not, try spinning the line cord plug around 180 degrees.

    Leave a comment:


  • SoulFetish
    started a topic Fix for ground loop hum in Gibson Falcon GA 19 RVT

    Fix for ground loop hum in Gibson Falcon GA 19 RVT

    So we got one of these amps in yesterday with the customer complaint about noise in the reverb. It wasn't in the reverb, and after an audio test to demonstrate the issue, it sounded like your run of the mill failing power supply cap.
    Here's where it gets a little interesting; After clipping in helpers on each of the nodes to see if we could identify a failing cap, the noise persisted. We checked all the tubes, but I was suspicious the noise was due to a grounding problem. The caps had actually been replaced at some point, but here is the schematic:
    Gibson Falcon Reverb
    So, I have to run, but I'll holler back at you guys later on today and finish the story. Sorry for the tease
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