Where is that feedback coming from?

I accidentally posted this in the wrong forum. I am reposting, with apologies, to the proper forum.

Well, I'm stumped! I have built cbgs with disc piezos, rod piezos and mag pups. I recently had a friend come over to play each box for marketing purposes. He brought a pedal board and we plugged it all in to one of my small 3w amps. We had issues with most of the boxes getting feedback from the amp. tried various setups and even unplugged the amp and let it run on battery, but still had feedback. I've been through the forum looking for answers but haven't seen this addressed with these parameters.

So, this morning I took several boxes and two amps down to the shop to see what I could figure out. Both amps are battery powered, and one of them is battery/AC-DC. All boxes sounded perfect when plugged in to the battery powered amps. As soon as I put AC to it I am getting feedback. Touch the cable and it knocks out the feedback, indicating a grounding issue. The boxes are all wired pretty much the same - a rod piezo built into the bridge, connected to a volume pot, connected to the output jack. Nothing fancy. The rod piezos are set into the bridge but not potted. Would potting in silicone help?

So, I am coming to the conclusion that my problem lies in the AC, not in the boxes.

Would the boxes cause feedback on battery power if they were improperly grounded? Is there a way to reduce or eliminate this feedback/hum? Is there a better way to test this to determine where the culprit is? I am thinking that the building wiring around us is the problem, not my boxes. Can I use a circuit tester or multimeter to test everything and, if so, what is the procedure and sequence?

It is frustrating as a box I lent to a friend had the same issue once they plugged it in to their Roland Micro Cube.

So, your disc piezo builds are hum free as are your mag pickup builds. Hummmmmmmmm

One has to ask why a disc piezo would not be affected by AC going to your amps, but the rod piezo is affected. Makes one take a suspicious look at that rod piezo. Could it be that the braided ground wire of the piezo rod isn't actually well grounded to the piezo rod? Might be tough to verify since the internal resistance of a piezo is crazy high anyway.

I know it would be a pain in the butt, but if you un-soldered the positive lead ot rod piezo and then soldered a disc in the circuit then if you no longer had the hum, I would definitely think about dumping that particular rod piezo. On the other hand, if you still have hum with the disc in the circuit, then the pot or jack become suspect. Remember, none of these components are super high grade. There is likely a high degree of variance within a batch and sometimes the mismatches stack up and cause a problem.

You've built guitars without hum prior to this so something is not functioning in this circuit. It's a matter of elimination. You can nail it with a little more testing.

Replies

Steve Griggs July 12, 2016 at 10:41pm

This may sound a bit simple, and I may have missed it in all the replies, but why not pick up the git...and the amps...and run over to somebody else's house and see if the problem goes along for the ride, or if it stays home. If it stays home...its in the ac wiring and has nuttin' to do with the git at all.
John Sawyer July 7, 2016 at 7:46pm

I see you have tried different amps, but (maybe this is silly). Did you use the same instrument cable? I had a bad one once that was poorly shielded... Laying it on the concrete would make it buzz...
- Michael Zack > John Sawyer July 7, 2016 at 10:13pm
  
  I have tried other cables, but haven't noticed a difference. Funny thing is that I don't get the hum with my disc piezo pickups, only with the rod piezo pickups.
  - RTZGUITARS > Michael Zack July 8, 2016 at 5:03pm
    
    Don't have time to read all the posts so these may have been said.
    
    If you cut the rod yourself you may have a short try another rod.
    
    Lights and other ac equipment like soldering iron can cause hum.
    
    CBN just put up a post about how to put a Capacitor on a piezo rod input to tame feerdback. It works check all wire, joints and pots.
    
    Use a metal tailpiece to ground strings, really helps on picking up thing like computer and monitor noise. Good luck.
- Tom T > John Sawyer July 7, 2016 at 8:00pm
  
  Joh
darryl kernaghan July 4, 2016 at 8:17pm

Make sure to turn off any flouro lights while you test,they are a great hum inducer
Michael Zack July 4, 2016 at 8:15pm

Update - I have uploaded a photo of my wiring. As you can see it is not very complicated. I re-soldered the connections with no change. I then disconnected the volume pot and connected the piezo direct to the jack - still getting a 60 cycle hum.
- Fomhorach > Michael Zack July 10, 2016 at 3:04am
  
  Use shielded wire to run from your pot to your jack
- Tom T > Michael Zack July 4, 2016 at 9:15pm
  
  So, your disc piezo builds are hum free as are your mag pickup builds. Hummmmmmmmm
  
  One has to ask why a disc piezo would not be affected by AC going to your amps, but the rod piezo is affected. Makes one take a suspicious look at that rod piezo. Could it be that the braided ground wire of the piezo rod isn't actually well grounded to the piezo rod? Might be tough to verify since the internal resistance of a piezo is crazy high anyway.
  
  I know it would be a pain in the butt, but if you un-soldered the positive lead ot rod piezo and then soldered a disc in the circuit then if you no longer had the hum, I would definitely think about dumping that particular rod piezo. On the other hand, if you still have hum with the disc in the circuit, then the pot or jack become suspect. Remember, none of these components are super high grade. There is likely a high degree of variance within a batch and sometimes the mismatches stack up and cause a problem.
  
  You've built guitars without hum prior to this so something is not functioning in this circuit. It's a matter of elimination. You can nail it with a little more testing.
  - Ron "Oily" Sprague > Tom T July 5, 2016 at 12:50am
    
    I'd take the volume pot out of the circuit temporarily. If you wire the rod piezo direct to the Jack, do you still get the 60-cycle hum? If so, then you have a bad ground with the braided part of the shielded wire. If the hum goes away without the vol pot in the circuit, then I'd suspect either (1) the solder joints on the vol pot, or (2) more likely you've created an antenna with the wires going from pot to jack. Twist them together down their entire length.
    
    I also notice that you use all three tabs on the vol pot. In many wiring diagrams and actual pots from guitars I've repaired, the ground tab is frequently bent over to contact the back of the pot, which has been previously scratched down to base metal to remove the thin plastic coating so as to improve the connection, and all the grounds are soldered there. This would remove one jumper wire and a solder joint that could be giving you trouble. Also, although I can't be sure from the pic, the braiding coming from the piezo needs to be soldered to ground.

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