18Watt.com

Hi guys,

I wired up an A/B box out of spare parts lying around. 20awg teflon wire, a Hammond box, metal switchcraft jacks, and a Carling DPDT.

I'm tried it with my two Marshalls and also using it to switch between the two channels in a standard 18w. I'm getting a ton of hum.

What did I do wrong? Thanks!

http://www.fulltone.com/qaframe.html

Click on building your own a/b box.

Norm

Hi Norm,

Thanks, but unless I'm missing something, I think I have it wired up just like that (sans LEDs).

It sounds like you have a ground loop happening. Both amplifier grounds are connected together at your A/B box, right? Perfect scenario for a ground loop.

Can't say what the deal is with the channel switching...

how can I rewire it to eliminate the ground loop? Would you expect moving the ground from "A" directly to the switch to make any difference?

Hey PJM...

I checked your web site.. Awsome work! Great info! I LOVE your home made tube coolers! Great job!

(sorry Brian, for my OT reply - hope you get your A/B box in order - COUSE I WANT TO MAKE ONE AS WELL ) - let us know if it worked!

Gabi.

You might try only grounding one jack to the aluminum enclosure and isolating the rest. Your signal ground will still be connected via the switch, but they won't be connected through the box.

Ethan

No prob, Gabi. I just had fun looking through his site, as well. VERY cool.

Ethan, pardon my inexperience, but what do you mean by "isolating" two of the jacks not grounded via the chassis directly? Run a ground wire from the middle/bottom switch lug seperately to each?

THANKS!

Brian,

Typically, the sleeve of a female jack will make contact with the enclosure or chassis. This creates the possibility of looping grounds through the chassis itself. You can, however, buy insulating bushings made of plastic or fiber that will isolate the jack from the chassis. In this way, you can ensure that your ground is only coming from one source. Many plastic enclosed jacks have this feature built in.
Hope that helps to clear it up.

Ethan

Ah yes! Thanks Ethan. I have some spare (plastic) Cliff jacks I can swap out for two of the jacks.

Thanks very much.