Fender Showman hum when connected to device with AC power

[bbbbass]

I have a '67 Showman. I installed a three wire AC cord (and I've verified it's installed correctly: brown goes to the fuse holder to the power switch to PT black wire and blue goes to the other PT black wire and the green goes to ground. All this is confirmed with a meter). It works fine with a passive guitar plugged in directly or with battery powered effects. However, as soon as I plug in my Alembic Series II equipped Starfire bass, or any other AC powered effects, I get hum. An Ebtech hum eliminator cures the problem, but I'd rather not use it.

Any ideas?

Thanks!

[HotBluePlates]

... as soon as I plug in ... any other AC powered effects, I get hum. An Ebtech hum eliminator cures the problem, but I'd rather not use it.

Everything is working exactly as it should.


When you plug in a.c. powered effects, your cable's shield/ground connects the chasses of the two pieces of gear together. Now ground currents have 2 choices of path to follow to get back to outlet ground: through the amp's power cord or through the effect's power cord.


The only safe solution to breaking the big ground loop just created is to isolate the ground of one piece of equipment from the other. The Ebtech does this with an isolation transformer in the path of the power cord. That's why it stops the hum.


Theoretically, you could modify one piece of gear to have a ground-lift, but that's a hassle in an amp which wasn't designed from the start to have a ground lift (the entire power and signal circuit of the amp has to be isolated from the chassis with only one connection at a lift switch bypassed with a cap and diode or resistor).


You could also theoretically move the transformer to be in-line with one of your connecting cords (i.e., bass into splitter, 1st cord from splitter into effect, 2nd cord from splitter into transformer then 3rd cord from transformer to amp).


So the existing Ebtech is very likely the easiest solution.

[uki]

What about the ground switch in the amp, maybe that can do the trick, have you tried that? But don't forget about the possible shock when touching a microphone or some other device.

[HotBluePlates]

What about the ground switch in the amp, maybe that can do the trick ...

Now that there's a 3-wire power cord, the old Ground switch does nothing useful. That is purely a relic from the 2-wire power cord (and uncertain outlet orientation).

[bbbbass]

What about the ground switch in the amp, maybe that can do the trick, have you tried that? But don't forget about the possible shock when touching a microphone or some other device.

The ground switch is now an output impedance switch (Mercury Magnetics output transformer).

HBP, I hear what you are saying, except that none of the rest of my gear has this kind of problem to this degree, including the '67 Bandmaster which has exactly the same kind of 3 wire AC cable. Also, if my Alembic power supply is plugged into the same power strip as the Showman, then the difference in ground potential should be minimal enough that it should be barely audible.

Thanks for chiming in.

The search goes on....