Question about an Ampeg V4 (late 70s) - polarity switch and getting a shock

[James_E]

Just got one of these off CL locally here. Sounds great!

My question is related to the polarity switch, the 3 prong adapter for power (looks like this was one with 3 prong originally) and an issue I had.

I had it on, playing my bass, and the polarity switch was down. My markbass head was also plugged in to the same power bar as the v4, but was not turned on. When I touched the metal top of the markbass head while touching the strings on the bass, I got a shock.

I then proceeded to follow the instructions in the v4 manual (found one online) that says to set the polarity switch to the position that has least amount of noise with a cable plugged into the instrument input but no instrument on the other end. This position for me is UP, not DOWN like I had it to start.

Since I don't like being shocked.. I have not redone the "touch the strings while touching my markbass head at the same time" experiment.

Questions are: is this expected if I had the polarity switch wrong? Does the polarity switch do ANYTHING if the power supply is 3 prong? If not.. then why is it there?

I'm going to get a tech to look at the amp anyway, but was curious what you experts here think.

[jastacey]

If the amp had a polarity switch, it most likely didn't originally come with a grounded 3 prong plug .... get it serviced promptly ... IMHO

[James_E]

Thanks, I will be taking it to a tech.

Note also that the back outlet for powering other devices is also a 3 prong.. I know they made these with 3 prongs from the factory late in the 70s. I'm so confused.

[coreyfyfe]

Originally Posted by James_E

Thanks, I will be taking it to a tech.

Note also that the back outlet for powering other devices is also a 3 prong.. I know they made these with 3 prongs from the factory late in the 70s. I'm so confused.

My V4 - I believe it was a 72 or 73 - had a 3 prong outlet on the back/3 prong cord and a polarity switch as does my 74 SVT. Maybe by that time they had gone to 3 prong plugs but not all outlets were 3 prong so it was still included? If you're getting shocked it could be bad grounding in the wiring - either of your amp or whatever you're plugged into. Definitely have a tech give the amp a once-over to be sure.

[beans-on-toast]

Many modern amps have polarity switches and grounded power cords. For example, have a look at the back of an Ampeg CL or VR. It is also detailed in their manuals.

Your late 70's amp did originally come with the three wire power cord. Earlier V4 models didn't. Set the switch so that you don't get a shock.

The capacitor connected to the polarity switch could be leaking and causing your problem.

[James_E]

Tried the test quick tonight with polarity switch UP. No shock.

Still going to get this into a tech though.