Introduction: Vintage Voltage for Old Equipment

I work on a lot of vintage tube/valve guitar amps, and the older ones expect mains voltage somewhere in the 115-117 VAC range. Modern North American mains usually are significantly higher these days, often in the 124-126 volt range. Using higher-than-designed-for mains can cause all sorts of troubles for old equipment, including too high heater and B+ voltages (which may threaten marginally spec'd capacitors).

Wouldn't it be nice to have an inexpensive little box that cut the mains voltage down by 5 or 10%? Well, here it is!

This instructable shows how to make a "vintage voltage" adapter (as described on the GEOFEX website), using a transformer and parts easily obtained at the local "big box" hardware store. I highly recommend you read the GEOFEX article for background before continuing.

NOTE: This project involves dangerous and potentially lethal mains voltages. Do not attempt this if you do not have experience working with 120V.

NOTE: Most solid state equipment made from the 1970s on has no problem with slightly higher mains voltages.

Apologies, this article is North-American-centric, because I have no experience with other voltages worldwide. :)

Step 1: Obtain the Parts

Below are the parts I got at the hardware store:
  • an outlet box
  • an outlet extension box (no bottom)
  • an outlet box cover (with one switch and one outlet) (1)
  • a replacement "tool power cord"
  • an outlet
  • two cable clamps

Other parts you'll need to mail order or locate otherwise include:
  • a center-tapped 12v6 transformer, 3 to 5 A secondary (e.g., All Electronics TX-125)
  • a fuse holder (e.g., Mouser 576-03453LS4X for 3AG size fuse)
  • A fuse, suitably sized for your transformer (3 to 5A).
  • wire, screws, nuts, lock washers, solder, crimp-on-connectors, etc.

The crimp-ons are optional, but make for a neater job.

(1) The ideal cover would be one outlet, one "blank," but I was unable to find one.

Step 2: Fit the Transformer Into the Outlet Box

Use the transformer to mark mounting positions in the outlet box, and drill holes appropriately sized for your hardware and transformer.
  • Mount it to one side to allow room for the power wires, fuse and outlet.
  • Allow sufficient space so that the exposed terminals are in no danger of touching the side of the box.
  • Use lock washers so that the nuts will not come loose.

Step 3: Mount the Fuse Holder

Using a standard clamp, mount the fuse holder as shown.

Using the clamp for the fuse holder is admittedly a bit of a kludge, but these boxes are punched all the away around, and it is difficult to drill large holes in the punch slugs without dislodging them.

Be careful tightening the fuse side clamp, snug so it won't come loose, but not so tight that you crack the fuse holder housing. Some "Plumber's Goop" might be a good idea to fix it in place..

Step 4: Attach and Connect the Cord and Fuse Holder

  • Using a standard clamp, mount the mains cord opposite the fuse holder as shown.
  • Connect the hot (black) wire to one fuse holder terminal.
  • Connect the other fuse holder terminal to one of the transformer primary lugs.
  • Connect the neutral (white) wire to the other transformer primary lug.

The fuse value should be no more than the secondary current rating, 5 A in this example.

Step 5: Figure Out the Transformer Secondary Phase

One connection will add (produce higher voltage) and one connection will subtract (produce lower voltage). We want to determine which is that latter connection.

This process is described in detail at the GEOFEX site mentioned earlier, but basically:

  • Connect the Hot (black) side of the primary to either end of the secondary.
  • Plug in your partially assembled VVA. Measure the voltage across the primary, e.g., 125 VAC.
  • Measure the voltage from Neutral (white) to the other (unconnected) end of the secondary.

Unplug the unit!

If the measured voltage is less than than the primary voltage, you're done this step.

If this measured voltage is more than than the primary voltage move the hot connection to the other end of the secondary.

Step 6: Break the Hot Outlet Link

Using needle nose pliers, grasp the link connecting the two gold colored screws on the outlet.

Bend it back and forth until it breaks off.

Step 7: Connect the Outlet

Note: Make sure to allow enough outlet wire length to include the extension box, which will fit on top of the main box.

  • Connect a white wire from Neutral to one of the silver colored screws on the outlet.
  • Connect a wire from the transformer secondary center tap to one gold-colored screw.
  • Connect a wire from the remaining unconnected transformer secondary tap to the other gold-colored screw.
  • Connect the mains cord green wire to the green screw.

Step 8: Connect the Extension Box

Firmly screw the extension box to the main box.

Step 9: Mount the Outlet to the Top Plate

Using the supplied hardware, mount the outlet to the top plate.

Add a "star washer" under one or both screws to ensure the box itself is properly grounded.

Step 10: Cover the Unused Switch Hole.

Cut a piece of sheet tin about 3 inches by 3/4 inch in size.

Drill two holes in the tin spaced as per those in a normal light switch.

Mount the tin to the cover to block the open switch hole.

Step 11: Mount the Top Plate to the Box

Mount the top plate to the box.

Plug it in and measure the voltages at the hot (smallest) slot of each half outlet with respect to ground (the box itself).

One should be 6 to 7 volts less than the current mains value, and the other should be about 13-14 volts less.

Step 12: Label the Outlet

Add labels so you can remember which half outlet is which.

You're done!