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. :)

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

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!



    • Indoor Lighting Contest

      Indoor Lighting Contest
    • Make It Fly Challenge

      Make It Fly Challenge
    • Growing Beyond Earth Maker Contest

      Growing Beyond Earth Maker Contest

    33 Discussions


    10 years ago on Introduction

    How much power can you get from this device ? If i right you are increasing a reference voltage of the AC and actually it may cause some problems on some old devices because you are increasing a potential difference between ground and neutral. Could you add the circuit schematics please?

    10 replies

    Reply 10 years ago on Introduction

    If the primary opens, the "buck" voltage from the secondary goes away, and it becomes an overweight straight-through outlet box. :) --mark


    Reply 8 years ago on Introduction

    Would a surge suppressor/protector prevent possible damage? If so what would be the most important spec to look at?


    Reply 10 years ago on Introduction

    it will act like a ballast (resistor/inductor thingy and may produce a pulse of up to 220v (the same as a ballast uses this high voltage to help start the lamp) and destroy all the valves/cap.etc.!!


    If the primary opens, the primary becomes a very high impedance "load" to the secondary.  There's no telling what voltage the secondary will assume but it could be a significant percentage of the source.  Lets say for argument it assumes 30% of the source or 36V.  If the transformer secondary is a 12V winding then the voltage presented to the primary could be as much as 120/12 x 36 or 360.  Sparks could fly!  More could be present if the secondary is a 6V winding.


    I don't understand your argument (not being contrary, looking for enlightenment. ;)  I understand that an unloaded secondary can cause crazy high voltages to be reflected onto the primary, but it seems to me if the primary opens (in the middle somewhere, as opposed to being a complete coil with no load) the secondary will end up acting like an inductor, since no current can flow in the primary.


    My mistake. I forgot that both ends of the primary winding are tied to somewhere and thus constrained.  Even if it opens.  You are correct.  The secondary will become an ordinary inductor.
    Sorry for the confusion.


    Reply 10 years ago on Introduction

    The schematic is available on the GEOFEX website cited in the intro. In this design, the ground and neutral are "straight through," only the hot wire has any additional impedance.


    9 years ago on Introduction

    Looks fun, cheaper than using a variable transformer... :)

    Reminds me of something the fella at a local electrical repair shop told me about this town's power, up until the 60s, before the town was attatched to the national grid, half of the town was powered by DC, and the other half was AC, as we had two power stations here (part of one is still in existence, but obviously no longer used for power generation, it's a "professional clothing" warehouse these days)... :)

    Makes me wonder how we ever got along with such crazy electricity supplies here in the UK... :P


    10 years ago on Introduction

    do you know that if you do the opposite you will have up to 137v (make small tools more powerful)! also do you know that it must not exceed the current rating e.g. 20w transformer means max 180w out from the unit otherwise it might overheat and start a fire and or make the case live!


    Reply 10 years ago on Introduction

    Funny guy. Of course, the same concepts apply everywhere, but I have no idea if, for example, high mains voltage is a problem in other places to begin with... Plus, for all I know, the standard mains wiring colors elsewhere could be brown and blue. ;)


    Reply 10 years ago on Introduction

    It is,but electricians keep using red and black ARGH Also,on my UPS the outlet was wired up American colors while the input connector was wired up EU colors :)


    10 years ago on Introduction

    wait...what does this do? from what i see inside is a transformer being used as an inductor to make 110 volts...right?

    2 replies

    Reply 10 years ago on Introduction

    Actually the transformer secondary is in series with the line 120V, but out of phase. So, for example, 124 volts minus 12 volts will give you 112 volts net. This is similar to putting DC sources + to + , they effectively subtract from each other.


    10 years ago on Introduction

    My friend has an early 80's Sovtek Mig100 head that keeps inexplicably popping tubes... could this be the problem?