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.

Comments

[twocvbloke]

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

[maresal]

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?

[ste5442]

I think the max power you can consume from one of the outlets depends on the rating of the secondary winding on the transformer.

Do you know what happens if the primary goes open circuit (a common failure mode of mains transformers)?

PCBPolice Electronics Forum - we need some users....please!

[mhuss]

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

[BasinStreetDesign]

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.

[mhuss]

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.

[BasinStreetDesign]

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.

[ste5442]

With the full voltage presented to your vintage stuff?

[12V]

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.!!

[mhuss]

yes

[Taller76]

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

[mhuss]

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.

[12V]

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!

[Derin]

I don't have any experience with 120V because I use 220V ;)

[mhuss]

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. ;)

[Derin]

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 :)

[ReCreate]

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

[mhuss]

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.

[ReCreate]

sigh...rephrase that last part

[pfingers]

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

[mhuss]

Not likely. IIRC, those old Sovteks use fairly high voltages internally, and tubes manufactured these days are not quite as rugged as those made in the 70s and before. I suggest he take it to a tech and get it checked out. --mark

[nolte919]

Great Instructable. I don't suppose you'd give the Digikey part numbers for the fuse and fuse holder as well as the All Electronics part number for the transformer. Especially since All Electronics doesn't list transformers like "12v6" but rather like "12 V.C.T."

[mhuss]

I meant to -- I'll try to remember when I have the data handy. :)

[mhuss]

The mains voltage being 5 to 10% high is not an issue at all for most or all solid-state equipment.

Some vacuum tube/valve equipment, due to the high internal DC voltages used, are potentially subject to overstress.

Relatively little tube equipment has been made since the 70s, mostly guitar amps and high-end stereo equipment.

Some folks might use this box to deliberately under-volt their tube guitar amp to "brown" their sound. lol

--mark

[guitarman63mm]

Is this insinuating that my 4 track and stereo setup isn't safe in its current home? They're both from the seventies....a Teac A-2340 and a Sony STR-6055, if that helps.

[perlpower]

This is more for 20's-50's radios, your 70's stuff (even if tube) should work just fine.

[perlpower]

Sorry not just radios, but stereos too. Still most stuff from the seventies should work fine

[gmoon]

Nice, I'm adding this to the "Vacuum Tube" group... Actually, a lot of older tube EQ expects only 110V AC. You could do this with diodes, too. But since a high percentage of that EQ would benefit from an isolation transformer, this is great thing to have around.

[gmoon]

I just checked out the GEOFEX link, and that transformer not wired to isolate. Oh well.

[p9inter]

If i was receiving electricity at a voltage higher then 120 volts I would complain to the power company. Most equipment and components are only rated to 125 volts.

[mhuss]

That's exactly right, the secondary is out of phase (by selection) with the primary, and connected in series, so that the voltages buck (cancel) each other.

[kelseymh]

Excellent, well written Instructable! Besides the how-to details, you cover safety issues clearly but not excessively, and incorporate appropriate engineering safeguards into the design.

[Phil B]

As best I understand it, this does not work so much like a traditional transformer, but the current in the primary and secondary oppose one another and cause a reduction in available voltage at the output. Someone correct me if I am wrong, please. Thanks.