DIY bucking transformer for vintage amps

[Heemis]

Sorry for the amp question, but I just need a quick confirmation.

My house AC voltage is 125, which my vintage princeton hates.  I want to build a voltage adapter a la this article at geo: http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

I found this transformer on Mouser which I think will suit my needs: http://www.mouser.com/Search/ProductDetail.aspx?R=VPS12-2000virtualkey55310000virtualkey553-VPS12-2000

Just wondering if anyone can confirm that this will work as I expect...

Also, what value fuse should I use?  The value that the transformer is rated for, or the value of the amp being plugged in?

[Heemis]

Actually, I think this one will be even better, bringing 125 down to 117 on the nose.  The one thing that concerns me is the 115ac primary rating... does that matter?

http://www.mouser.com/Search/ProductDetail.aspx?R=166M8virtualkey54600000virtualkey546-166M8

[R.G.]

What is the AC input power rating shown on the plate on the back of your amp?

[Heemis]

Both of the vintage amps I would be using it with specify 117 volts.

[R.G.]

But how many amperes?

That tells you about the total current the bucking transformer will need to provide on its secondary.

[Heemis]

Oh sorry!  I just noticed you said input power, not voltage.  It's .7 amps.  The transformer I'm looking at now is rated for 3amps at 8.5volts on the secondary.  I was thinking that would be enough power to run my Princeton (.7 amps) and my Pro Reverb (1.5 amps) safely off the same line.  In this case should I use a 3amp slo-blow fuse in the box before the bucking transformer?

[R.G.]

Item 1: Do NOT try this unless you already know how to do AC mains power wiring safely. You could die, start a fire, or kill or injure someone else, even years after the mistake.

Item 2: I prefer to make the amplifier match modern mains, not to make an adapter box. If they were my amps, I would put a small bucking transformer for just the amplifier inside each amp. This considerably simplifies issues of enclosures and safety wiring, fusing, and so on for the added transformers. This is my personal preference for my own amps.

Item 3: If you choose to do one external box, yes, you will have to separately fuse the external box, as well as making it meet all safe wiring requirements.

[Heemis]

I've done a bit of AC mains wiring and I am very diligent in terms of safety, it will be wired properly!

I guess I like the idea of a dedicated box so I can use it for different amps and take it with me for rehearsal/gigs.  Also the ability to switch between the full 8.5v drop and 4.25v drop from the CT is appealing as well if I encounter voltage lower than the 125v at home (rehearsal space voltage is around 120).

Pardon my ignorance, but I guess my main question is why do all the transformers I am looking at specify 115v on the primary when modern voltage is clearly higher than that?  Why are the primaries not listed as 120v?

[R.G.]

I've done a bit of AC mains wiring and I am very diligent in terms of safety, it will be wired properly!

Good! I hate to lose you guys once I have you all trained.   

I guess I like the idea of a dedicated box so I can use it for different amps and take it with me for rehearsal/gigs.  Also the ability to switch between the full 8.5v drop and 4.25v drop from the CT is appealing as well if I encounter voltage lower than the 125v at home (rehearsal space voltage is around 120).

Then build a big one. Go for at least 5A, better 10A. Build it into a box with a main switch, indicator light and fuse for the box, and then a duplex outlet in the box. Use a 12Vct/10A transformer, and hook the secondary in series/aiding with the primary, take the outputs from the CT and the top of the primary. This makes for an autotransformer which will pass 10A easily enough to the outlets. The output voltage gets a little smaller. With a 120:12Vct hooked this way, you have 22 units of "voltage" hooked in series, and you can pick 21/22 or 20/22 of the AC power line as an output. That's 119 or 113V out for a 125V line in.

Pardon my ignorance, but I guess my main question is why do all the transformers I am looking at specify 115v on the primary when modern voltage is clearly higher than that?  Why are the primaries not listed as 120?

Largely because the world got sloppy about what line voltage is, at least in the USA. They got used to putting 115V on everything, still do. The plain fact is, such trannies have to tolerate at least 125Vac without melting down, no matter what they call the AC primary nominal. In the USA at least, the power companies put out anything between 110 and 125 (and I've seen 128 on my power mains) and call it "in spec", in spite of whatever standards exist. Don't sweat the "115" designation. It should work on 120. That's one advantage of the series-aiding secondary across the entire AC line as above. The actual primary of the trannie used as a buck runs at a lower volts/turn - 113 in the case of the 125V in and a "120:12" transformer. This changes a little - not much - if you buy a 115:12Vac transformer for a bucker.

[Heemis]

Thanks so much for clearing that all up, I tend to over-analyze things before I start a project.

I was actually looking at some electrical boxes in which I could fit the tranny and a duplex outlet, seems like the best option...

Thanks again!

[PRR]

> I think this one will be even better, bringing 125 down to 117 on the nose.  The one thing that concerns me is the 115ac primary

If it is nominal 115V in 117V out, then it will convert 125V to 127V. (It does NOT "force" 117V out, it is a "lever" with a 1.017 ratio.)

Wired in reverse, 125V in makes 123V out. Not enuff diff to matter.

While there are many-tap step-down transformers, they are uncommon (costly).

If you find a 8VAC part (you did), you "know" you can wire it for 125V in to either 117V or 133V out (so *check* it before you plug that amp in!!).

The 166M8 is rated 3 Amps on the 8.5V winding. It should be safe for 117V at 3 Amps or 350VA out. A 2-6V6 Princeton will suck 60W-80W from the power line. This part is 4 times bigger than you really need. 8VAC 1A would work for the Princeton. You may want to be generous in case you ever find a Twin, but 2A would be ample.

A 12VAC part is easier to find. Today at 125V it will give you 113V. This was a *very* common outlet voltage in The Old Days, and the Fenders ate it up. Also as you can see, wall voltages have been rising 0.2V per year for a century, so in a couple decades we will have 129V and a 12V knock-down will be 117V.
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> why do all the transformers I am looking at specify 115v

You have been finding Hammond. They have not made radical changes to their designs since the 1950s, when 115 was still quite common. Also much of the rest of the world has used 230V; with dual-winding this is two 115V windings.

The "115V" parts of today WILL stand 125V 130V. There's a limit, and Hammond still uses the old turn-count, but the core iron has improved even faster than line voltage has gone up, so they are very conservative.

[PRR]

You can also try a Variac. Weber VST used to stock them?

These can go 0% to 100% (or 110%) of the incoming voltage. So your little 7% knock-down can be a bit twitchy to find. You probably want one with a voltmeter (if you can). Don't trust the voltmeter, check it with your bench meter and mark the "117V" point.

An advantage is that you can go to my old office (108V) or a rural Texas farm-house (sometimes 130V), and dial-up your 117V.

[Heemis]

Thanks for the additional info PRR!

I think I'm gonna split the difference and go for a 12v 2amp part, that should give me enough options for now and in the future, I do also have a Pro Reverb and Bassman from the same era, so those should be happy with the 2A rating.

[R.G.]

I had to resist saying "variac" for several minutes. IMHO, variacs are a Bad Thing for musicians. They are great for finely setting conditions, and have a wide range of settings - some of which are damaging.

I think that discrete switch settings for AC power are better for musicians. Fewer chances for error when the musician needs to be concentrating on music, not technology.

Appropo of which, it's worth looking here:
http://www.mpja.com/Digital-Panel-Meters/products/52/
It is handy to know that you can get a digital panel meter suitable for installation inside a voltage changing box for under $10.

[Heemis]

I had considered a Variac, but like you say RG, I am really looking for a "set it and forget it" type of thing.  Also, scary to think of accidentally boosting the voltage by bumping the knob, especially at a gig where things are not always very controlled.

I was actually looking through my parts bin for an analog voltage meter to maybe throw in the box (found one but alas it was DC).  These digital meters are definitely a good option though!  I have one of those "kill a watt" meters for quick voltage and current readings, but it would be super handy not to have to lug it to gigs.

[KazooMan]

I made a bucking transformer to get the proper voltage to run a 60's Klempt tape echo.  It worked great.  Then I was checking voltages while working on another unit and found them off again.  It seems that the line voltage around here varies quite a bit from day to day. 

Check the line voltage in your area to see if it is constant throughout the day and over a period of several days.

[Heemis]

I've measured it many times, and it rarely shifts more than a volt or two.  I'm mostly just looking to get the amps into the correct range to keep the heater voltages correct.  My Princeton Reverb is running almost all of it's original tubes still, and I'd like to prolong their lives as much as possible!  Plate voltage is also a concern with the Princeton, which I'm rectifying now (no pun intended) with a 5Y3GT rather than the original GZ34.  Plate voltage is less of a concern for my Pro Reverb which is running Sylvania 6L6GCs which can handle the ~460vdc on the plates my wall voltage + a GZ34 is throwing at them.

[PRR]

> resist saying "variac" for several minutes.

Took me several minutes to remember the word.....

Agree. Good bench tool, bad performance tool.

> an analog voltage meter to maybe throw in the box (found one but alas it was DC).

For this purpose, a rectifier turns AC into DC. If you found a 150V DC meter, just put a full-wave-bridge rectifier unit between wall and meter (and stand WAY back for first smoke-test). If it is a lower voltage unit you will need some resistance too.

FWIW: HomeDepot still carries the classic $9.98 needle-type passive VOM. Leave it on 150VAC. Preferably back-wire the probe jacks and seal the fronts against finger shock. Nice big scale. Usually pretty accurate. The 1mA loading is insignificant here. Main drawback is that it does not like to be banged-around.

> digital panel meter suitable for installation inside a voltage changing box for under $10.

Cute.

Even cuter is a suppresed-zero meter which goes 90V-130V or so. Radio Shed used to have a needle-type plug-in which spread 100V-140V over half the dial. With bare movements you can do such a thing with a Zener.

FWIW: I found a combo Volt/Amp meter with backlight for $15 (dropped to $13 after I bought two to meter my house).
ebay link
The panel unit is well made. The CT doughnut is funky home-made, and I did some beefing-up before I hid it near my fusebox. Would be less problematic in a single box (without remote display). The Volts is meant for 230V; it reads my 125V-108V fine but only to the nearest Volt. Which is really good enough for jazz, but I do miss having the tenth-Volt jiggle of my Fluke. This one the Amps goes to 100A, he has others for 50A. On paper the CT accuracy should lessen at very low current, but mine seems spot-on at 10A and reads sub-Amp currents believably. Shipping is free but takes a couple weeks.
meters
DIN-rail Digital AC140-300V (really 100V-350V) 0-100A 'LCD' (really LED) -- fairly finger-safe without box

[Heemis]

So I finished up my box this weekend, got it all into this 4x4 electrical box.  The switch is a 3-way light switch, which is essentially a SPDT, so it switches between the center tap and the full secondary.



Oddly, it's dropping the voltage quite a bit more than I expected.  At the -10 setting I'm dropping roughly 13-14 volts, and at the -5 setting it's dropping about 7-8 volts.  This is the transformer I used: http://www.mouser.com/Search/ProductDetail.aspx?R=166L10virtualkey54600000virtualkey546-166L10

Any idea why it's dropping so much more voltage than it's supposed to?

Some more pics:

[R.G.]

So I finished up my box this weekend, got it all into this 4x4 electrical box.  The switch is a 3-way light switch, which is essentially a SPDT, so it switches between the center tap and the full secondary.
...
Oddly, it's dropping the voltage quite a bit more than I expected.  At the -10 setting I'm dropping roughly 13-14 volts, and at the -5 setting it's dropping about 7-8 volts.  This is the transformer I used: http://www.mouser.com/Search/ProductDetail.aspx?R=166L10virtualkey54600000virtualkey546-166L10

Any idea why it's dropping so much more voltage than it's supposed to?

Nice work. If I were quibbling, I'd say to put a neon indicator on it so there's a light on when it's alive, but that's just my personal paranoia.

As to why it's dropping more than it's supposed, I can speculate it's due to either the internal hookup - there are a couple of ways to do this - or the fact that secondary voltage is higher with no load than under heavy load. In this case, the secondary voltage is subtracted from the mains voltage, so for some hookups, the secondary will subtract less from the mains. Depends on the exact hookup.