Overhauling a late-50s Telefunken.......

One of the first things I found was the multi-voltage PT replaced with one marked "110 only". If I had left this alone, I might have had to add a bucking transformer later. Another "feature" of this transformer was that the leadouts were nothing more than the small-gauge winding wire. (fragile--must fix)

I pulled the transformer apart, added some more robust leadouts, and then added a "bucking winding". Here's the schematic:

This can be viewed as a winding that subtracts 10volts from the supply voltage and applies the result to the existing primary. OR: Just imagine that it is an extension of the primary......
The first step was to find the scale factor for the transformer---I wound a few turns of wire around the existing stack and came up with 4.1 turns / volt. So, to get a 10-volt winding, I needed 41 turns. Threading that much magnet wire through the gap** would have been a MAJOR chore, so I elected to disassemble the thing so I could add the new turn with my coil winder,

Once back together, a quick test gave me the "polarity" of the new winding. Here's the new transformer with new leadouts for primary and the HV. (the yellow leads are the buck winding.)


**the gap between the existing winding assembly and the inside of the core opening---this was less than 1/8"

Awesome, that is a great idea.
one question, what test you use to determine the polarity?

Whoever replaced the transformer may have been using "110V" colloquially, with the transformer primary actually wound for 115 or 117 V, which would have been the prevailing USA voltages when the radio was new in the mid-50s (plus, it presumably was a few years before the power transformer shelled out).

I have a household electrician friend who has been in the business since the 1960s who still refers to all in-house AC as either "One-ten" or "Two-twenty". I suppose he got this from his father, whom he worked for for years before taking over the shop upon the senior man's retirement. It actually was "110V" when his old man got into the business in the 1930s.

Not to say that the bucking winding isn't needed or desirable in this age of 124V convenience-outlet voltages, but the labeling ambiguity makes me wonder what is really needed voltage-wise.

Good points.....

Physically, the transformer looks "stock"--until you notice how it's wired.

My theory is that the repair person had an inventory of genuine Telefunken transformers---this one could have been intended for sets to be sold in the US.

I had considered putting a tap in the buck winding, so as to have more control---but I got lazy. If the voltage comes out on the low side, I can adjust the series resistor that gets added when the Selenium rectifier gets replaced with Silicon.

The AC source accommodations of German radios could probably provide enough content to write a small book on the topic. From early on (1930s at least) it seemed like the German radio industry's AC power strategy was to be "all things to all places" so they had overbuilt power transformers that could operate reliably on 50 Hz and a tapped primary that could be set to accept almost any AC service on the face of the earth.

Sometime in the late 1950s some unsung German design-to-cost engineer must have noticed that they had reached a "tipping point" with respect to their massive North American quantity exports- and that less-expensive (untapped primaries, less core iron) custom transformers could add pfennigs to the manufacturers' coffers for each radio shipped there. So "117 V 60 Hz" radios came into being, probably around 1960.

Other economic motivating factors were the big recession of 1958 (which mortally wounded Ford's Edsel and Philco); and the massive rollout of consumer stereo, the additional complexity of which necessitated cost-cutting in other product areas.

JMO, your reality may vary according to your favorite Dr.Who.

Was this a problem in need of a solution or a solution in need of a problem? I mean, did you measure the transformer voltages under load with nominal 120-volt line power applied to see if the transformer voltages were really in need of being bucked down?

Line voltages in most parts of the US have not been 110 volts since the 1920s. It was increased to 115 volts in the 1930s. By 1940, many areas had 117 volt power and that became the nominal standard around the country after WW-2. They raised the ante to 120 volts in the 1960s. Yet there were some who persist in referring to line voltage as "110 volt power" to this day. The fact that somebody wrote "110 volts" on a transformer in grease pencil doesn't mean it would be over-volted at 120-V.

Well, yes- that's entirely accurate as I understand the history of USA distributed AC. But it doesn't really diminish the basic idea of the thread, which is: I have a transformer rated for X voltage; how can I get it to reliably operate at X+Y line voltage? The bucking winding seems like a sound premise, although one which I personally would prefer to deal with by simply denying that it exists (hey, I'm a geezer!).

I prefer to call it a "defensive maneuver". I did not want to get the set all done and then decide I needed a bucking transformer. As found, the set did not give me the choice of turning a switch to correct for the actual line voltage.

"Whup! They it is!"

I have always been a proponent of bucking transformers when voltage reduction is needed; it's the best way to get a fixed voltage drop that doesn't change much with load current. It is also quite efficient and does not heat things up like a dropping resistor. The only thing I'd wonder about here is heating of the radio power transformer. I have yet to see a German radio power transformer which was not exquisitely engineered to have not an ounce of copper or steel more than it needed to do its job. A bucking winding subtracts a smaller secondary voltage from the primary, but the current through the primary remains more or less the same since the primary impedance is lowered by the de-magnetizing effect of the bucking winding. This is why the arrangement gives good regulation. But depending on how the other windings of the transformer are loaded, it may contribute to heating of the transformer. One would have to do some temperature runs with the transformer in the radio to see if the bucking winding makes it hotter or if the reduced voltage to the rest of the circuitry lowers the load currents enough that the transformer stays the same temperature or even runs cooler.

It is also possible to connect the additional winding in series with the primary, in phase, like an autotransformer. This simply adds turns to the primary. Since for any transformer, Vp/Np = Vs/Ns, it follows that increasing the primary turns by a certain percentage, while leaving the secondary turns unchanged, will reduce the secondary voltage by roughly the same percentage. But since there is no de-magnetizing effect this way the primary current is likewise reduced and the transformer **should** stay cooler. Voltage regulation, however, will be poorer than with the bucking connection.

"in series---in phase". That's the plan.
I don't see how the overall dissipation will be be adversely affected. I see it as equivalent to simply increasing the number of turns in the primary. To be sure, that does increase resistive losses.

German PTs marginal? I have yet to see the PT get even slightly warm on any German set that's gone through here....about ten 50s tube-type, two S/S, and one pre-war

PS: "in series, in phase" IS the bucking connection.....

The voltages in the two windings have to be out of phase in order to buck. If they are in phase they add and you get a boost transformer instead. (I am well aware that if you go by start/finish winding terminology it appears to be the opposite because of the inherent 180 degree voltage phase flip in the transformer, but you still have to get a voltage subtraction or it’s not a bucking transformer.)

I’ve fixed more than 50 European radios over many years and their power transformers tend to run normal temperatures for organic varnish and cloth or paper insulation systems. Yes I have come across some that burned up due to being left on after capacitors shorted; they’re not indestructible. But as that old varnish and whatever insulation continues to age and get closer to failure with every page turn of the calendar, I see no reason to push it harder than necessary.

Many ways to think of it....

the primary and the buck winding are IN phase---therefore, their voltages ADD to get the supply voltage

So let's get the terminology straight. "Bucking transformer," "boosting transformer" and "autotransformer" are similar, but not identical things. They are different winding connections which produce different outcomes.

Dots adjactent to the windings on transformer schematics by definition indicate the instantaneous voltage polarity. In other words, the dotted ends will both be positive or both negative at the same instant in time, relative to their other ends. So your schematic shows the two windings in series adding, with the power connected across the outside terminals. Sorry but you do not have a bucking transformer in that case, and calling it something it is not just confuses everybody. What you have is a transformer with few more turns added to the primary.

OK, this is thought provoking.

Right, the dots are joined for a conventional bucking transformer connection.
But Mark's winding increases the length of the 120 volt primary. This increase on the primary side reduces the secondary output by 10 volts.

I think it is a buck/boost winding, depending on how connected.

Ahhhh---nothing like a good semantics debate to round out the day.....

First---since I invented** this approach, I can call it anything I like.....n'est-ce pas???

All seriousness aside, this set indeed does not now have a "bucking transformer"----it has a factory transformer with an added conventional winding that provides a voltage roughly 1/10 of the primary voltage. This winding can do any number of things---including reducing the output voltages if the primary is connected to a service running at 120-125 volts. The effect is the same as if a true bucking transformer were added in front of the factory transformer.

It can--of course--also be used for boost---eg if the user finds themselves in Japan with just 100 volts supplied by the utility.

**patent application not yet filed---now's your chance to make a killing.......

Let us know the patent number when they grant it!

It's called habit. I say 110/220 or 120/240 depending on which one comes to mind at the moment.

As for the topic at hand, I check voltages before making any changes. The idea seems like a good one if there's no room for an external bucker.

RRM