Really OT!


This matter is quite OT but I'm pretty sure that somebody could help me to understand, thanks to the presence on this board of many engineers, most of them experienced with "granma" or "granpa'" engineering equipments....

A fiend of mine found, while restoring a garage in a old building a strange (for me) and unusual electrical machine: a rotary DC DC transformer, made in 1945 (2nd worldwar time).

It is a small size object transportable inside a wooded box. I think it was used for rise a DC voltage from a 12V source (maybe the battery of a car)>>>> "input" 12V DC, 10A to (output) 500V DC (50mA) and intermediate 275V DC (110mA)..... apart any consideration about the working principle of this rotating machine, its efficiency (120W input and 30W output!?) and its tranforming rate, my personal question and curiosity regards the use of this machine, I mean which kind of equipment was designed for?
Where is requested a DC Hig voltage with a few mA?

I can think only to a valve radio transceiver (cathod voltage)....but I'm not sure

I tried to search for this equipmant on the net, but actually I was not able to find anything, what do you think about?


Edited: 19 May 2012, 1:28 p.m.


You have a very common device used, as you suggest, to generate the high voltage needed for vacuum tube electronics. These devices are small motor-generators, with a single common stationary field winding for both motor and generator end. In English usage it is called a dynamotor, while in German it is an Umformer. Typical efficiency is about 50 percent.

I collect vintage vacuum tube radio sets, especially those of World War II vintage. I own at least 200 dynamotors in various sizes and the equipment in which they were used. (They take up a lot more room than does a calculator collection.)

The devices were rather expensive, with a small 28-Vdc input/250-Vdc 65-mA output unit for a small six-tube receiver costing the 2012 equivalent of US $500 in the mid-1940s. That's why the most common use was in military electronics and high-quality commercial radio sets. Common consumer gear like older vacuum tube car radios almost always used much cheaper vibrator-type high voltage supplies.

Edited: 19 May 2012, 2:04 p.m. after one or more responses were posted



You beat me to it and with a much more detailed answer than I had :)

I used to see surplus Dynamotors advertised in the back of Radio Electronics magazine back in the 70's. I never had one but I did repair a number of vibrator type tube radios (mostly from 50's Ford cars) when I was younger. A funny side note. On the first Ford car radios that used "transistors" (around 1960), they only used them to replace the vibrator part of the HV supply. The rest of the radio was still a tried-and-true tube design. Within a year or two, the entire radio was transitory based.


There was one final stage of vacuum tube car radio design before transistors took over everything in the mid-1960s. That was a very popular receiver design starting in the late-1950s that used new space charge tubes designed for operation at 12-Vdc plate and screen voltage. Only the audio output stage used transistors in these models. There was no need for any voltage in the set above 12-Vdc.

I have a couple such radios that I picked out from an auto junk yard in 1965 that are still some of the best performing AM broadcast sets I've ever owned. Car radios back then always had an RF amplifier input stage and were the most sensitive yet selective AM entertainment sets being made 50 years ago. Kind of ugly, though, if used out of the car! :-) Mine were made before the early Cold War
CONELRAD system was replaced in 1963. CONELRAD station markers at 640 and 1240 kHz are shown on the dial.

Edited: 19 May 2012, 2:25 p.m.


Now that you mention it I do remember coming across some some early 60's tube car radios that had a transistor A.F. Amplifier output design. I did not realize that the tube plates ran on 12 volts. And I certainly remember the Civil Defence markers at each end of the AM dial. I mostly worked on restoring 1955-1957 Ford "Town and Country" motorized auto-seek car radios for my dad. As you mentioned, most of these tube designs were very sensitive. The "Country" selection of these radios would attenuate the antenna input so the radio would not stop on strong stations during seek.

I have always been amazed at the ingenuity of electrical engineers that we're able to design so much functionality with so little. This applies equally to the history of technology whether it was radios, tv, spacecraft or of course calculators.

Thanks for the links. Remembering this has been a real trip down memory lane.


Mike, Steve, thank-you both, here's a pict of the beast, excuse me the poor quality!

When I find a couple of hours I' will try to clean and to check it...if I move the rotor by hands it's quite free, but signs of wear and time are evidents, who knows if it will work again?

I wrong the year, 1944 and not '45!

Thank-you again, newer and newer I should find for a "dynamotor".... on the label it's written rotary transformer!

And the only rotary transformer on wiki is



Interesting design, there are two commutators on the output side so that would suggest that the armature has 3 separate windings one for the 12 motor and then one each for the two output voltages.

A few years back I toured a British designed Tribal class destroyer and on those most of the electrical system was DC and it had two motor generator units on board one to supply some AC power when at sea and another to supply the DC power to the ship when in port.

When I first started in the computer service business more than 30 years ago, I remember seeing mainframe systems that had multi hundred amp power supplies in them, and they where attached to a motor generator set that had a 60Hz three phase motor driving a 600Hz alternator that supplied the systems AC input.


It will almost certainly work again, at least for a short while, if the brushes are in good condition. You can check brushes by unscrewing the brush caps and withdrawing the brush/spring assembly, at which time you can check for adequate brush length. Do this one cap at a time, because you don't want to get the wrong brushes, even the one for the opposite polarity, mixed up. It's even recommended to avoid rotating the brush in the holder when replacing it.

You also will want to put a little lubricating oil at the bearing assemblies on the armature (rotor) ends before test starting. Of course, the voltages generated by this device are potentially lethal, so be careful after start-up!

If you were going to run this machine for any significant time, the end bearings would need clean-out of old lubricant and repacking with new light grease. There are some suggestions in the article associated with the "dynamotor" link in my first reply above.

It would probably be best to operate your unit for only a short demonstration, all things considered, unless you will be doing a comprehensive proper cleaning and bearing repack. It will have a rather heavy start-up surge until it comes up to speed.

I don't recognize your unit as one used in US service, yet it appears military due to the Fungacide treatment note, common on WWII military gear destined for use in the Pacific theater. Are there any markings on the box? As you stated, it has obviously one low voltage input winding/commutator/brush set, and two high voltage output windings/commutators/brush sets.

Enjoy! I love the old vacuum tube electronics. It's amazing what was accomplished in equipment that used very few active devices. Maybe I can find a 1950s Cold War S.A.G.E. computer some day! :-)


..."input" 12V DC, 10A to (output) 500V DC (50mA) and intermediate 275V DC (110mA)..... apart any consideration about...its efficiency (120W input and 30W output!?)...

The efficiency is much better than that. The rated output power is 25W from the 500V winding and 30.25W from the 275V winding. So it's 120W input producing 55.25W output...46 percent rated efficiency. That's very close to the 50 percent efficiency typical of these devices. Actual in-service efficiency may be even better than rated.

I have identified your Rotary Transformer as the one used in the Supply Unit Number 1 for the WWII UK Wireless Set Number 19 Mark II. I have most of a Canadian version of this set in my collection.

Edited: 19 May 2012, 4:24 p.m. after one or more responses were posted


Thank-you Mike

about the maintenace:

the only and the first thing I've cheched was, yesterday, one of the brush, just one and it seems in a excellent shape!

I don't remember any label or writing on the box and anything related to the U.S. or the Brithis Army, but tomorrow I'll find the time to check it much better.

about the efficiency:

I did not consider the two output toghether and I've calculated only each one at time....apolgize, thinking like it was a static machine (electrical AC transformer I mean)with only one winding in the output and intermediate feeding points

Apart electrical and mechanical losses, quite half of the input energy is transformed in mechanical.... actually is a motor (dynamo) it will be interesting to calculate the output torque available.....

Edited: 19 May 2012, 4:17 p.m. after one or more responses were posted


Hi aurelio,

I have identified the equipment your device was used in. Please see my edited posting above yours.



Hi Mike, you are great!!!!!

SI, it is!




I have identified your Rotary Transformer as the one used in the Supply Unit Number 1 for the WWII UK Wireless Set Number 19 Mark II. I have most of a Canadian version of this set in my collection.

Good grief! I've actually used a 19 Set (as we called them). Back in the late 60's, early 70's, I was into amateur radio, and our Radio Society got together for a meeting with the local Territorial Army Signals unit. Even as late as this (1970 or so) they were still using the 19 set, in the back of a Land Rover. I would hope that the standing army was better equipped!


--- Les


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