I tested the transformer with a couple of diodes on the HV winding, and guess what? Half-wave rectification. So, Jeff at Gear Page was right, with the way this thing is wound, it will only make half-wave rectification when hooked up to a rectifier.
The Dukane amp that this xfrmr came out of was running half-wave its whole life because of a mistake at the factory.
I dissected the windings of the transformer (photo at right) and unsoldered the tiny wires (using my very valuable magnifying hat) and then connected the center tap to the CORRECT ends of the windings, which Joe (it was his first day) had screwed up in 1965.
Behold! 736 VAC across the HV winding. Excellent. A new transformer is > $100.
At this point I had to start remembering transformer physics and stuff. The picture on the left over there is how a typical HV tap is designed for use with a tube rectifier (or a solid-state rectifier, for that matter). The secondary winding has a center tap exactly in the middle of the winding, which results in the two voltages to the center tap (Va and Vb) being equal to each other, but of opposite phase; I was seeing the correct Va and Vb of 350 Volts, but across the whole winding I wasn't seeing 350 + 350 = 700, I was seeing 350 + 350 = 0.
I started thinking about polarity. engineers use the "Dot Convention" to indicate polarity on transformer windings. The polarity has to do with the direction that the winding goes around the core. The "Dot" is placed on the "+" end of the winding. If the primary winding and the secondary winding are in the same direction, i.e., wound the same direction around the core, then they are "in phase" - the voltage sine waves will have no phase shift, as in the top figure of the drawing on the left. If the secondary, for instance, is wound in the opposite direction, then the polarity will be opposite, and the Voltage sine will be exactly 180 degrees out of phase, as in the bottom of the drawing on the left.
In the case of a center-tapped winding, like my weird transformer, the two halves of the HV winding should be equal voltage and opposite phase, and their combined voltage should equal Va + Vb. But the Dukane transformer refused to do the math.
At this point I enlisted the help of the wizards over at The Gear Page, and Jeff suggested that what I thought was a center-tapped winding was actually two identical windings joined in series, with a "center tap" where the two windings came together, and that the polarity dots of each of the windings were either both at the center tap, or both at the outside taps.
Jeff was right, that explains it. With two separate windings, with opposite polarity, and with a "center tap", you will see exactly the behavior I was seeing. The two winding are each delivering 350 VAC, but when you measure across them, the voltages cancel each other out, because they are equal and opposite.
If you have read this far, then I know what you are thinking, because I was thinking it too: then why don't the voltages cancel on the truly center-tapped single winding, like the second picture above? The answer is because it is a single winding, and it has just one polarity - so when you are reading across the whole winding, you are seeing the whole (Va + Vb ) voltage.
Look at the series of drawings on the left. FIG. 1 is an in-phase polarity of a primary and a secondary.
FIG. 2 is two separate but equal windings on the secondary. The bottom one is in phase with the primary, and the top one is out of phase with the primary. They both will read 350 VAC.
FIG. 3 is the perplexing and bothersome Dukane transformer. The two windings of FIG. 2 are now joined in series with a "center tap". We see 350 VAC across the top winding and also 350 VAC across the bottom winding, but across both windings in series we see 0 VAC, because the polarity of the two windings are exactly opposing each other. ( Since the windings are not perfect, we actually see a small AC voltage, maybe 500 -600 mV in this case).
This type of connection of multiple windings is called "subtractive". If the windings were in phase, the connection would be called "additive". This is very important in certain cases, such as polyphase transformer circuits, but it doesn't usually come up in little tube amp transformers. Until now.
I still don't know why Dukane chose to do this, unless it was a mistake at the factory. Dukane also made dual-rectifier amplifiers, but I don't see how this would be a benefit in that case. Sometimes the polarity of secondary windings can be important in split-phase circuits where you try to balance the neutral leg (the center tap), but, again, it doesn't really apply here, as far as I can tell.
If you can explain why the transformer was designed this way, please send me a note and let me know. It bothers me.
The Transformer that Doesn't Know How to Add
This rusty lump is the power transformer from a Dukane 1D460A amplifier. I was testing it to use in another project when I found something unusual. This led to a whole bunch of research on transformers and winding polarities, and magnetic flux, and neutral current, and other things related below.
The ID on the transformer bell end is 710-4103-03. I can't find any specifications or diagrams of this transformer anywhere on the web. I do have the original amp schematic, though, so I know all the wire colors and taps, etc. It is a standard power transformer for a two-5881 amplifier, with a 5U4GB rectifier and a bias tap, and 6.3 VAC for the filaments and 5 VAC for the rectifier cathode and high voltage for the rectifier plates, with a center tap. One thing wasn't ordinary - the primary had taps for either 117 VAC or 127 VAC; that was not common in 1956. Useful, though.
So, I tested the resistance everywhere and between all the leads to make sure the windings were close to what that should be, and not open or shorted to anything. The transformer seemed to be in good shape.
I hooked the primary up to the Variac and ran it up to 120 VAC and checked all voltages on the secondary. Everything was in spec, or close enough. But, wait! I checked the two red wires of the high voltage winding and got zero volts! How is that possible? Between the center taps and the ends I got 350 VAC, as expected; I checked about 6-8 times to be sure I wasn't seeing things. Nope - no voltage across the HV winding. (Actually about 500 mV).
So I unhooked everything and checked all the resistances again, thinking I got a winding wire colors wrong or something. Same as before. I then measured the inductance of the wingings as well as possible (a LCR meter is not accurate with an iron core transformer), and that wasn't any help. I tried the "Super-Secret Transformer Test" for winding shorts, and the transformer was fine - no shorts.