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A guy on a German forum measured the frequency response of over a dozen interstage transformers in 2016. The Thordarson R200 is among them.
Had to open an account there to see the pictures and sadly, the one with the measurement setup is not online any more. Here's the description, translated by DeepL:
Hello,
I have recently started collecting single-ended interstage transformers from the early days of tube technology. Since I plan to use the transformers for listening to music, I measured their frequency response. The measurement was not performed under laboratory conditions but “live” in the amplifier.
For the measurement, the transformers were operated with a 112a at an operating point of 140V/5mA and the frequency response was measured with ARTA. https://artalabs.hr/ Here is a picture of the “measurement setup”: The content cannot be displayed because it is no longer available.
This is a 3-stage amplifier in which the coupling elements between the stages are “octal-socketed” and can therefore be replaced. Each stage has a control instrument for Vh, Vg, Ia, Va, and Vb. This is very helpful when operating old (and often unreliable) triodes in order to monitor the correct operating points. The first stage consists of a 112A that is RC-coupled to the second stage. The slot for the second stage contains an octal plug with crocodile clips soldered to it. Here, the transformer to be tested is connected to the 4 terminals B(attery), P(late), F(ilament), G(rid). A 71a in anode-base circuitry is at work in the output stage. If the second stage is also RC-coupled, the frequency response is (approximately) linear. Given the (sometimes disastrous) frequency responses of the transformers, I don't want to haggle over +-1dB...
Thordarson R-200 (Ratio=2), which also has quite good bass for its time (around 1925) !
Looks like its first resonance is at ~7.5 kHz. Maybe this can help in finding an equivalent one or even re-winding the original which, depending on the construction, might not be that hard. Here's the complete page in a PDF for the other test candidates: https://files.fatbox.moe/rvm7ui.pdf
We only do toroidal inductors, transformers, etc. at my work but there's a sister outfit that does ferrite E-cores, not sure about laminated but it wouldn't hurt to ask. If resistance, # of turns and wire gauge can be teased out, there might be a chance to rebuild him, we have the technology... :) Some transformers use special winding techniques to reduce interwinding capacitance in order to raise the resonance frequency. Those are a whole different can of worms.
Yah, it's a big-ass heavy transformer, so the fidelity was most likely really good.
One thing you need to remember about this transformer is it's also the bias for the tube plates - in this case, specifically, the first AF amp and the detector tube, so any transformer would also need to have those particular characteristics (whatever they may be.) It's not just a input/output device.
It's not worth having rewound, that would exceed the value of device by a wide margin.
What's ultimately going to happen is one of these three things:
1: I'll see one of these transformers at a show and buy it. If I don't see one at Dayton or Butler this year, then that's the cutoff point. 2: I'll see if the shell can be taken off without destroying the device, and check/repair broken connections inside. 3: It will go to the Early Television Museum as a auction donation. Many better equipped people show up there.
Well, fingers crossed you find one or get this one back to working condition.
The more I think about it, the more it's looking like the Early Television Museum may get a piece to sell.