This is from my private collection. You will never find this again. This is the heater transformer of an ELPO P508 tube tester, which tester is in the same category as AVO Mk4.
This transformer offers any possible heater voltage from 0.63 to 110 Volt. Incredible. I do not know the current for each voltage, but the ELPO P508 is a VERY BIG tube tester, so I am sure it will be enough. What makes this transformer also professional, it is totally oversized, this is to reduce voltage drop at load. So though even a large tube has still may be only 20 Watt or so heater power, the core of this transformer I estimate 170 Watt or so. It is really oversized. So what output current it can supply, I can not tell you, but it will be sufficient for many tube.
Though the primary has taps for 110V 129V and 220V it can work from any voltage from 58V to 233 Volt. At the high end of the primary winding, you find the big steps of the taps. So set it for 220V for Europe.
Then, at the low end is also a series of taps, in the schematic connected with switch P1. With those you have an auto-transformer effect, and you can set it for any deviation of the mains voltage in steps of 4.2V in both directions. You do not need this switch of course when you already KNOW your mains voltage. So for 230V connect the mains to Connection "1" and "12" in the schematic. Though such a rotary switch is a nice option to add still, so you have 2% steps for variation of the primary voltage. Like this you can adjust the heater voltage you get, more precise to the mains variations and also to the load. You do not have to use the compensation, but if you do, you can just connect a simply a cheap 100V AC panel meter to the 114V heater winding, and add a potentiometer in series with the meter, so it indicates "100" at 114V. Like this, it will become a calibration in %. So it indicates "100" when the auto-transformer tap is set correct. Even so, you can compensate for high load. So a 2Amp 6.3V tube will be a higher load than a 60mA heater tube. This way, you can quickly calibrate it.
I think there was NO BETTER TRANFORMER every made, that's why I bought it myself. But time flies, and the project was one too much for me.
Some of the smaller ones, ripped from a defective Hickok or so, have a too small core, and the voltage changes a lot under load, with larger tubes. Or is too high unloaded. Not so with this BIG core. Besides they do not have these auto-transformer taps for compensation. Dimensions are appr 85 x 85 x 100mm. It is larger than the pictures suggest.
This transformer was tested by me for each winding, it is 100% good, and no rust or damage of any kind. I just have too much tube stuff around, and some has to be sold. I can email you a complete schematic of the ELPO P508.
The small resistors as you see in the auto transformer section of the schematic are only needed when you use make-before-break contacts. I recommend just a normal switch, so one which is "open" when it is in-between, and then the resistors are not needed.
Sorry for the price, it is 175 Euro, but such transformers are rare and expensive. If you go to a transformer winder with such requirement, they probably have not even a way to build it.