Albert, and all,
Thanks for the clarifications. I wish you success in trying the #3(?) transformer with the new design with less winding turns!
I have just one doubt left before starting to wind the #3 transformer - which I will wind on a separate bobbin, to avoid destroying #2, which at least works for a good while before heating up. Maybe I can even find a core with a smaller airgap.
The doubt is what exact voltage to wind the primary for.
What I did in versions #1 and #2 was winding it for what I thought would be 200V rms, which should have made the plate swing between roughly 120 and 680V. That sounds like a reasonably efficient use of the tube's capabilities, along with acceptable voltage headroom against line voltage variations.
After the voltage drop in the HV circuitry turned out to be far less than I thought, the transformer actually operates at a lower rms voltage, roughly 150V rms. This results in poorer efficiency, requiring more plate current, in exchange for a very generous voltage headroom.
Designing for 200V rms seems technically better to me - but John's measurements confirm that the original transformer works at a low level, roughly 140V rms!
So, should I try to copy the same voltage levels Tektronix used originally, or should I try to use a slightly higher primary voltage, to reduce the load on the tube? The point is that since I don't have the original core, and can't do a universal winding, anyway my transformers are very different from the original, and making them work the best way possible is more important than copying the exact primary voltage of the original. If my transformer ends up more lossy than the original, operating the primary at higher voltage, lower current, gives some margin to tolerate this higher loss.
I checked the datasheet of the oscillator tube. Even with 250V at the screen, the plate voltage can swing down to around 60V before the screen starts to take a large current. But in the Tek 310 the screen never goes up above about 100V, and the nominal operation level is around 50V. So I would expect the tube to be able to pull the plate voltage down to significantly less than 60V before the screen will start robbing a significant current. That makes it possible to use a somewhat higher primary voltage than the original design does, although the tolerance to low line voltage would be less.
Any ideas, comments?
What's clear so far is that I will wind the secondaries for 1206 and 1312V rms. The primary+feedback rating could be anything between 140+70V and 200+100V rms, and is what remains in question. And a finer detail could be that if winding it for 200V, the feedback winding spec might remain at 70V, resulting in 200+70V rms.
The original spec seems to be 140+70 or 150+75V rms.
As to my problems with the 564 I'd better leave that alone for the moment, it's confusing to mix it with the present topic.
It's probably best to start a separate thread for it.
Manfred
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