> At the very least you will need to add base resistors to Q3 and Q4;
> otherwise, the one with the lowest Vbe will win.

Yes.
You may note that at the top I said -

    > I have taken some liberties in simplification which will probably
    > not be justified in practice eg Q2 and Q4 will quite possibly need
    > separate base drive "spreading" resistors

Good to see that someone's actually looked at it closely enough to have
understood this point!
Any thoughts on aspects of the circuits (hoped for) operation which I
haven't covered?

Note my post of a few minutes ago adding D3 from rh end of L to rh end of
RBUK1 thereby (hopefully) adding proper cycle control.

regards

        Russell McMahon

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