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If you run a drill bit into the base of a 4x1k or 4x1.5k to enlarge the hole slightly, you'll also wind up grinding out that dimpled keyway that aligns the tube with the notched socket shaft. Of course, for a tube that's otherwise unusable, that's mostly-inconsequential.

Steve, K0XP

If an amp uses FU728 by design and has an actual 728 socket stem, a 4cx1000 will not just fit in. The center shaft is too wide in diameter. ?It may be possible to enlarge the center hole on the 4cx1000 with the right size drill bit to make it fit. ?The FU728 with a larger hole will fit a real 4cx1000 socket without an issue. ?Change filament voltage of course either way..

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On Sunday, November 26, 2023, 6:49 AM, Adrian Fewster <vk4tux@...> wrote:

Alek, see?

Can dropin for a FU-728 with filament voltage changed to 6v ..

73

vk4tux