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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: Have old 10K OPT's for Pentode EL84: want to add UL posted by -3db on November 12, 2006 at 14:41:45:
If it could, there would be no need for UL trannies. The screen needs a very low impedance path, which a resistive divider alone cannot provide, even using a cap.The nearest I've seen to a UL connection without a tranny tap was using a cathode follower buffer, with the grid of the CF fed by a resistive divider between OP pentode plate and ground. This gives you the 43% or whatever ratio you need and it applies to both DC and signal conditions. The cathode of the CF goes to screen grid, with a resistor also to ground. The plate of the CF goes to a voltage which needs to be higher than B+, because B+ would not be high enough (the plate voltage can swing from approx. 0v to 2 x B+, because of the action of the OP transformer).
I don't know how well this arrangement would work in practice but I imagine an SF MOSFET could do the job just as well as a CF triode without the heater supply hassles. Maybe somebody else has tried it?
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Follow Ups
- Can't be done with resistors & caps - Ray Moth 22:50:58 11/12/06 (1)
- no need... - NoWay 18:27:47 11/13/06 (0)
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