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My BDT preamp

I've been doing this with a BDT beam deflection tube (6AR8), the input signal goes to a pair of deflection plates (like a scope) that deflects an electron beam between to plates. The regular grid can be used to adjust the current through the tube which changes the gain. At higher currents the tube is very linear, some distortion starts creeping in at low currents (ie volume turned way down).

In the last few days I resurected this and updated it with a much better PS (low DCR flywheel etc) and a pair of Lundahl PP line out transformers, the results are stunning. Way better than any other preamp I've done before.

Next I'm going to be experimanting with separate accelerator and plate supplies. It works best with the accelerator between 200 and 250V, but with the plate that high its reaching its dissipation limit at the high currents I want to use. So I'm going to try accelerator at 250V and plate much lower and see what happens.

John S.

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