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RE: BDT Preamp Schematics

Posted by John Swenson on May 14, 2008 at 15:12:35:
Hi Evangelos!

I use a Cascode bias of 100V with a B+ of around 130V or so. The exact value is not important.

The maximum idle current through a plate is about 12mA, so at 30V drop across the cascode MOSFETS they are only disipating 360mW, which is fine without heatsinks.

On the transformer front all that will have to get answered by Dave, I really don't know the details.

As to the complexity, most is in the PS. You can certainly try building it with a different PS configuration. I tried a bunch and this sounded the best, but that doesn't mean something else is going to sound BAD.

My original version had both accelerator and plates running at around 180V, this was really too high for the plates, they got too hot at maximum current. With 100V on the plates they stay much cooler. The tube was designed for an accelerator voltage of 250V and it really does work best there. You can certainly run it lower, but then the maximum current is less. This is important to getting a wide gain range. At very low currents the tube will start to distort on high level inputs. By keeping the max current (ie max gain) high you can still have the minimum gain be quite low before the distortion sets in.

Thanks for the interest,

John S.