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RE: Allen - would your cascode work with DHTs?

Posted by PakProtector on September 15, 2009 at 05:41:02:
The cascode gain is set by the lower tube gm and plate load. The swing available( w/o getting into the curvy part at left hand side of plate curves ) depends on the upper grid voltage and how much more or less voltage is required to flow current across it v. the bottom tube.

Take a look at GE's 6BQ7 cascode curves. The cascode plate curves are constructable with some single triode curves; take the single triode curves, and superimpose a second set shifted to the right by the upper grid voltage. The horizontal part starts with the intersection of shifted g=0v with the original grid line. using the 6BQ7 curves, it takes 125V to flow 20 mA on the single triode curves; the cascode turns horizontal at 250. 10 mA is reached at around 75V, so add 75 to the 125V upper grid voltage and that is where you'll see the -1.2V grid line turn horizontal.

The cascode creates a pentode-ish set of plate curves, and they can be extended closer to the upper grid/gate voltage with a device that requires less voltage across it than the lower tube. Lower mu tubes will require more lower plate voltage swing though...and that will effect the composite curves too. Lower voltage drop and high gm is nearly the exclusive domain of the SS device. Triode/MOSFET cascode works very well here.
cheers,
Douglas