On 24 Oct 2005 00:56:14 -0700, "MarkAren"
wrote:

Hi All,

I am contemplating the design of a class C amp for 144MHz using wound
output inductors and a split stator (butterfly) tuning cap. This seems
to be a very common approach in the good old days for QQV06-40As, but I
don't think that I have seen such a design for 250Bs in push pull.

The issue of voltage rating of the o/p cap is of course an issue, and
the output coupling loop will need to be well insulated (assume 1500VDC
on the anodes).

Any obvious reason that this isn't a common approach ?

The loaded Q (and the circulating current) of the tank will be
necessarily high because of the tube output capacitance. The
efficiency is dependent upon the loaded to unloaded Q ratio. The
tuned line approach can offer higher unloaded Q and a physically
larger structure to dissipate the heat generated by the inevitable
loss.

Output coupling will likely be problematic as well.

I ran a single tube with a wound plate inductor and the efficiency was
pretty low, but it was just a 100W driver so size was more important
than efficiency.