Tree N6TR,
Fine on your linear. The 4-250As work fb. I found that I got better
linearity running in Class AB2, with a 200 ohm grid swamping resistor,
and a 4:1 transmission line transformer. By making the input to the grid
itself through a low pass filter at 200ohms, you can absorb the input
capacity of the tubes into the output capacitor of the filter, and get a
much better input match. I run with a regulated 300volt supply on the
screen, with a bleeder of about 20mA, but I've never seen any negative
screen current. I too use negative supply filtering, with a tuned choke,
but the 'fuse' in my case is a piece of about 40 gauge enamelled wire
supported between two large ceramic standoffs in the positive line. It's
gone once in ten years. I don't have surge limitation on the filaments,
and have lost some tubes (old used ones) through them going grid -
filament short apparently at switch on. I use a shunt regulated bias
supply, so both screen and control grid are held very stable. It works
very well. And if you come across some 4-400s, you can plug them in to
get a bit more, although looking at the characteristics, the only
difference appears to be that the 4-400 has a bigger plate structure.
One advantage of big glass tubes of this sort is that they have a lovely
friendly red glow at full dissipation, and you can SEE things working!
73
Peter G3RZP
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