Wynand says;
>Any comments about these [4-250] in a grounded grid configuration?
They should perform like 4-400's in GG, except the plate dissipation is less.
Although figures have been published in the older ARRL Handbooks for 4-125 and
4-400 in GG, they didn't publish for 4-250. However, studying the curves, it
becomes pretty apparent that the filament/grid/screen grid assemblies are pretty
much the same for 4-250s and 4-400s. Certainly, the filaments are the same - 5
volts at 14.1 amps., which gives a good indication as to the emission
availability.
Although you have the advantage of no screen or bias supply, you have to provide
tuned input, which could be a pain on a multiband amp. (Which is why Rich said
that GG isn't practical for a 9 band amp) For that reason, I prefer to use
grounded cathode - switching inductors and capacitors that are going to carry
fairly high currents is a bit messy, compared with a couple of mains
transformers, some diodes and a few transistors......Some would recommend a
G2DAF to get around this, but how good these tubes are for it, I wouldn't know,
and personally, I don't like the DAF circuit.
I would recommend a fan to cool the filament seals unless your using proper
Eimac sockets and chimneys. The Mullard version (QY4-250A) doesn't have the
metal base rim that the Eimacs have for base cooling, and they can get into
trouble (despite what the data sheet says) if you assume that the seals will
stay cool enough on their own.
The orange glow at max dissipation is good for lighting as well as heating the
room.........
They like lots of volts on the plates, too - 3600 to 3900 is good.
All the strictures about filament volts apply too. 5volts, +/-5% at most.
(well, the plus is more of a problem than the minus). Step start isn't a bad
idea either, unless you have a transformer with high leakage reactance to keep
the maximum current down. Or use a 120 volt xfmr for the fils, with a big series
dropper resistor from the 240volt line in the primary. Note it will need to be
about a 200 watt resistor, though.
73
Peter G3RZP
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