4-250A. Spares for British Army D13 transmitter also Marconi NT201 naval
tx.
Nice tubes, I use a pair of them, theyglow a nice red at full dissipation,
go nicely with 3600 or so on the anode. Design is older than I am - the
1948 ARRL handbook has a design using a 6L6 tri-tet xtal osc driving one.
Must have pushed some current through the crystal!
I find the linearity better in AB2 when used with RF nfb, shunt regulated
bias supply and regulated screen supply.
The 'little brother', the 4-125 was used in some 2 metre amplifiers in the
50's: I heard somewhere that there were some CAA ground tx's using4-250As
in the 108 - 136 MHz aircraft band.
Are they 4-250A with the metal base ring, or the Mullard/Philips/Siemens
types without (QY4-250 - and some US number in 6000 series)? Either way,
you need to make sure you keep the filament seals cool. They're the same
fil/grid/screen structure as the 4-400.
vy 73
Peter G3RZP
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