According to my full size Eimac manual ( the yellow one with the multi page
spec loose leaf sheets) the only 4CX250 style tube with decent IMD is the
4CX250R/7580 which was used in a lot of military equipment including the
AN/URC-32 that I worked on in the USN.
At Ep of 1500 in AB1 3rd/5th is -30/35. However at 2000V it slips to -23/27.
This is a 2 tone measurement which disagrees with Toms claim of Eimac using
a single tone. Maybe they swing both ways (;
In any case the -23/27 is what Ive experienced with that tube at 2KV in a 2M
amp before building the 3CX1000A7 and the generic 250 is a bit worse. This
was in the dark days and I used VR tubes for screen regulation. However in
this crowded area my 600W often got wide reports even tho the CE-100V TX was
rated at -30 ( A pair of 6550's that the audiopiles drool over)and the 6360
transverter output couldnt be that bad?? Those reports partially prompted
( and another 3+ dB) the 3CX1000A7 with a rebiased brick in between the xvtr
and amp. The old xvtr was replaced about 15 years ago by a MM unit from your
side of the pond that I improved the RX a bit.
The RCA 8122 was about 3dB better in AB2 at 2500V and another 3 dB better
with Ians regulators. A pair of 8122's is in the NCL-2000 which I converted
to 6M in 1964 while I worked at National on the development of that amp and
still use today.
Ive also upgraded a few 6N2 Viking Thunderbolt amps with Ians kits so I do
have more than a passing aquaintance with tetrodes.
Carl
KM1H
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