Hello Ulli and all,
The 4CX1500A was designed, as Rich intimates, basically as a Class C
amplifier.........note: I did not say linear amplifier although Eimac
mentions its use as one. Using it as a linear will take some careful
implementation or you will not be too popular with your neighbors! My
Eimac Power Grid catalog does not give its IMD spec on the 3rds but, as I
recall it is in the upper -20dB region, as referenced to one tone, that
is behind the more modern tubes for linear application. If you want to
clean it up a bit, run it in GG, Class AB1 as Collins did with their
30S-1 amplifier. That eliminates the neutralization problem, too.
Since it lacks the shielding of its pentode cousin, the 5CX1500A,
neutralization will or should be utilized if it is to be passive grid
driven in AB1. Otherwise, its interelectrode capacitance will cause
sufficient regeneration to additionally degrade its relatively poor
linearity to start with.
Further, out of respect for your neighbors, it would be wise NOT to run
it at the level that you may be intimating or it will probably tear up
the band! For CW, it should be a good performer.
Lastly, if you wish to run it in AB1 with the optimum linearity, you will
need to run the quiescent anode current as high as possible, without
exceeding the anode dissipation spec, thereby generating lots of heat,
requiring an oversize blower.......make that noisy! Remember that its
filament is 5V @ 38.5A so you have 192.5 Watts to handle before applying
any other power. Of course, its instant heating filament may be somewhat
of an advantage.
GL/73,
Marv, W6FR
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