Bob,
The key phrase in the "P" version of tubes is "caveat emptor". As Rich
says, the pulse rated tubes often are different only in that a longer
anode insulator allows for higher anode voltage. I believe this is true
for the 3CX1500A7 v. 3CPX1500A7. Yet I know of one radar designer who
runs the standard 3CX1500A7 at 50kW pk, so maybe the "P" version isn't
always required.
I am more familiar with the 'X800' and 'X5000' and there's a different
story there. The 3CPX800A7 looks to me as if it could be dropped in
place of the 3CX800A7 without difficulties. Same filament voltage,
slightly higher anode voltage, and, interestingly, slightly lower
interelectrode capacitance and slightly higher maximum frequency without
derating. Odd thing, though - when you look at the two physically, the
pulse-rated tube actually has a shorter insulator between anode and
grid, despite the higher anode voltage rating. I know several hams
who've gotten the pulse-rated '800 and put it in amps designed for the
straight 3CX800A7 and changed not a thing.
The '5000 is altogether different. The 3CPX5000A7 is actually a
3CX15,000B7 with a smaller anode. If you compare specs on the 3CX
versus 3CPX version, you'll see that the "P" version requires higher
filament voltage and current, and the grid is massive in comparison (50
watts dissipation, if I recall). So in that case, even though they're
physically identical, you cannot drop one in place of the other.
So, basically - make sure you have both data sheets before you use one
in place of the other.
Regards,
Dave W8NF
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