Basing things on efficiency means that at best you are looking at maybe 60%. Which means that 1500 watts dissipation really is 'pushing' the tube to about what it's going to be capable of and still have a reasonable life span. At that, you are going to be producing LOTS of heat. (Sounds silly, but keeping coffee hot setting over the vents of an AL-1500 is not a joke. And, no, don't do that, it's not a smart idea.)
Mixing input/output power figures is also one of those silly things unless you know how those figures relate to each other. A very rough rule of thumb is that about half of the input power is probably the output power of an amplifier of typical efficiency, maybe.
Ignoring carrier, and even using Pep figures (if a constant tone input is used for testing), the output power levels will be exactly the same for AM mode as if the power were measured in RMS/average units. That's perfectly normal since the peak envelope power is -constant-. The 'trick' is getting a -constant- output, and you can't do that by humanly produced sounds, whistling, etc. So, you end up seeing bigger numbers by using Pep. It also means that the resulting power figures are not very legitimate for any particular use except having 'bigger numbers'. They just don't tell you squat about what's happening. But if that's what you are looking for, then knock yourself out! Unfortunately, that's also what you are doing to that tube, knocking it out. It get's very apparent after you've bought two or three of those 3CX1500's, so why not save your self some trouble to start with? That's also the reason I said 'tickle' that thing instead of 'driving' it. (grid dissipation)
Depending on what you are used to operating, there are some tubes that can be 'abused' and there are some that can't be. The 3CX1500 is one'a them 'non-abusable' tubes. That does not mean that it's unsuitable for amplifiers, it certainly is. It does say that you had better pay attention to what's happening or you will regret it.
- 'Doc