Mr. Clean,
I'll apologize to start with because you won't like this answer.
There are a couple of things that I take for granted as an amplifier user. One of those things is that the signal fed to that amplifier has to be constructed properly. In the case of AM mode, that means about a 4:1 modulation to carrier ratio. Which 'translates' to about a 2:1 power ratio. I also don't try tuning an amplifier with a less than 'normal' input signal. For AM, that means that the input signal isn't just the carrier, it's the carrier and modulation. I also use an input signal to the mic input of the radio that's a constant level (mini-signal generator). That means that I'm tuning that amplifier at full input power levels for maximum output power. That's not really true unless I've done it before on that particular antenna and frequency, so that I know it will take it. If I haven't 'been there' before, I'll start off at lower input power and work up to normal, or maximum power input. That procedure will work well for any combination I can think of (radio/amplifier/antenna/etc.).
So how do you know when you get to the 'max' point? On an amplifier such as that 'AL-1200' it's pretty easy, you pay attention to the meters on that amplifier, NOT just the watt meter measuring output power! Those meters will tell you plate voltage (fairly constant), and grid current (the biggy!). A 3CX1200(A7?) has definite limits for grid current and you do NOT want to exceed them! All tubes have those limits, it helps a huge lot if you know what they are for the particular tube(s) you are using. It's an even better idea to never get close to those limits, thing last a lot longer that way.
There are some 'catches' in all that. I'm not using a typical CB radio. Once the proper modulation level is set, I don't mess with it to try to get 'more' signal, it stays put at probably 85% modulation. It takes a little time if you have to 'work up' to full power for the first time, but not all that much time at all. It ain't, and never will be "plug-n-play". Having found out the hard way about those 'limits' for grid currents, etc, I do pay attention to them. (Want a few 'dead' tubes?? I got some. A '3cx1200' won't work, but '3-500's make real nice decorative patio lights.
) That "mini-signal generator" is just a tone generator, nothing very expensive. They are very nice thingys to have around. If you really wanna get 'fancy', try a 'two-tone' generator. Or if you're as cheap as me, a telephone dial-tone will work.
Don't want to run full power? Drop the input power and re-tune.
A 3cx1200 is a pretty 'forgiving' tube. The '3cx1500' is not forgiving at all. And there are gobs of other tubes that I have no experience with that fall all around those two.
You can take 'short cuts', but only after you are familiar with the behavior of your particular set up. Taking one of those 'short cuts' before becoming familiar with things is a sure way of adding to that string of decorative patio lights, you know?
- 'Doc