You probably have too much voltage on your 6550 plates as your B+ is rising to a high level before the cathodes heat up and the tubes begin passing current. B+ will decrease after the cathodes heat up and tubes start passing current.
You can hook a voltmeter (rated to 600V minimum) to your B+ and see what the plate voltage is doing as the amp warms up. With hot cathodes, the tubes will begin conducting current immediately and the crackling sound will not occur.
Make sure you have a proper 5AR4/GZ34 (not 5U4G) rectifier tube that warms up slowly and does not allow B+ voltage to your plates before the output tubes heat up. If you are using solid state rectifiers, then you are abusing your output tubes unless you allow the cathodes to fully heat up before applying B+ to the output tube plates.
I had this trouble with a Heath W5M that came with 5U4G rectifiers that was applying B+ to my plates before the output tubes could conduct. B+ would rise to around 550V before the tubes started conducting!!! The tubes would start to conduct, and the B+ voltage would drop to around 450V. Switching to Sovtek 5AR4 rectifiers solved my problem. The previous owner was ignorant but willing to experiment, and he had made several "mods" that I had to set straight. If you buy used gear from musicians and audiophiles that like to fool around and experiment more than reading technical books, then you had better go through the gear and look for any mods they may have done.
If your amp has solid state rectifiers, then you had better have a delay circuit that waits for cathode heat to come up before applying B+ or learn how to turn on with amp in standby and wait. You can always install a standby switch and allow the tubes to heat for at least one minute before switching on the B+ supply. Any tech can make this mod if the amp didn't come with it.
As I did with my own preamp design, you can use a 555 timer IC chip and a relay switch to apply B+ after a reasonable length of time, and I went further and had an output delay that switches on the output after the circuit has settled down after about 45 seconds. This is necessary to prevent damaging anything that is powered through a direct coupled solid state amp with output down to zero or near zero Hz.
Transformer coupled amps do not have response down to to DC, so that would not be necessary in your case. But many a good audiophile solid state amp and speakers have been damaged by hooking up a vintage tube preamp that slews voltage for a very long time as it warms up. It takes time for the preamp output coupling cap to charge and stabilize. The input side charges to 180V and the output side is tied to ground by a high value resistor on my preamp. That's a big slew, when it only takes about 2V at the input to send a power amp to full output voltage.
Since electrical power can fail and come back on after a few seconds or minutes, I rather let an automatic circuit power up my tube gear properly that have to operate a standby switch to start up and shut things down properly.....which I can't do if I step out of the room when power fails.
My tubed Conrad-Johnson control preamp and phono preamp both have these startup circuits from the factory, and all top quality modern audiophile tube amps should have them as it protects what are often very rare and expensive tubes and speakers....and DC coupled solid state amps that are not protected against huge input offsets. Crackling tubes are telling you they are being highly stressed and you will suffer premature failure at some point in time. There are plenty of schematics available online for installing auto startup circuits in tube amps. When tubed guitar power amps went from 5AR4/GZ34 rectifiers to solid state rectifiers, the delay circuitry and output muting circuitry failed to show up at the same time. That costs more money, and guitarists rarely are willing to pay what a C-J or Audio Research owner has to pay for a tube amp.
Hope you have good quality fast blow fuses installed in your 6550 B+ supply rails, as you will be in need of them once a crackling tube decides its time to short out internally during startup.
