If you don't have an electronics background, strongly consider taking the amp to a qualified technician. This is for your safety and also for the amp's safety.
Not a lot your can do without the necessary background and test equipment. Here are things you can consider.
Check the bias. The indicators should be green with the amp idling. It's normally for red to flash when you play. If the bias is set close to the red threshold, red may flicker when the amp makes the noise.
Try moving the amp off the speaker. Sounds like the crackle may relate to vibration.
Check each jack and control to see if they wiggle. If yes, do they make noise? Probably job for a tech.
Try inserting a jumper from the Pre Out to the Power Amp in. Also try the jumper in the effects loop. Sometimes a switching contact get dirty or bent. If this is the problem you can use the jumper until the next time you have the amp serviced.
The following requires working inside the amp. Tubes get scalding hot and the SVT has lethal voltages inside. If you decide to proceed, let the amp cool off and then reseat all of the tubes. If the noise continues, try the chopstick test. This involves lightly tapping each tube with the amp set to operate and controls set to a normal playing level. The idea is to identify tubes that may be mechanically compromised.
If you identify a bad small signal tube, you can replace all of the 12AX7s with another 12AX7, 7025, ECC83, etc. Before replacing the 12AU7 drivers tubes, set the bias for minimum current in the output tubes. The reason to do this is because the bias goes through the drivers. If you install stronger tubes, the o6550s may pull too much current. After replacing the drivers, set the bias per the owner's manual.
If you identify a bad 6550, you could try rolling in another 6550. However, the tubes really should be reasonably matched. Ideally you would install a matched trio of tubes on the side with the noisy tubes. Before pulling the noisy tube, set the bias for minimum current and then perform the bias procedure after installing the new tube(s).
SVT CLs monitor all of the output tubes individually. The tubes are organized in two sets of three. Each trio of tubes has a dedicated circuit to adjust and monitor the bias current.
For green all three tubes in the circuit must be pulling at least 15mA. Red occurs if one or more tubes pulls 30mA or more. So as you see the CL is not super picky about matching.
If you get Red before Green the matching is really bad. If you get Red but can't get Green, one of the tubes is dead or extremely weak.
My advice for the future. Assume a tube amp will need a new set of output tubes. This means budgeting about $500 for an SVT to cover a matched sextet of 6550s and a bench fee to set the bias and give the amp a quick check up. The actual cost may be more or maybe the amp will give you years of service before it has a problem.
