Popping Sounds

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Popping is almost always an arcing problem. The high voltage in the amp has found some path that cannot stand the high voltages and discharges suddenly through that path. The arc current is high, but cannot be sustained by the power supply, so the voltage drops a little, the arc extinguishes, and it takes some time for the power supply voltage to build back up to where the arc will start again.

Popping is often associated with the time when you flip the standby switch. In standby, the current drain from the power supply is less, so the voltage rises, causing more voltage stress. When the standby switch is thrown, the higher-than-normal voltage can break over things that stand the normal stress of operating voltage.

The tubes themselves will sometimes develop internal, intermittent arcs. Do some tube swapping. Start at the front (preamp) end of the amp and pull a tube, listen, pull a tube, listen. When you find one where pulling it makes the popping stop and a new tube makes it quit completely, you're done.
Arcing power tube socket
The B+ may be arcing across the surfaces of the output tube sockets themselves. This is often the case when an amplifier has a lot of dust and dirt inside it. In some cases, the arc can be started by a few seconds of playing without a load on the amp, which causes large spikes on the plates of the output tubes. Contamination of the tube socket surface can let an arc get started, and the arc itself burns the surface of a plastic tube socket body. This leaves a carbon residue in the path of the arc, burned remains of the trail of the arc; the carbon residue is itself somewhat conductive, so in the future, there is a ready made path for the next arc. You have to replace the socket if this is the case.
Intermittent switch
A switch, often the standby switch, can develop internal arcs
Intermittent breakdown of coupling cap or a popping resistor
Sometimes a signal coupling capacitor just can't take it anymore, and it starts breaking down intermittently. Resistors, particularly those delightful, brown sound carbon composition ones, sometimes develop internal pops. Proceed as for tubes. In this case, a new replacement tube will NOT make the noise quit permanently, and you have to figure out which component is causing it.
Intermittent breakdown of output transformer or choke
The filter choke, if your amp has one, and output transformer are connected to the highest voltages in the amp. If they are old (can you say "vintage"? I thought you could) and if they get hot, the insulation on the wires inside can start being intermittent. Internal shorts that clear will cause popping. Shorts that don't clear will pop a fuse, usually. Sometimes it'll just cause smoke.
broken resistor or capacitor or R/C lead/ wire
This is one where the problem is not caused from the B+ breaking down insulation. Mechanical damage can break a part and leave enough pressure on the pieces so it mostly makes contact and kind of functions. Vibration will cause it to open momentarily, causing a pop.
Heat from resistor or output tube melting solder
This is a fun one. Some of the power supply dropping resistors, output tube cathode biasing resistors or the connections on the output tube sockets themselves get so hot that they melt the solder that attaches the leads. Even more interesting is when they just soften it so it gets grainy and any vibration (speakers, anyone?) makes a cold, grainy solder joint. This bad joint can pop and arc, sputter, hiss, rectify AM radio, do lots of nead stuff. Once you find the bad joint, you'll also have to find out why it was so hot.

You can sometimes leave the amplifier turned on and turn out or dim the lights in your workroom, and see arcing happining. On pops that happen when the amplifier is touched or jarred, you can (gently, now) tap the chassis with a rubber hammer or wooden stick, being careful not to break anything or touch the amplifier with your hands as you do this. This often makes a mechanically-motivated arc happen, and you can see where it is, and deal with it when the lights are on. BE VERY CAREFUL NOT TO TOUCH THE INSIDES OF THE AMP IN THE DARK - THE HAZARDOUS VOLTAGES ARE JUST AS DANGEROUS IN THE DARK.