Please Help, ST-120 Snap/Pop Sound On Initial Start Up

When I finished assembling the ST-120, I continued to follow the instructions for initial start up.  I turned down all four bias pots all the way counterclockwise.  Then I turned on the amp with only the three driver tubes (no rectifier tube and no output tubes).  The three driver tubes lit up perfectly and I left the amp on for just over a minute and they stayed lit until I turned the amp off.  Then I shut the amp off and unplugged it.

Then I plugged in the speakers and I put in the rectifier tube and only the two left output tubes (both of the right output tubes were left out).  Then I plugged the amp back in and turned it on.  Both of the KT88's lit up but seemed to stay a little dimmer than the driver tubes.  After roughly one minute there was a loud snap accompanied by what might have been a blue arc in or near the rectifier tube (not sure tho).  The sound was loud enough that I wondered if it came through my speakers. It sounded like those little white paper "Snap & Pop's" that they sell at the fireworks tent that you throw on the floor and they pop (but maybe a little louder).

EDIT:  ...also, I just noticed that the center valve on the driver board has a black burn mark around the center of the top of the valve.  It's in that area that is normally silver looking centered around that nipple on top of the glass.  The black burn mark is shaped roughly like an egg or football and is about 5/8 inch long by 1/4 inch wide.

When I finished the last step of assembly, right before screwing the chassis together, I checked each and every connection to see if it was a solid solder, and they all were.  Now I'm going to open the amp up and check each connection for any errors.  Any help you can offer would be hugely appreciated.

EDIT2: https://www.dropbox.com/sh/mad57ttj2keyo3y/SFRJGzPDPi
that's a link to the photos if the wiring I did inside the ST-120, just in case it helps you to help me  ;-)

EDIT3:  could it have been just a loud turn on thump?  I'm afraid to turn it back on (with the rectifier tube in it) for fear that it could further damage other parts that are not currently damaged.  Also, I ran that test to see whether or not it is a high voltage problem as follows;
Details of the test:  
I put in all seven tubes (excluding the rectifier tube) and then I plugged the amp in and then I turned the amp on and left the amp on for three (3) minutes based on the stop watch in my cell phone.  I ran that test twice with about 5 minutes of rest between each test.  I had the speakers hooked up as well.  All seven (7) tubes lit up as expected with all having the same level of brightness.  All seven valves stayed perfectly lit until I turned the amp off.  I checked the fuse and it is perfectly intact showing no signs of stress.

Kind thanks,
Eddie

Did the amp play properly after your last comment- Edit3? If not, you may have just lost a rectifier tube. It's a good idea to have a few spares, need to weed out the weak ones. Double check the 1000 Ohm resistors on your output tubes. On my Mark III's when I have a arc in the rectifier accompanied with a noise through the speakers the fuse blows due to a short in the tube, but that must not have happened in your case.

My kit supplied rectifier tube (China not Sovtek) did not last through the start up process at the time I thought it was something I may have done but the more I learn the more I think not.
I have went through two rectifiers with less than 140 hours on my ST-120 and have few spares on hand 
Bob and others will tell you it is the tube most likely to go in these amps.
I would go through the instructions double checking every wire to wire point and solder connections before doing anything else.

I am assuming that you do not have access to a variac. I would do the "yellow sheet diode mod" as described in the sticky section. If needed, your local Radio Shack is a source for HV silicon diodes. I would also put a 100W incandescent bulb in series with your power cord on your next power up. With all tubes in place and "disposable" speakers connected the bulb should initially light up bright then dim down a good bit if all is well. If the bulb stays bright, or goes out you have more debug to do. This procedure reduces the chances of a catastrophic failure if you have a short or unexpected dc path.