I rechecked all the voltage values (pg. 59 MP Manual) with the chassis upside down. I got a couple of bad values. Terminal 4 read 0 instead of 7-9, and terminal 6 read 252 instead of 110-140. All other measurements were good. I powered off the MP. After 15 minutes, I turned the chassis right side up and powered up. Turns out the bad measurements are a separate issue: the tube in socket A doesn't light up and there is no sound from the right channel. The left channel is fine. I swapped tubes and remeasured. This time, terminal 4 is 6.54 and terminal 6 is 111. There is no sound from the left channel and the right channel is fine. So the tube is blown.
Both 6V6 tubes were working fine before. I'll just get a replacement pair. It will take some time before they get delivered.
Also, after performing the voltage test I'm sure that my MP survived the spark undamaged.
Questions:
1) How do I test filament power?
2) Can anyone recommend particular 6V6 tubes or their equivalent?