{Collins} 75A-4 problem

My troubleshooting skills leave much to be desired, so I'm asking you experts for some help.
I have a 75A-4 that I bought used in 1961.  It was beautifully restored last year by Howard Mills.  He did the caps, his standard mods for early serial number A4s and realigned it. 
It's worked great for a year, but the other day it developed a problem.  The signal level suddenly dropped considerably, though the radio could still receive, detect, and amplify strong signals.

I checked tubes and found that V9, the 6BA6 final IF amplifier, was bad. Put in a new one (tested good) and got some loud thumps in the audio, along with the S-meter going high with each thump.  No signals could be heard.  Started to smell something burning so I removed power.

I discovered that R55, the 150 ohm resistor from pin 7 of V9 (cathode) to ground, had suffered heat damage.   The new tube was now bad.

I replaced R55, checked its resistance and put in a new tube.  As I powered up, I again got loud audio thumps, and I could see V9 flashing. I'm assuming something connected to this tube's socket has failed. 

What's  a good way to identify what's gone wrong?

    Jim - K6XZ

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