1958 Ford Tube radio popping noise

Thankyou very much for your reply.  Makes perfect sense.    Maybe I'll try experimenting by putting a capacitor in parallel to see if it drains the surge faster.

Back in the 60s Hybrid car radios did thump. I think that they had caps that discharged fast but the transistors picked up the power off surge before they lost power. I will do some research on that. I know my dad bought a new 1960s Ford Fairlane and It also POP'd or thumped either it was when it switch on or off. I don't remember which. Solid state was very new and designs were not fully understood.

I have an tube radio with one output transistor. radio that was given to me for a 1958 Ford.  Model # 84MF.  Sounds great problem is when I turn it off it makes one popping sound from speaker. 
I have it on the bench with the covers removed.  Hope someone can help me solve this problem.
Thanks