First of all, when music is input, whether the circuit works or not is determined by the level of the music input at that time.

So it may be because the music mode circuit works for a while and does not work for a while (the frequency is determined by Determined by music input), if there is a load (secondary iron adder) at the moment when the triode is working, there will be a surge. At this time, the current of the triode is very high. After many cycles (music causes the triode to conduct back and forth), the triode will be damaged

When there is no music, the current will be large at the moment when the switch is closed, but it will work stably afterwards.

I think the transistor may have been damaged by multiple surges