Well, wasn't expecting this. A Philips model 123 which I use about once a week died on me, so I took it to the bench. The very first test revealed the culprit, that being the speaker transformer. It's integral with the speaker, a Rola 6H. My stock of speaker transformers is almost zero, but luckily at the back of the drawer I found one of the same shape and size. It even had "6M5" written on it, so I knew had a good replacement. The only thing I didn't know was the speaker impedence, not being marked on the original speaker or the transformer. So I had to just use it and see what happened.

The old transformer was riveted on, so the drill was used. The transformer was replaced and upon testing, the radio now worked. I had even got the feedback connection the right way around.

Luckily the power supply hadn't been damaged by the overload, which may have been going for up to 20 minutes (or more).

You are lucky. The123L here was recovered from a recycle depot. The feedback was disconnected and the cone had been drilled & wrecked. I replaced the speaker with a NOS one out of a Mitsubishi Magna.

The greatest risk to the 6M5 is its screen, when the plate goes open circuit. You can in desperation use 6CK6 but you have to connect pin six to the cathode.

I have plenty of spare 6M5's, about 40 of them from memory. One of them is missing an unused pin, so if I ever use it I must make sure it's the correct orientation. The 6M5 is incredibly reliable, and it's quite rare to ever have to replace one.

With the feedback disconnected, the tone control (switch) won't work.

If the feedback is reversed, like many Philips with it, it will oscillate. 6CK6 is a TV video tube and by the characteristics, is a modified version of a 6M5. The oldies were great as repurposing tubes.

Like 2A7, 6A7, 6A8 were merely heater & base changes. There was of course a metal 6A8.

In the latest resurrection (1936) Howard (Aus). It had an open audio transformer and the 6V6 was u/s. As were other valves & one was missing.

Never did get a circuit for it.