Hi Marc, thankyou so much for this information as it seems every circuit I have looked at with similar tube line up is different to this circuit.
It has a cathode resistor which is 175 ohm. Could you please tell me what voltage I should typically see?

The 6M5 has a cathode resistor - that's the grey WW one above its brown bypass cap,
The 6N8 also has a cathode resistor. it's 2k.
Its G2 is supplied by a voltage divider - I can't see the upper value but the resistor to ground is 600k. What is the voltage at that point?
The audio diode load is that 500k resistor and it is returned to the 6N8 cathode, as it should be.
There is back bias but that is only used for the RF stages bias.
The 16μF 1st filter cap negative is returned to the back bias resistor, as normal.

I don't see anything unusual in this circuit, but I can't see it all.

Remember when you are measuring resistors (which I don't do btw, I measure voltages) those resistors have a tolerance of 20%.

I assume you have replaced those black caps. If not, at the very least you should replace the one that connects to G1 of the 6M5.

Assuming you have done that, what is the voltage on G1 of the 6M5 as the fault develops? It should remain at zero.
Because of the unfortunate choice of pinout on the 6M5 it is vulnerable to leakage from G2 which is next to it. Early examples had silver plated pins and suffer from sliver migration across the glass, eventually resulting in crackles and bangs.

I haven't had much time but yes I knew the grey ww is the cathode resistor for 6M5 which I was measuring earlier. I'll go find the time to do some more testing to work this out. Thankyou again for your help.

Wire-wound resistors ARE inductive but the Q is very low and there is a whopping great 25μF cap across it so any inductance will be well and truly swamped. Don't worry about it!

I just edited that post, yes I was wrong about the inductance. Thank you for your information.

-5 volts bias on 6M5 is fine.

Hi Ian, what I have since found is when I hook my DMM onto G1 the fault stays away. The voltage stays on 0.3V and just keeps playing. As soon as I remove it the fault returns. Also when the fault occurs, it has to be turned off for some time before it will emit any sound again, but with my DMM hooked up the start up delay is very short. Also the radio is now working quite well with test lead attached which otherwise it does not. I'll check around that area and see what I find.

Easy one! Very valuable clue you found.

There is a resistor, probably 500k, that goes from 6M5 G1 to ground. It is open circuit. Replace it.

The meter is doing the job of that resistor at the moment.

The only problem with that is the capacitor is the only component connected to that pin.

Well there SHOULD be a resistor there!
Don't know what happened to it.
Put one in!

I will as well as a couple 1 meg ones.

Is there a connection to a tone control pot from that point by any chance?

Is so, they may have been relying on the track of the pot to provide the resistance. It might be OC or the far end of the track ground left off when the pot or its associated cap was replaced.

Back a few steps. If the voltage on the plate is the most common 250VDC. Then the tube data says with Rk 170 Ohms, then the Plate will draw 36mA and the Screen 5.2mA giving a cathode current of 41,2mA. then you go to ohms law where V= IR which gives seven volts across the cathode resistor.

As noted with no grid leak the tube will not be happy and have no control. The electrical position and wiring of the volume control are now substantially in question. It may have been the grid leak. 500K to 1Meg. Grid leak resistors on output tubes tend to have a high attrition rate, as do plate resistors on the first AF tube. It may have been removed to replace it and some one had a senior moment & forgot to fit a new one?

Well.....I had a look and there is a wire going from the tone control to pins 8 and 9 on 6M5 and then a resistor which has both legs soldered to this same point.

That also sounds weird.

If the tone control has its origin on the Plate Pin 7 then it is normally wired rheostat (one end open circuit) with the wiper going to ground via a capacitor, other wise its a DC path and a short. Pins eight and nine are dead pins.

HMV quite often do not have their tone control on the plate and it might be a multi-position switch.

It is not unusual to have an additional pF value cap also from plate to ground to decouple RF. Some tubes are not particularly fussy about the frequency they amplify.