Bias voltage too low.
Faulty 6M5, o/c resistor to grid, leaky coupling cap or fault in back bias cap/resistors.
JJ

Yep Jim that's exactly what I would have said. I assume you'd have already changed out the paper caps and checked the G1 resistor, if so the 6M5 is low emission. There's not much else there to cause a problem. The low value resistors of that vintage in the back bias look like carbon resistors on the outside but are actually wire-wound internally so they should be OK.

That circuit has an unusual bass-boost negative feedback to the screen on the 6N8 audio amp. If that cap was leaky or the screen dropper resistor was high it would sound very sketchy indeed as you turned it up..

Jimb, you mentioned that you are using a 20,000 ohm per volt meter and if you used this to measure the grid bias, you will get a lower than actual reading because of the meter loading.
The model 42 circuit from Kevin specifies a 1000 ohm per volt meter which would present an even higher load, but this would be taken into account with the circuit voltages given.
To accurately measure voltage in a high impedance circuit, (grid bias, AVC etc.), you need a VTVM or a digital meter. It is because of this loading that the circuit does not specify the voltage on G1.
Harold

I will measure it again using the digital meter.
I assumed the readings were all taken on an analogue meter.
I forgot about vacuum tube volt meters.looking at the escutcheon panel it should have had a 9 x6 speaker not this tiny 5 inch speaker .I have a 9x6 I will try if no improvement I will leave the five inch in place.It is not my radio.

Regards Jimb

A more useful thing to know would be the voltage from the centre tap of the transformer to ground, i.e. the back bias. It'd expect to see about -7 volts there if the 6M5 is OK.

If the 5" speaker isn't properly baffled air will leak past it, the cone will flap and it'll sound awful. Maybe all it needs is a piece of cardboard around the speaker cut to fill in the gap. The negative feedback would have been tuned to suit the 6 x 9, it should sound better.

Ok with a digital meter the grid bias reading on the 6M5 is minus 4.3 volts. At this stage I have not changed the 8mfd electro on the plate circuit of the 6N8. It is my intention to do so.I am waiting for an order arriving from Carl. I have exhausted my current supply.
Regards Jimb.

That's closer to a realistic value for a 6M5 assuming it's 50% emission which it probably is. Should still be good for 1 or 2 watts which is plenty.

2 watts should be loud on a good speaker. The speaker, or its baffling, could be your problem.

I assume you checked voltages around the 6N8? Plate and screen? Somewhere between 50 and 150 volts?

If the cap from the volume control tap was leaky the 1st audio amp would be biassed off as you turned up the volume and that would cause distortion on high volume settings. The volume control would be scratchy too. And the plate voltage would go up with increasing volume setting irrespective of audio content.

I just ran my Kriesler chassis that has a 6M5 driving a 5" speaker.

A good 6M5 causes 6 volts bias to be dropped across the BB resistor.
That valve drives 2 watts into the speaker, and is loud when I baffle the speaker..

A weak 6M5 causes 3.5 volt bias and sounds horrible with about 0.75 watt into the speaker and is as weak as p---s.

I am guessing your 6M5 is down on emission like the guys are saying.
Test: plug in another 6M5!

Fred.

Ok all fixed . The problem was C31 8mfd electrolytic on the plate supply of of the audio amp 6N8 was open circuit. I fully intended to replace it when supplies arrived . I did have an RB type 10mfd. Electro I used this and up it came,sounds good . The bias remains at minus 4.5 volts .I am happy with the result. Although I have a 6x9 speaker I have chosen to leave the 5 inch in there and keep my spares for another day . I think I am being too soft I have been fixing these for this guy for less than cost for the reasons I stated before, they are keeping my mind active and giving me something constructive to do. I am not doing immaculate chassis restorations I am just making them work hopefully reliably with paper caps and the electrolytics replaced and Carl's nice power cords fitted, plus anything else stopping them from working well.
I think I need to be charging something! He is not short of a quid and is selling his house in Arncliffe he will be very comfortablably off.
What are your thoughts on this? We are not so comfortably off both on aged pension, however money does not excite me, we are getting by ok and I have never been motivated by money. What is fair and reasonable?
Thank you all once and again for helpful comments you are all a good bunch of people and I appreciate all your help.
Kind Regards.Jimb

Just because your retired does not mean you work for free and as I've said before people would be happy to have their radio working again and let's face it ,not many left who can do it so yes your time is money.
I won't restore a finish for free unless its a good friend.
It takes a long time to restore a finish correctly and materials cost a fortune now , it aint 1980 any more and everything costs big bucks so unless it's a good friend I charge .
Pete

Hi Pete .
Hope you are feeling well again now. The stuff you do requires endless paintance and care apart from the expense of the materials you are using. I admire your level of paintance and care I am afraid the older I get the less patienc I have.
Plus the fact my condition makes anything requiring paint or varnish insane it all ends up either on the ceiling or under my armpits it is not a good look trust me.
Regards Jimb.

Hi Jim ,
Yes I'm well thanks
I Dont do much of that type of work anymore,but I will always like it and this spring and and summer I will be starting back up again . I have been running around buying up Nitro and tints and lots of 2k getting ready for the warmer weather. It's only something I do part time anyway.
I don't know what I will do for a living from here on in , probably just buying selling and fixing things up at home to sell.
I think the job market is probably a thing in the past for me now, A good job would be near impossible to get now in this new world so I best paddle my own boat .

Pete