Another thought: if the problem is an anode bend related, then the distortion would be present at any volume control position, as the fault would be pre volume pot. ....but it is volume related. Very little distortion at very low volume, but as it increases so does distortion. This tends me to think it's output stage or bias related?

My position here, is to establish if a Monkey has got at it? That speaker does not seem to be consistent with the set. That is why I want to now what it is using instead of the field coil, if the speaker was an electrodynamic & indications are that it was.

If "B" Voltage is wrong nothing else will be right & that spells distortion. I had an STC from this era not that long ago and I ended up increasing the bias on the 6D6 as it was overloading the anode bend detector.

Such an overload can also be caused by too much signal as these have no AGC. Often these have a "Local / DX switch to reduce that. It was missing in the one I had? 68pF was the series cap I found best in series with the antenna on the Local station 11Km away.

It is one thing to speculate, but if it has old wax paper caps, & electrolytics and unchecked resistors, I consider fault finding futile until that has been addressed.

Marc

Hi Marcc....

I've re-capped and replaced several dodgy resistors.
The radio works, but performance is not good, audio output is low and distorts at much above a whisper.
B+ rail is around 240 v which seems ok. 220 v on anode of output valve, similar on screen grid.

The above was done about 15 years ago......... and its entirely possible I may have mis-read a resistor value and replaced with incorrect value.

What I really need to do is get hold of a circuit of this receiver, so I can check off the values on paper against what is there.

The circuits on the link are not this 4 valve model, but at least they use a 6D6 and a 42, so its start point - something to go by.

And to add to that, check for a dropping resistor or choke in place of the field coil of the original speaker.

The most common "B" Voltage is 250V and 20% either side of the known "B" Voltage is considered normal. I would not mess with diodes. We don't even know if the circuit is right & that may only add another layer of not needed faults. I would not interfere with the circuit until we work out what it is.

My normal approach with a set with no known circuit (and I get a few, a Monkey with STC 59 chassis being the last) is to use the Autocad to print the major components in the position that they are in within the chassis & then play join the dots. The CAD had some valves & I drew the others I needed.

One thing that needs to be done once parts have been changed in the RF section, is alignment. if its out the set can be quite deaf. The method I use utilises an Oscilloscope, as the measuring device and it doubles as the signal tracer if there is a problem with distortion. It would be ideal in your situation.

Lets do this in a logical order, "shotgunning" can only get you into strife.

Marc

I agree. Don't start making non-original modifications until you've sorted out what's actually wrong with it. Mods shouldn't be done unless you understand exactly how the radio works, and there's no alternative.