Civilian Wartime Radio - Further Notes

[Terry]

I agree, Jeffrey, the actual value is 390Ω + 180Ω, so  570Ω in total - Bob included the circuit in Post #45, by the way.

Do take a look at it because to my way of thinking, it is weird in the extreme! The cathodes of both RF valves are grounded but are provided with grid bias, but not via a resistor in the earthy side of the PSU, it is in the earthy side of the main smoothing capacitor, so there is going to be a nice bit of 100Hz ripple superimposed on it!

The 6V6 has cathode bias but the bias resistor is split and the earthy end of the volume control/grid leak returned to the tap, so most of the bias is lost! This point is also the reference for the earthy side of the IF detector coil and the full output valve cathode bias is fed to the AGC line so that, in the absence of AGC (or enough of it) both RF valves are almost certainly fed with positive grid bias - or am I overlooking something?

[Terry]

I think on those sets the detector will not properly work if the cathode isn't bypassed by a value near its original, the biasing for the detector comes from the cathode of the 6V6 so too much AC content will cause problems.

I think Bob has got himself confused here. Jeffrey beat me to it over the appalling frequency response that would result from such a small value cathode bypass and, as I note from the circuit that he shows another electrolytic, the 8uF HT decoupler, changed to 0.1uF, I wonder if he forgot which electrolytic was which and assumed, incorrectly, that he hadn't noted the change on the circuit diagram.

In view of the performance of the set that he is now claiming - and I have no reason to doubt what he says - that would explain things.

[pwdrive]

Any 100Hz ripple on the AGC line will be very small, it's decoupled by a 4.7meg resistor and a 0.1uF.

Lawrence.

[Murphyv310]

Hi.
I see what you mean Terry, I've been looking at the official manual and not Bob's changes, there could be a bit more hum with the 0.1uf, I'd be inclined to fit a cap of the correct value.

[Sparks]

Terry,
You are quite right. The V3 cathode bypass is 22uF, as I intended. It is the screen decoupler that I left off inadvertantly, and put a 0.1uF in its place!
Bob

[Sparks]

I took it to the North West Model Shipwrights Society last Saturday, but never got there!      There was a police road block between 0600 and 1230, so I gave up and came home.    Took it to the RAF welfare society on Tuesday, (at their request after seeing it on Facebook)  but there was minimal interest showed, although I got the obligatory clapping at the end.   Not one of them came over and looked closely at it, and another asked "what are valves?"    I guess there is not much interest in this sort of thing outside vintage radio forums and societies!    
Bob

[Murphyv310]

Hi Bob.
That doesn't surprise me. I have found that today there seems to be little interest in vintage electronics outwith the forums we are members of. For a long time I personally have tried to kidle some interest in young people with little or no success, the sad fact is we will eventually pass away and there will be very few to pass on how interesting the hobby and vintage electronics actually is.
In fact audiophiles are good as exponents of valve technology as are musicians that prefer the old technology, Radio & TV generates very little interest in comparison.

[Sparks]

I decided to try and make my own oscillator coil.    Having measured the original, it was 100uH and 6uH.    I cut a small paxolin panel and fitted it with four double solder tags.    Across the top ones, I soldered a 100uH choke, round which had been wound 60 turns of 34 swg enamelled copper wire.   This winding was pure guesswork, and it metered at 20uH, but I decided to try it anyway.      The results are completely satisfactory.    It performs just as well as the original.      I even guessed at the correct way round for the extra coil windings.   If it had not oscillated, I would just have swapped the wires over.    
   

[Murphyv310]

Nice neat job.