I'll start a thread here about researching the Autodyne valve convertor.
I have kicked it off by selecting a set of tuning coils and trying them in my prototype chassis using hi gain 9 pin TV valves.
That gave me a set of results described in the first article.
Having made 3 examples of the autodyne circuit using a range of valves and coils I am getting a feeling for how the thing works.
My next move will be to use a set of RCA metal octal valves, 6SS7, 6SH7, 6AC7, 6SQ7 and try them with the latest coil set.
But a lot is still a mystery as discussed and I welcome any comment on what I am doing.

Autodyne Research 2022 - Part 1
Autodyne Research 2022 - Part 2
Autodyne Research 2022 - Part 3


Fred.

Document uploaded.

Actually, I did find an excellent article on Autodynes in a back issue of "Radio and Hobbies".
November 1943 page 17.
Neville Williams discussed bringing discarded radio sets back to life.
This was War time and the shortage of components and with most production going to the war effort the civilian population had to make do with whatever they could find.
So resurrecting old sets with Autodyne convertors was worth writing an article about.
In that article Neville covered a lot of practical tips with coils, IFT's circuit arrangements but all at practical level.
No theory of operation was really gone into.
Very interesting article.

Fred.

I read your article late last night, fascinating again as always.

I suppose you could have tried a few other pin-compatible valves to see how they performed, such as 6BX6, 6BY7, 6BW7 and others.

Looking forward to the story of the octal valves.

If you want to experiment? I did some work on a couple of American Midwest Autodyne room heaters (circa1936). The most unusual one I think went to around 60MHz? They used a separate exciter & one trick was to switch in a new set of oscillator tube cathode resistors on the oscillator, believe it, or not, is a "Colpitts". That may be worth toying with?

The universal widget tube 6BL8 "frequency changer" may be an interesting tube to play with.

I have a Crystal locked Sig Gen (455kHz) I made for a modulation experiment. That uses 3/4 of a Quad NAND gate into a delay line to create a modified square wave as the Oscillator and for the mixer a dual gate FET wired as a buffer amp. Audio in one gate RF into the other.

Marc

Well, I had a lovely time testing my Autodyne and making it work.
Really the job changed more into making a set of war time metal valves work as a Superhet and I learnt a lot from the process.
I have documented the journey into a PDF and will send that to Brad to pin to the top of the thread.
Any comment welcome!
Fred.

Part 2 uploaded.

Interesting as always.

Hello again Fred,

The metal cased valves, are they part of your success this time? I have no experience with these valves but I am assuming their envelopes are earthed. I also recall some of your previous problems were with spontaneous/unwanted oscillation due to sensitivity.

was this the reason you went for metal, to screen unwanted feedback? Just wondering.

Hi BBTV the metal tube certainly does help in screening between stages.
Saves you from fitting separate shields.
With a grounded cathode most of the pins on the base can be linked to ground.
I used the metals because they represented an evolution of design past the older G shaped glass enveloped types.
They were a step up in performance from the 1930's designs and of course were produced in the millions for the war effort.
I was curious about the characteristics of each type.
If not for the war I think they would have been replaced by the GT glass models quickly.
Like the European EF50 metal the 6AC7 gave high gain and high bandwidth in a rugged plug in package.
I have a handful of them and thought why not?
My guess is RCA were meeting a government specification in developing the type and the metal build would pass vibration and shock testing that a glass envelope would fail.
It is amazing the vacuum seals are still holding, out of the dozen or so only one failed to work, a 6V6.
That valve acted as a dead short to the HT.
Fred.

I got a completely out of the blue thought Fred.

What would happen if one was to methodically replace a metal bottle with a glass one? One at a time of course.

Not sure why that came into my head, only drink light beers these days. But seriously, is is worth doing just for the purpose of the exercise?

BBTV having tried to run other sets RF and IF stages with no shields, the answer would be instability!
The equivalents in the GT envelopes are not internally screened.
As well there is no glass version of the 6AC7, the closest would be a 6SJ7 with less gain.
Once you move to later internally screened valves the situation is better.
Fred.

Thanks Fred,

I suspected this would be the case. My question was initiated because I have a few 'metals' but no practical experience with them.

I can now appreciate their value in design.

Thank you.

Well, I have made a complete set but I don't think much of its appearance.
I'll send a "part 3" to Brad to pin to the first post along with parts 1 and 2.

At the end of that I have a picture showing what the complete set looks like, so far, with the comment Ugh!.

Now I am open to advice please, mainly about the front panel.
The cabinet itself looks fine, having a honey wood colour and a bit of grain showing.
The set and speaker fit in there with no problems.
where the set looks horrible is from the front.

I started with a sort of cross shape window like speaker aperture and a cut out for the knobs to stick through at the bottom.
Either the outlines are too rough and that looks offensive or its the colour combination.

Black may not have been the best choice for a 1930's domestic table radio?
Looking at archive photos most sets seem to have same colour wood fronts or maybe a wood contrast colour.

The knobs maybe should be wood colour and ornamental round ?

Fire away with advice if you like, I'm no artist so am clue less.
Maybe make a steel panel, or a wood grain panel?

Fred.

Part 3 uploaded.