Two Valve TRF

[Aerodyne]

I'd have thought the iron powder would have effectively acted as a shorted turn. Ferrite cores contain iron dust but the particles are electrically isolated from each other by the ceramic support medium. Your answer provides a much greater induction value, apparently. Neat experiment!
-Tony

[Variometer]

Tony,
Each end of the glass was sealed with plastic padding, so there was no contact between the iron powder & the metal caps. I expected the inductance to be greater, but as usual, I had no particular reasoning or knowledge to be certain. Anyway, having read your reply, I poured a quantity of powder onto a piece of paper & stuck the meter prods in it. Even on the 20M range, the powder showed as a perfect insulator! It is extremely fine, as fine a flour & is a light grey colour. I suppose it is iron powder, someone gave me a tube of it some time ago & it has a home-made label "iron powder" on it.
I don't know whether insulation is a normal property of it, or whether it has been treated in some way. I have just held a magnet to the side of the tune & it is strongly attracted by it, so it looks like it is what it says on the label.
Bob

[Norm_N_Tam]

Originally Posted by Variometer

Further experiment. I took two empty 1 1/4 inch fuses. The first one I wrapped the glass section full of 35 swg enamelled wire. On the inductance meter, this came out at 17.7uH. I placed it in the holder next to the reaction choke (leaving the 47uH in the other holder). I found that the set then operated well into the sw bands, but nowhere near as loud!
The second fuse, I removed the caps & filled with iron powder, sealing off the ends with plastic padding before replacing the metal caps. I then wound it full of 35 swg enamelled wire exactly like the first fuse. It tested at 55.6uH & worked quite well in the circuit. Slightly different coverage than with the original choke, but all on good volume.
Bob

What do you think of Mike Branca's idea:

'The trick in getting high audio from a regen is to use a lot of AF inductance in the plate (anode) circuit. An interstage tube transformer or Hi Z choke can be used but the greatest output that I got was from an old auto ignition coil. Tried a couple and even a mower magneto coil but a TV flyback would not work.'

It seems he tried some resourceful ways to get more volume out of his receiver.

Norman

[Variometer]

I have never heard of that method before, & don't really know the theory behind it. I have a couple of Unidyne circuits from the 1920s that claim to work with ONLY an LT battery, but I have never tried building one.
Bob

[Aerodyne]

Light grey is the colour of so-called 'black' mild steel, gained as it left the rolling mill (never passed through dies to clean the faces). The grey colour is in fact heat-developed 'rust', in effect corrosion and I guess this powder of yours has been subjected to heat to create the grey surface. It is likely to be this that is acting as an insulator, I believe.
As far as interstage inductance goes, when I was nobbut a boy - as they say in Yorkshire - I built a two-valve set and used an RS 5:1 interstage transformer, but the results were mediocre. I was then given a hefty Telsen 5:1 transformer, resplendent in Bakelite case, with plated brass terminal posts. It worked wonderfully! Moral? experiment, I suppose, and don't assume that the letters RS (Radiospares as was then) arer actually a true guarantee of top quality.
I further guess that the Telsen component may have had many more turns on both primary and secondary, maintaining the ratio but increasing the inductance.

-Tony

[Variometer]

Tony,
Thanks for the info on the powder. I was surprised at how much it did insulate, even with the meter prods almost touching, it still showed infinity of the 20M range!
I do have a number of "real" intervalve transformers, but am generally reluctant to use them becuase I often write about what I build and am always aware that readers may not be able to find a suitable transformer. Apart from that, they are all fairly large. Fortunately, valves are still around in great numbers, but I have my favourites, acorns, subminiatures, the small B7G & B9A types with 6.3 volt heaters. Also have a load of the B7G battery valves, DAF91, DK91 etc, but I still prefer the 6.3V heater types. EF91, EF80 & EF95 all perform very well with low HT.
Bob

[kalee20]

Really nice looking project Bob, displaying your usual craftsman's approach!

Regarding the Ferrite / Iron Powder, I'd agree with Tony Aerodyne that the particles in Bob's mass of iron powder are insulated from each other by oxidation induced by heat at the iron mill. Conventional iron powder cores are indeed made from finely ground iron powder, mixed with an organic binder resin so that they can be moulded. However, this is NOT the same as ferrite!

Ferrite is a ceramic material (more or less homogeneous) made from manganese-iron-oxide, and it is inherently non conducting and also inherently high permeability. Iron powder (because of its distributed air gap) is much lower permeability.

When used as a central slug in a coil, because the magnetic circuit return path is through air on the outside of the coil, you'd not see much difference between them. Both are used as tuning coil adjustable slug material.