This would revolutionize DIY tube amplifiers. (probably can get audio to Mhz bandwidth too)

What I'm talking about is an Amorphous Metal Serve On Litz (AMSOL) transformer. A what?? OK, here goes:

Here's a technique that a home Diy-er could use to make an unconventional output xfmr with likely VERY good performance results and NO complicated design problems (like interleaving or sectioning, in fact, not even any winding required!). Start with a spool of Litz wire with maybe 20 strands of enamel insulated wire in it. (I see Litz wire on Ebay regularly) Unserved Litz is fine. (the serving on Litz is the fiber wrapping around the wire strands to hold them together and provide extra insulation)

Now you need a spool of approx. say 1/4 to 1/2 inch wide amorphous magnetic alloy tape. Most likely you will have to buy this new from the manufacturer.(Allied) (See below about avoiding all this hassle) This stuff is very thin. Be careful when handling this stuff, its got razor sharp edges on it. Now you need a rig that can spin the tape spool around the wire as the wire gets pulled thru. Essentially, one would be putting an amorphous film serve on the Litz wire. Can experiment with how dense a spiral layer gets put on by altering wire pull rate versus tape spool spin rate. But unlikely to need a layer thicker than half the radius of the Litz wire.

This process should be a piece of cake for any commercial Litz wire manufacturer, so hopefully one can eventually just buy this stuff off the shelf, ready made on a spool. (I'm going to call a Lits wire manufacturer Monday and discuss this. I think this is clearly in their interest to offer this kind of product.)

The final output xfmr. would be configured by connecting half of the Litz wire strands (the little wires in the Litz) in series for the primary and half the strands in parallel for the secondary. Should probably use triple or higher enameled wire strands to reduce capacitance between "turns", but at least leakage inductance will be near zilch. (its the product of leakage inductance and distributed capacitance that determins xfmr bandwidth) Another approach to lower distributed capacitance further would be to use a large central wire for the secondary, surrounded by individually insulated small wires in a twisted shell. Can optimize distributed capacitance that way.

The whole final length of the Litz assembly could be coiled up for compactness, but this won't have any effect on magnetic coupling unless wrapped around an additional magnetic core. The equivalent of this Litz technique is often used to make broadband RF transformers using ferrite beads strung on multiple wires.

You have to get enough inductance in the primary to handle the lowest frequency. While this technique does not get to take full advantage of a large number of turns to get inductance, it does take advantage of minimal magnetic path length around the wire circumference, which will provide a large boost to inductance.

Inductance = u*N*N*Area/(magn. path length)

My first quick analysis led me to believe their would still be an overall disadvantage for getting the same primary inductance as a conventional E core design achieves using the same amount of magnetic material (volume or weight). However, on closer inspection, I have found the two are equivalent.

Here's a quick calc. to illustrate. Lets say we are talking about a 1000 turn primary transformer on E core with a 10 to 1 turns ratio. The Litz version gets 10 wires for primary, and 10 for secondary. Assuming the E core secondary uses about the same winding space as the primary, this would be equivalent to 2000 turns on the E core. So, magnetic path lengths in each case are roughly scaled as 2000 for E and 20 for Litz. Now looking at turns, the inductance is proportional to turns squared, N*N. So 1,000,000 factor for E (primary turns) and 100 factor for Litz. We seemingly come up a factor of 100 short for the Litz inductance:

(20/2000)*(1,000,000/100)* A/A'= 100

Whats missing here, however, is the area factors "A" and "A' " are not the same in these two cases for the same volume of material. With magnetic path length reduced by a factor of 100 we are free to increase one dimension of the area factor by 100 to get the same volume of material. This means the Litz wire length gets increased from the length of one turn on the E core winding to 100 times that length.

Interestingly, this ends up with the same primary wire length and resistance as the E core version, since 100 lengths of 10 wires in the Litz is the same as 1000 turns on the E core. So both the Litz and the E core are identical in inductance and resistance for the same weight of core. But the Litz version has far far better control of leakage inductance.

Since one is likely to coil the Litz version up for convenience sake anyway, a further improvement is possible yet. By coiling the Litz on a large toroid core, say an old 20 amp variac core, the magnetic material area "A" can be doubled, tripled or more. One can either go for extended low frequency response this way, or one can shorten the Litz wire to get the same design inductance, for a cost saving on the Litz/amorphous metal purchase.

So, how much work does the DIYer have to do to make an output transformer with this idea? If the pre-assembled Litz wire/amorphous metal serve is available off the shelf on a spool, you just have to figure the length needed to get your required design primary inductance and order a spool of that amount. You will want the correct number of wire strands in the Litz for the design turns ratio plus any other windings. With extra strands, one can make a flexible, variable impedance with taps xfmr., or go for additional windings like partial cathode feedback or isolated screen grid UL. Actual construction consists of soldering 10 wires (or so) together for a 10 to 1 turns ratio xfmr. and optionally coiling up the Litz (before soldering!) on a variac core. 20 minutes, my guess.

I doubt I will ever build another conventional E core or toroidal output xfmr., this simply blows them all away!

Don