Rewind Dynaco mains Tx for 240V

[joebog1]

50 C is perfect!!!!. I wouldn't be changing a thing,
( except maybe the voltages on your heater windings ).
Today's commercial transformers are more usually class C which means 100 C is perfectly normal.
Some industrial applications run twice that temperature !!!.

Cheers

Joe

[retailer]

After unwinding the filament supplies I spent a few hours looking for a way to reduce the winding height - slightly smaller wire - the maths showed that I could get both 17 turn filament windings on the one layer - bobbin 33.5mm wide 0.9mm wire (actual diam 0.95) but in practise I found I could only get 16 turns on for each filament winding.
The end solution was to use 0.67wire (actual diam 0.71) and wind 2 wires together for each filament supply, overall current carrying capacity was just a shade under that of a 1mm diam wire, but the final winding height was reduced enough for me to finish off with 2 layers of insulation and still fit the laminations without help of the G-clamp.
With the transformer assembled on the bench the no load magnetising current is 50mA, once bolted in and powered up the HT sits at 370V and each filament supply measures 6.6V, I'm reasonably happy with the final result.

[joebog1]

Excellent result!!. Both in your construction and the presentation of the finished xformer, INCLUDING the external flux band.
I can see where you added the new iron in the centre of your stack. I would have blown a coat of flat black enamel over those to hide them.
Otherwise its a superb effort. Especially the stacking which looks like it was done by a stacking machine.

I won't comment in future, You can do a better job than I can.

Joe

[dougietamson]

Originally Posted by retailer

After unwinding the filament supplies I spent a few hours looking for a way to reduce the winding height - slightly smaller wire - the maths showed that I could get both 17 turn filament windings on the one layer - bobbin 33.5mm wide 0.9mm wire (actual diam 0.95) but in practise I found I could only get 16 turns on for each filament winding.
The end solution was to use 0.67wire (actual diam 0.71) and wind 2 wires together for each filament supply, overall current carrying capacity was just a shade under that of a 1mm diam wire, but the final winding height was reduced enough for me to finish off with 2 layers of insulation and still fit the laminations without help of the G-clamp.
With the transformer assembled on the bench the no load magnetising current is 50mA, once bolted in and powered up the HT sits at 370V and each filament supply measures 6.6V, I'm reasonably happy with the final result.

That does look good, nice work.

Joe/Ed, is there a rule of thumb we can use with AWG (off the top of my head, there is a relationship between each gauge going up/down) where we can sub a single wire to use bifilar or trifilar etc.
Eyeballing the filament wind work retailer did it looks like 19AWG can be subbed with 22AWG, 3 gauge difference,

If we take it further, as a minimum for an OT secondary, can we use, eg 5 x 23AWG winds wired in parallel (sandwiched between pri winds) for a single 16AWG wind.
The rule of thumb = required gauge (eg 16AWG) + 5AWG (parallel winds) + 2AWG (constant?) = 23AWG

Doug

[joebog1]

Ideally, what you need is the same current capacity using two or more wires, that the original design only a single wire.
Yes your rule of thumb is OK, but I have not seen a chart that gives you the parallel currents. You have to work it out yourself. That is why a transformer is ALWAYS designed on paper first.
It can get very expensive if you make mistakes!!!.

Joe

[joebog1]

Just as an addition, you can see why I use .75mm or 1mm thick fibreglass sheet as the bobbin. If it is neatly constructed, and doesnt have any gaps, ESPECIALLY at the corners where the insulation is stretched to the max, .75mm is thick enough, although I dont have specific breakdown voltages for it.
I have never had a failure. Also when reading wire guage tables, "mostly " wire tables are for bare copper wire NOT enamelled wire, so you will always make the mistake of not adding the insulation to the equation. You need a set of micrometers to measure wire properly. I measure it WITH the insulation first, then burn off the insulation with a little gas torch, clean the wire very carefully with soft steel wool and measure that. If you have ten layers of wire, thats 20 layers of insulation you have to add.

Joe

[Ed_Dinning]

Hi Retailer, I think Joe has covered it very well.

My UK/ Europe wire tables list both bare wire diameters an grade2 wire (double insulated types) there is also a Grade 1 type which is between the 2 and is a single insulated conductor. There are also other types of thinner or thicker enamal by various manufacturers, you need to consult their sheets for actual sizes.

On many designs the current density for copper is taken as 3A/ mm^2 of bare wire: pro rata for different diameters and can be read across from mm to SWG & AWG etc.

In the larger sizes of wire it is possible to get square section wire, this will give a better packing factor in the wind.

Note that additional insulation should be used between filament windings if one is feeding the main valves and the other is for the rectifier heater.

Nice transformer by the way.

Especially on mains transformer ensure the gaps between the "E" & "I" laminations are as tight as possible as this will reduce Mag current


Cheers, Ed

[joebog1]

So in light of the discussion I have scanned what information I have that is scanable. By that I mean I have a large probably A2 wire chart that I cant scan.

What I give here is what I have and should help aspiring transformer builders. On saying that, there is much to learn so that you wont kill yourself, or somebody else.
Specifically some time should be dedicated to learning the insulation properties of all the components you will use in construction of any transformers you might make or attempt to produce.
I will help where I can, and if I have specific knowledge of any particular bit of information you might be looking for.
I would suggest strongly that power transformers for analogue oscilloscopes be left to those competant to design and produce the same.

Audiocyclopedia version:

Joe

[joebog1]

Internet version:

Joe

[joebog1]

Engineering Tables from Google flop:

Joe

[joebog1]

The Radiotron Designers Handbook:

Lots of data here.

Joe

[joebog1]

Last page of Radiotron Designers Handbook.

Joe

[joebog1]

OOPPPS !!! Sorry mods, that didnt come out as planned. I dont even quite understand what I did

Joe

[dougietamson]

Originally Posted by joebog1

Last page of Radiotron Designers Handbook.

Joe

Bonzer mate

[joebog1]

Remeber Doug, Its a guide only. Different manufacturers apply different amounts of insulation. Back in the good ole days we used enamel. Today its horrible solderable wood varnish. Called Polyurethane.
It apparently works but goes against my grain. If you can solder through it, how hot can it run ?.
NO way you can solder through enamel insulation.
YET its not as highly rated.

Oh well ( Hank B Marvin )

Joe