Re: Interstage Audio coupling transformer
 

Exactly as Mike Cobaltblue says - the turns needed is set by the inductance needed for the frequencies that the thing has to handle (so 5,000 turns and 15,000 turns rather than 12 turns and 36 turns).

I saw the photos and thought, what a horrid-looking transformer! But they do tend to work a bit better than you'd expect. Take care of the laminations, they will be high-permeability material which you really need.

One good bit of luck - the transformer does not have to pass DC current (some intervalve transformers do), else there would be other complications such as non-interleaved laminations and a small air gap.

Handling this sort of wire is not for the faint-hearted. You need a steady rate of rotation and a tensioner, else any jerk will snap the wire. You really don't want to handle the wire - it's probably failed because the original winder had sweaty hands, or maybe wound it after eating their lunch which finished up with a raw orange, and contamination has caused corrosion over the years.

All the same, it is do-able (I have wound intervalve transformers). But expect to spend a few hours at it.

Re: Interstage Audio coupling transformer
 

If you measure the wire diameter (and the insulation thickness if there is insulation between layers) with a micrometer and then the cross section of the winding area and volume of the winding can be fairly closely determined from its shape, you can then calculate the number of turns of that wire that will fit in there. I have done this many times where a transformer with fine wire has gone O/C. If there is a DC resistance figure, from a schematic , it helps to check you have got it about right. (Try to estimate the cross sectional area and average length of a turn that the wire uses is the main task).

The thing is to acquire wire that is as close as possible to the original diameter, and put in in a cross sectional area that the original wire had to occupy. (you probably won't be using the insulation between layers).The average length of a turn will end up close enough.

For example, for some fixed sized winding cross sectional area, number of turns that will fit there is inversely proportional to the square of the radius of the wire and the resistance of the wire is inversely proportional to the square of the radius of the wire too. This means that the resistance you end up with is inversely proportional to the 4th power of the radius of the wire, so a small error in the wire diameter has a big effect on the resistance, and the inductance too which is proportional to the square of the number of turns you put there.

Or, you can unwind it and count the turns, that for very fine wire, can be time consuming and tricky.

Re: Interstage Audio coupling transformer
 

Are Ok so they are keeping the impedance relatively high so they don’t load the previous stage. Thanks Mike

Re: Interstage Audio coupling transformer
 

I think can certainly measure the wire diameter with a micrometer. When buying the new wire is it gauged for just the copper or is it sized for the copper plus the enamel.

I was going to try and unwind it now I have softened it with acetone it feels like it wants to unwind. I have bought a cheap electronic counter module from eBay so I will attempt to build an unwinding machine.

I have also put a mark where it’s centre tapped so if all else fails I will wind until I hit the mark. I don’t see I have anything to loose but my time.

Thanks for the replies and excellent advice. I will keep you all posted. I hope to get it working but it’s the journey that’s the interest for me. Never tried to wind a transformer and it’s nice to find something new in life.

Regards Chris

Re: Interstage Audio coupling transformer
 

I measured the wire diameter best I could this morning using a couple of micrometres and both of them say its close to 0.085mm in diameter including the enamel. I think this has the closest standard as AWG 39 (0.0897mm) The only other wire available is AWG 40 (0.080). Based on experience can anyone suggest is it better to go thicker or thinner than my nominal 0.085mm. My thought was to go a bit thicker as it may be slightly stronger and then just recalculate the number of turns slightly.

Regards Chris

Re: Interstage Audio coupling transformer
 

AES have a Labor day sale on until the 3rd Sept - 10% off everything.
I just ordered 2 intervalve transformers.
Peter

Re: Interstage Audio coupling transformer
 

Just the copper. Enamel increases the OD by a certain amount, according to whether it is Grade 1 or Grade 2 thickness, but in the absence of other info, I tend to estimate an increase of 10%.

Re: Interstage Audio coupling transformer
 

I went ahead and bought the 39 and the 40 AWG so I will measure them when they arrive and make a decision. I am kind of at the stage of thinking I will just have ago and do my best I cannot make it any worse as the set doesn’t work. Valves are amazing tolerant devices.

Thanks Regards Chris

Re: Interstage Audio coupling transformer
 

I dragged one of my 238's out and checked the coupling transformer and checked the dc resistance of the coupling transformers total 3.64K tap 1.49K battery end hope that helps.

Re: Interstage Audio coupling transformer
 

Thanks Mike that’s good to know. Hoping to have ago this weekend at winding a new transformer if the wire arrives in time.

Regards Chris

Re: Interstage Audio coupling transformer
 

Hi Chris from wire tables 0.085 diameter over the enamel is close to 0.063 coppeer diameter, a standard size in grade 2 from wires.co.uk
A small drum should be about £15 and will do all you need.

Ed

Re: Interstage Audio coupling transformer
 

That's the spirit!

A couple of bits of advice: don't forget that as an auto-transformer the two windings are 2:1 in turns ratio, so that when in series you get your 3:1 step up. You might as well wind turns on until the bobbin is 1/3 full, make a tap, and then continue winding till full. That way you get maximum inductance with the wire you have.

When you bring out your terminations, leave a loop of slack as strain-relief. A break at the end is a minor inconvenience. A break at the tap or at the beginning is a disaster. If you can, double back the wire on itself a couple of times and twist, forming a cord of 3 strands or 5 strands, just for the first turn, a turn either side of the tap, and the last turn, as reinforcement.

Have fun, and put up photos!

Re: Interstage Audio coupling transformer
 

Many thanks for the advice I’m still a bit confused regarding the wire size if that includes the enamel coating or not but it will become clear when my order arrives. When I measured the wire this included the coating. I will have to have another look at the website and check out how it’s specified.

On the bobbin I have marked the holes where the centre tap comes out and also where the winding finished. But I am going to try and count the wire off the core. Most of the turns are on the core first with possibly just the last third on the outside off the bobbin.

I will probably need a bit more advice before I’m finished so i will reports back with some pictures.

Thanks Regards Chris

Re: Interstage Audio coupling transformer
 

Good suggestion for this sort of rewind because if the turns numbers do not match the original, its much better that there are more of them, or the transformers low frequency response will be more limited. Even though the resistance might be higher with more turns, it doesn't matter greatly as the transformer is not in the job of transferring power. If a bobbin is hand crafted out of transformer card, or FR4 fibreglass ( I Araldite them together), since it will be jumble wound, without insulation layers, it may be possible to get the same number of turns with a grade thicker wire, or more turns even with the original size wire.

Re: Interstage Audio coupling transformer
 

Pulls up a seat and gets popcorn :thumbsup:

Re: Interstage Audio coupling transformer
 

Thanks, Argus! (It's not actually a case of 'more turns is always better' because as turns increases, winding self-capacitance also increases so as low-frequency bass response improves, high-frequency response drops off. But this is unlikely to be the limiting factor with the wire Chris has.)

Chris - another thing to get right, is make sure you have a winding mandrel which supports the side cheeks of your bobbin. By the time you have filled the bobbin with wire, the pressure on the side cheeks is pretty high and if unsupported during winding, you are likely to find that they have spread apart so much during winding that you can't fully insert the laminations. And that also kills inductance and hurts low-frequency response!

Re: Interstage Audio coupling transformer
 

Thanks for the hints and tips. Thanks for the tips for supporting the bobbin on the mandrel I would not have thought about that. Unfortunately the wire I ordered won’t be here until next week but I can at least wind off the bobbin.

I think I may have to change my plan for winding coil as I don’t think the mandrel will fit the small hole in the bobbin. May have to put it on the lathe or the milling machine with some type of home brew clutch so I don’t break the wire.

A cheap electronic counter arrived today so I am going to build something to count the turns off the old coil.

I will post some pictures over the weekend

Thanks again for all the help and encouragement

Regards Chris

Re: Interstage Audio coupling transformer
 

Yes the advice about supporting the bobbin cheeks while you wind is *very* pertinent. I once wound an inverter transformer (to get 450V@150mA from 12VDC) using a colour LOPT as the core - made up nice Paxolin-sheet bobbins but the unsupported cheeks bulged when I wound them and then when the two cores were fitted there was about a 1mm gap between the ends.

Which contributed to about a 10% reduction in efficiency!

Re: Interstage Audio coupling transformer
 

Having removed the Interstage transformer I wanted to ensure that the radio would tune. Given I had found two open circuit transformers I feared for the tuning coils and hadn’t had any audio out So I have retained the -4.5 volt grid bias to the pentode output via a 500k resistor and also capacitive coupled to the detector output. The radio tunes and it’s as loud as I would ever want a radio to be. Sound quality is about as good as any 1930s radio I have owned. Now I’m still going to rewind the transformer, but cannot help thinking why did they bother to use a transformer when capacitor coupling works well and would have been cheaper.

Chris

Re: Interstage Audio coupling transformer
 

I think it was a common fix, if the secondary was O/C, to use capacity coupling. As Ed says, you could buy RC coupling units too.
Transformers were presumably a problem even from the early days.