Transformer windings question. - UK Vintage Radio Repair and Restoration Discussion Forum

Techman · 21st Sep 2014, 7:21 pm

I have been today to collect a faulty transformer from a linear amplifier. The reason for this is that I have in the distant past had these, now over thirty year old valve amplifiers, pass though my hands for repair. These units were made in Italy to run on 220 volts, so it was always the case that they were run on a bit of an overload - as well as being built down to a price. In the past I used to just replace these transformers, but that was in the days when I had quite a stock of transformers to substitute. I generally used to fit two separate transformers, one for the HT and another small one for the valve heater.

There are a few of these amps starting to turn up and as well as all the usual faults, the burnt out transformer is likely to show up too. Infact, I have one at the moment which is heavily modified with a non-original transformer set up. I had been asking around on the radio if anyone had an old transformer that had burnt out, that I could have to 'experiment' with, as all the ones that I had replaced thirty years ago, I had long since chucked, thinking that I wouldn't be 'into' this sort of thing any more. So, someone had a transformer that I could have to play with, but the wires have been cut away from the original board and I have no reference as to which winding was which, other than the obvious heater winding. I wish I'd taken a note of the wiring from the last 'original' one I had for repair.

As would be expected, the heater winding is likely to be ok with a very low resistace of 0.1 ohm. This winding which has yellow lead out wires is wound over the winding on the lower bobbin that has red lead out wires and it is this winding that is burnt and reading just over 1 ohm. This winding seems to be wound with a 'double wire, ie, two thinner wires in parallel. The upper bobin has a single winding which 'looks' ok and has black lead out wires and reads just over 11 ohms. I suspect that this winding is probably ok.

Looking back at my past notes, I see that I've noted that a working unit when run from a variac at 220 volts AC, the HT winding gives 300 volts at a current of 0.6 amps. This feeds a voltage doubler circuit. The heater current is sufficient to power a 6KD6 or an EL509.

The question that I'm asking is which one is likely to be the mains primary winding and which one is likely to be the HT winding? Is the heater winding likely to be wound over the HT winding or the mains primary winding on a transformer like this?

I'm looking into trying to take this transfomer apart to see if I can do a bit of re-winding. Any tips or advice from the transformer experts - or anyone else, for that matter please. Pictures below:-

julie_m · 21st Sep 2014, 7:34 pm

In a modern power transformer, the primary and secondary windings are separated by the centre cheek of the bobbin; so that would make the black wires the primary.

Techman · 21st Sep 2014, 7:38 pm

Ah, many thanks for that, AJS.

It's a shame that the 'probably' good primary will therefore still need some attention due to it needing some extra turns adding to match it to 240 volt mains.

Ed_Dinning · 21st Sep 2014, 8:17 pm

Hi Techman, this style of transformer has good isolation from the safety point of view, but poor coupling and regulation as the windings are side by side, not over each other. As has been said, the HT and heater windings will be on one side, the mains on the other.

It is quite common to use bi-filar winding for one of the windings. It can give a better "packing factor" and may also be economic if a large volume of that particular gauge is used.

On conventional transformers it was usual to wind mains pri; safety earth (copper) screen); HT secs; LT windings.

LT was usually put on last as it can be "lumpy" and is not good as a base for further windings.

It should be quite easy to re-design the transformer on modern lams and perhaps use a single, higher voltage winding in place of a voltage doubler.

PM me if you want more info.

Ed

21st Sep 2014, 7:21 pm	#1
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Transformer windings question. I have been today to collect a faulty transformer from a linear amplifier. The reason for this is that I have in the distant past had these, now over thirty year old valve amplifiers, pass though my hands for repair. These units were made in Italy to run on 220 volts, so it was always the case that they were run on a bit of an overload - as well as being built down to a price. In the past I used to just replace these transformers, but that was in the days when I had quite a stock of transformers to substitute. I generally used to fit two separate transformers, one for the HT and another small one for the valve heater. There are a few of these amps starting to turn up and as well as all the usual faults, the burnt out transformer is likely to show up too. Infact, I have one at the moment which is heavily modified with a non-original transformer set up. I had been asking around on the radio if anyone had an old transformer that had burnt out, that I could have to 'experiment' with, as all the ones that I had replaced thirty years ago, I had long since chucked, thinking that I wouldn't be 'into' this sort of thing any more. So, someone had a transformer that I could have to play with, but the wires have been cut away from the original board and I have no reference as to which winding was which, other than the obvious heater winding. I wish I'd taken a note of the wiring from the last 'original' one I had for repair. As would be expected, the heater winding is likely to be ok with a very low resistace of 0.1 ohm. This winding which has yellow lead out wires is wound over the winding on the lower bobbin that has red lead out wires and it is this winding that is burnt and reading just over 1 ohm. This winding seems to be wound with a 'double wire, ie, two thinner wires in parallel. The upper bobin has a single winding which 'looks' ok and has black lead out wires and reads just over 11 ohms. I suspect that this winding is probably ok. Looking back at my past notes, I see that I've noted that a working unit when run from a variac at 220 volts AC, the HT winding gives 300 volts at a current of 0.6 amps. This feeds a voltage doubler circuit. The heater current is sufficient to power a 6KD6 or an EL509. The question that I'm asking is which one is likely to be the mains primary winding and which one is likely to be the HT winding? Is the heater winding likely to be wound over the HT winding or the mains primary winding on a transformer like this? I'm looking into trying to take this transfomer apart to see if I can do a bit of re-winding. Any tips or advice from the transformer experts - or anyone else, for that matter please. Pictures below:- Attached Thumbnails

21st Sep 2014, 7:34 pm	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Transformer windings question. In a modern power transformer, the primary and secondary windings are separated by the centre cheek of the bobbin; so that would make the black wires the primary. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

21st Sep 2014, 7:38 pm	#3
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Re: Transformer windings question. Ah, many thanks for that, AJS. It's a shame that the 'probably' good primary will therefore still need some attention due to it needing some extra turns adding to match it to 240 volt mains.

21st Sep 2014, 8:17 pm	#4
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Transformer windings question. Hi Techman, this style of transformer has good isolation from the safety point of view, but poor coupling and regulation as the windings are side by side, not over each other. As has been said, the HT and heater windings will be on one side, the mains on the other. It is quite common to use bi-filar winding for one of the windings. It can give a better "packing factor" and may also be economic if a large volume of that particular gauge is used. On conventional transformers it was usual to wind mains pri; safety earth (copper) screen); HT secs; LT windings. LT was usually put on last as it can be "lumpy" and is not good as a base for further windings. It should be quite easy to re-design the transformer on modern lams and perhaps use a single, higher voltage winding in place of a voltage doubler. PM me if you want more info. Ed