Insulation strength
Dear Mr. Lacombe,
Let me pour some more light on this topic.
1. The transformer wound with such paper insulation as I described can really withstand such voltages without breakdown.
2. It is true that the breakdown is likely to occur at the ends of winding layers. The solution is simple: I always leave a safe space of 3-5 mm at each end of the layer. This is an old practice, which is good for high-voltage windings of mains transformers too.
3. The thick paper insulation has two advantages: first, it makes an uniform surface to lay down the turns, it is essential when the wire gauges for primary and secondary sections differ too much; second, it lowers the transformer capacitance.
I tried mylar insulation of different thicknesses, but it appeared very difficult to maintain precise arrangement of turns.
4. The total thickness of all insulation layers in my transformers exceeds 5 mm and this do not prevent me from reaching the leakage inductance less than 3 microhenry(as referred to secondary) and -3dB point at 180 kHz.
5. I use no curing of impregnating because this increases the dielectric constant of insulation and, hence, the capacitances.
7 and final. I saw no high-voltage transients at overload in all my amplifiers. Moreover, any good tube amplifier must withstand deep voltage overload even at the condition of open circuit at the output! this is necessary to cope with loudspeakers which are often highly reactive loads.
You can see more on my designs at http://www.cortmi.com.ua/omak
Regards,
Denis.
Dear Mr. Lacombe,
Let me pour some more light on this topic.
1. The transformer wound with such paper insulation as I described can really withstand such voltages without breakdown.
2. It is true that the breakdown is likely to occur at the ends of winding layers. The solution is simple: I always leave a safe space of 3-5 mm at each end of the layer. This is an old practice, which is good for high-voltage windings of mains transformers too.
3. The thick paper insulation has two advantages: first, it makes an uniform surface to lay down the turns, it is essential when the wire gauges for primary and secondary sections differ too much; second, it lowers the transformer capacitance.
I tried mylar insulation of different thicknesses, but it appeared very difficult to maintain precise arrangement of turns.
4. The total thickness of all insulation layers in my transformers exceeds 5 mm and this do not prevent me from reaching the leakage inductance less than 3 microhenry(as referred to secondary) and -3dB point at 180 kHz.
5. I use no curing of impregnating because this increases the dielectric constant of insulation and, hence, the capacitances.
7 and final. I saw no high-voltage transients at overload in all my amplifiers. Moreover, any good tube amplifier must withstand deep voltage overload even at the condition of open circuit at the output! this is necessary to cope with loudspeakers which are often highly reactive loads.
You can see more on my designs at http://www.cortmi.com.ua/omak
Regards,
Denis.
Denis,
Many thanks for your explanations. I cann't disagree with any of them. Perhaps, I must insist on the necessity of a good impregnation in order to maintain correct reliability over very long time.
The conclusion is that design and construction of a good output transformer is not simple at all.
Regards, P.Lacombe
Many thanks for your explanations. I cann't disagree with any of them. Perhaps, I must insist on the necessity of a good impregnation in order to maintain correct reliability over very long time.
The conclusion is that design and construction of a good output transformer is not simple at all.
Regards, P.Lacombe
Transformer impregnation
There were no voltage breakdowns in my unimpregnated transformers altogether.
One more secret: I use special industrial brand of insulating paper used in cable manufacturing. This paper has specified electrical strength and dielectric constant. The unimpregnated transformers with this paper have absolutely negligible spread of electrical parameters. When I experimented with various kinds of impregnation, the frequency responce and symmetry have degraded, and the difference in complex transfer function between individual transformers was much wider.
Major cause for a voltage breakdown is a damage of the insulation layer. The risk of such a damage is much reduced thanks to use of dense and relatively thick paper. Moreover, each layer of insulation is to be carefully inspected before laying down the next layer of turns.
By the way, too many brands of paper should not be considered an insulation material.
There were no voltage breakdowns in my unimpregnated transformers altogether.
One more secret: I use special industrial brand of insulating paper used in cable manufacturing. This paper has specified electrical strength and dielectric constant. The unimpregnated transformers with this paper have absolutely negligible spread of electrical parameters. When I experimented with various kinds of impregnation, the frequency responce and symmetry have degraded, and the difference in complex transfer function between individual transformers was much wider.
Major cause for a voltage breakdown is a damage of the insulation layer. The risk of such a damage is much reduced thanks to use of dense and relatively thick paper. Moreover, each layer of insulation is to be carefully inspected before laying down the next layer of turns.
By the way, too many brands of paper should not be considered an insulation material.
As always I come into things a bit late, but then I guess it's better to be late than never. Eh? A while back I was looking for info like you on this subject and came across this site you may find usefull if nobody else has mentioned it yet...
http://www.mc-h.demon.co.uk/vtheory/vxformers2.htm
and
http://www.mc-h.demon.co.uk/vtheory/vxformers3.htm
also,
Sound Practices a few years ago did a reprint of an old electronics hobby mag article on winding your own. I forget the issue but I have the copy. If you are interested contact me and I'll post a copy of the article to you via snail mail...
tomcat
http://www.mc-h.demon.co.uk/vtheory/vxformers2.htm
and
http://www.mc-h.demon.co.uk/vtheory/vxformers3.htm
also,
Sound Practices a few years ago did a reprint of an old electronics hobby mag article on winding your own. I forget the issue but I have the copy. If you are interested contact me and I'll post a copy of the article to you via snail mail...
tomcat
Thanks for the explanations.
On my first transformer I used Mylar(why mylar increases capacitances?) and I found a lot of difficulties to do the windings.Especially after some layers and I also use a machine.I find interesting the use of thick paper for all purposes.
My next transformer is going to be with thick paper.You suggest to use the same paper between the layers of the primary and the changes between primary and secondary?Or at the second case a thicker one?
I don't understand exactly about impregnating.You don't suggest (Denis) to impregnate the insulation paper?What about to sunk the transformer into a paraffin oil or into varnish?
My last question is something that I read in Veen's book.He suggests to use a secondary impedance of 5 Ohms and not separate 4 and 8 Ohms.'Cause extra secondary taps impair the high frequency.What about to use four secondaries that can be connected in series or parallel to obtain the outputs you want?
Regards George.
On my first transformer I used Mylar(why mylar increases capacitances?) and I found a lot of difficulties to do the windings.Especially after some layers and I also use a machine.I find interesting the use of thick paper for all purposes.
My next transformer is going to be with thick paper.You suggest to use the same paper between the layers of the primary and the changes between primary and secondary?Or at the second case a thicker one?
I don't understand exactly about impregnating.You don't suggest (Denis) to impregnate the insulation paper?What about to sunk the transformer into a paraffin oil or into varnish?
My last question is something that I read in Veen's book.He suggests to use a secondary impedance of 5 Ohms and not separate 4 and 8 Ohms.'Cause extra secondary taps impair the high frequency.What about to use four secondaries that can be connected in series or parallel to obtain the outputs you want?
Regards George.
Transformer design tips
Mylar in fact has lower dielectric constant than the paper. Indeed the usually available mylar films are much thinner, therefore the capacitance will be higher. But the most serious problem with any mylar film is a gerat difficulty in turns alignment. This is difficult even with a good winding machine. Otherwise, perfect alignment with thick paper insulation is relatively easy even with manual winding. In any case be prepared for a lot of painstaking work.
Yes, I suggest using the same thickness of insulation between individual layers as well as between the sections.
It is essential to calculate the interlayer and total capacitance of the transformer before making the final decision on the sectioning. The simple increase in a number of interleaved sections do not broaden the frequency responce!
By the way, the primary use of impregnation in the mains transformers is the enhancement in heat conduction, not the electrical strength. The only exceptions are really high-voltage transformers.
Warning! Some varniches can dissolve the insulation of winding wire.
Menno Van Der Veen is right about the secondary taps! It is impossible to achieve the same transfer function at each tap. More probably one gets equally bad frequency responce than equally good!
From other side he is wrong about 5 Ohms!
The precise choise of turns ratio is a very complex problem involving the actual design of the output stage. There are great differences in load matching for triode vs. pentode or SE vs. PP output stages. Presently not too many tube amplifiers have correctly optimised output transformers.
In any case one should ensure at least threefold reserve of available plate current in respect to rated one. It should be remembered that any loudspeaker is not a constant active load, and it can draw enormous currents at transients.
For more help on transformer design, please e-mail me.
Regards,
Denis.
Mylar in fact has lower dielectric constant than the paper. Indeed the usually available mylar films are much thinner, therefore the capacitance will be higher. But the most serious problem with any mylar film is a gerat difficulty in turns alignment. This is difficult even with a good winding machine. Otherwise, perfect alignment with thick paper insulation is relatively easy even with manual winding. In any case be prepared for a lot of painstaking work.
Yes, I suggest using the same thickness of insulation between individual layers as well as between the sections.
It is essential to calculate the interlayer and total capacitance of the transformer before making the final decision on the sectioning. The simple increase in a number of interleaved sections do not broaden the frequency responce!
By the way, the primary use of impregnation in the mains transformers is the enhancement in heat conduction, not the electrical strength. The only exceptions are really high-voltage transformers.
Warning! Some varniches can dissolve the insulation of winding wire.
Menno Van Der Veen is right about the secondary taps! It is impossible to achieve the same transfer function at each tap. More probably one gets equally bad frequency responce than equally good!
From other side he is wrong about 5 Ohms!
The precise choise of turns ratio is a very complex problem involving the actual design of the output stage. There are great differences in load matching for triode vs. pentode or SE vs. PP output stages. Presently not too many tube amplifiers have correctly optimised output transformers.
In any case one should ensure at least threefold reserve of available plate current in respect to rated one. It should be remembered that any loudspeaker is not a constant active load, and it can draw enormous currents at transients.
For more help on transformer design, please e-mail me.
Regards,
Denis.
For what its worth, I just read that the Marin Logan SL3 electrostatic speakers use air-core transformers for their step-up inputs. I'd not realized that air-core transformers were a viable audio option. Might bear closer research.
Air cored transformers
I see no real advantages of air-cored step-up transformers. The ferromagnetic cores do not cause additional distortion or colouration if the transformer is designed properly.
From other side, an air-cores transformer may pick up too much air interference and will further complicate the task of getting low leakage inductance.
I see no real advantages of air-cored step-up transformers. The ferromagnetic cores do not cause additional distortion or colouration if the transformer is designed properly.
From other side, an air-cores transformer may pick up too much air interference and will further complicate the task of getting low leakage inductance.
Time goes by so fast
I have found the link that TVI provided to be very interesting and the technical information presented there quite useful.
I have not done any design work with tubes since the early 1970' but still find the subject quite interesting. And I still have some transformers from a scrapped out Fisher 500C and a single Dynaco 320 transformer or something like that laying around. But if I was to build a new tube amp using one of my old designs (more or less) I would use some of the new toriod transformers that have cathode windings. These look very attractive to me.
In my early days I used the center tap of the 16 ohm winding to do the same basic job in my push pull amplifiers and used balanced feed back from the output stage cathodes back towards the drivers and phase inverters. As long as 16 ohm speakers were used the circuit remained in balance quite nicely. Now of course this would not be practical since most loudspeakers are much lower in impedance to match up with solid state equipment.
It does seem that I used to have a lot more free time then I do now. When this was so I did in fact wind quite a number of output transformers by hand. I don't believe that I would ever attempt to do this again. Not only because of the amount of time needed to build a truly good transformer but also because I don’t have any good steel to do so. I have not seen anyone provide a link to a good source of steel.
The whole process of building my own output transformers is just to time consuming for me to even consider doing.
John Fassotte
Alaskan Audio
[Edited by alaskanaudio on 12-01-2001 at 07:38 AM]
I have found the link that TVI provided to be very interesting and the technical information presented there quite useful.
I have not done any design work with tubes since the early 1970' but still find the subject quite interesting. And I still have some transformers from a scrapped out Fisher 500C and a single Dynaco 320 transformer or something like that laying around. But if I was to build a new tube amp using one of my old designs (more or less) I would use some of the new toriod transformers that have cathode windings. These look very attractive to me.
In my early days I used the center tap of the 16 ohm winding to do the same basic job in my push pull amplifiers and used balanced feed back from the output stage cathodes back towards the drivers and phase inverters. As long as 16 ohm speakers were used the circuit remained in balance quite nicely. Now of course this would not be practical since most loudspeakers are much lower in impedance to match up with solid state equipment.
It does seem that I used to have a lot more free time then I do now. When this was so I did in fact wind quite a number of output transformers by hand. I don't believe that I would ever attempt to do this again. Not only because of the amount of time needed to build a truly good transformer but also because I don’t have any good steel to do so. I have not seen anyone provide a link to a good source of steel.
The whole process of building my own output transformers is just to time consuming for me to even consider doing.
John Fassotte
Alaskan Audio
[Edited by alaskanaudio on 12-01-2001 at 07:38 AM]
BLMN
blmn,
Hi, i now that topic is old but i saw your message and i´m Brazilian too so i´m asking if you can scan and send to me this material that you have about transformers (power and audio transformers) in portuguese?
I have a lot of material in English about this but i don´t understand much of that this writing there, I wait that you sees this message.
thanks,
blmn,
Hi, i now that topic is old but i saw your message and i´m Brazilian too so i´m asking if you can scan and send to me this material that you have about transformers (power and audio transformers) in portuguese?
I have a lot of material in English about this but i don´t understand much of that this writing there, I wait that you sees this message.
thanks,
Hello everybody,
I'm used to wind OPT's myself. I agree it is a lot of job, tedius and time consuming, but I think this is also rewarding.
But let's came to the question.
I used to wind trasformers like Williamson's. Tree sections for the primary and two for the secondary interlived. Sometimes I wound five sections primary and four secondary. The lasts provided a little broader band in the high side. And a lot of extra work.
Now I'm tring to test a new type based on the lay out of the five/four layer but splitting the core in two. I think it is better to see the picture of it.
I read about a suggestion to make one section for each tube. But a question arises in my mind. Is that balanced? What is going on the magnetic flux when the current flows in just a half of the nucleus when the PP tube one conducts and the other not? Yes Impedance and resistence are perfectly balanced, but what about flux? Is there a secret about connecting windings interleaved in a cross way betwen the two sections for maintainig good balance and aving a plain flux distribution?
Somebody knows? Tried it out? Am I making a stupid question, just try out and see (This would be a big waist o time if the results would be trash)?
Best regards.
Larry.
I'm used to wind OPT's myself. I agree it is a lot of job, tedius and time consuming, but I think this is also rewarding.
But let's came to the question.
I used to wind trasformers like Williamson's. Tree sections for the primary and two for the secondary interlived. Sometimes I wound five sections primary and four secondary. The lasts provided a little broader band in the high side. And a lot of extra work.
Now I'm tring to test a new type based on the lay out of the five/four layer but splitting the core in two. I think it is better to see the picture of it.
An externally hosted image should be here but it was not working when we last tested it.
I read about a suggestion to make one section for each tube. But a question arises in my mind. Is that balanced? What is going on the magnetic flux when the current flows in just a half of the nucleus when the PP tube one conducts and the other not? Yes Impedance and resistence are perfectly balanced, but what about flux? Is there a secret about connecting windings interleaved in a cross way betwen the two sections for maintainig good balance and aving a plain flux distribution?
Somebody knows? Tried it out? Am I making a stupid question, just try out and see (This would be a big waist o time if the results would be trash)?
Best regards.
Larry.
Hi Larry !
I've no definitive response to give, just wondering at the same thing !
At now, I plan to use a total of 4 primaries in serie, two in each section of the bobin.
Let me call'em 1A, 1B, 2A, 2B, the numbers for the order they are wound, and the letter for the the section of the bobin on wich they are wound.
One could use 1A + 2B for a plate and 2A + 1B for the other.
This leave possibility for a total of 6 interleaved secondaries ( 3 on each section ) tied either in serie, parallel or any combination that optimises the windings according to wire dia used.
Anyway, I beleive that the magnetic flux remains equally distributed.
I think that distributing windings on two half bobin will also help to reduce parasitic capacitance, wich is good for improving hi frequency behaviour.
Just some thoughts .
Ciao, Yves.
P.S.
He ! Livorno !
So many contacts with guys there in VHF radio when I lived near Nice !
I've no definitive response to give, just wondering at the same thing !
At now, I plan to use a total of 4 primaries in serie, two in each section of the bobin.
Let me call'em 1A, 1B, 2A, 2B, the numbers for the order they are wound, and the letter for the the section of the bobin on wich they are wound.
One could use 1A + 2B for a plate and 2A + 1B for the other.
This leave possibility for a total of 6 interleaved secondaries ( 3 on each section ) tied either in serie, parallel or any combination that optimises the windings according to wire dia used.
Anyway, I beleive that the magnetic flux remains equally distributed.
I think that distributing windings on two half bobin will also help to reduce parasitic capacitance, wich is good for improving hi frequency behaviour.
Just some thoughts .
Ciao, Yves.
P.S.
He ! Livorno !
So many contacts with guys there in VHF radio when I lived near Nice !
The original Williamson transformer used 2 sections winded on a split bobin, one for each tube and each section consisting of 5 primary sections and 4 secondary sections interleaved so obviously it works very well to use 2 separate sections, the main argument for doing so being to achieve best balance between the 2 tubes.
Regards Hans
hint for best top end, use 0.6mm polyester insulation and 5 secondary sections with 4 primary sections.
this will be the best transformer you have ever made, way in excess of most commercial ones.
this will be the best transformer you have ever made, way in excess of most commercial ones.
Hello folks,
Thank a lot for answers, all are correct, but I think only Yvesm got the point but it seems that he share my same opinion.
Yvesm, Livorno (Leghorn) is a place for artists, OM, CB, and any other kink kind of people 😀
To be clearer, I was concerned about what is going on the nucleus central column flux when current flows to the maximum to the anode of one of the two tubes of a class AB1 amp.
Think at it as if you have a transformer wound on one half but your load is still connected to the full both vertical divisions, since the output windings are allmost all in parallel. This is my concern.
It could be irrilevant but.... it lead me to stop building the transformer till I'm certain on wether it influences something or not.
What do you think? Do I need a psychiatric rehabilitation?
Cheers.
Larry.
Thank a lot for answers, all are correct, but I think only Yvesm got the point but it seems that he share my same opinion.
Yvesm, Livorno (Leghorn) is a place for artists, OM, CB, and any other kink kind of people 😀
To be clearer, I was concerned about what is going on the nucleus central column flux when current flows to the maximum to the anode of one of the two tubes of a class AB1 amp.
Think at it as if you have a transformer wound on one half but your load is still connected to the full both vertical divisions, since the output windings are allmost all in parallel. This is my concern.
It could be irrilevant but.... it lead me to stop building the transformer till I'm certain on wether it influences something or not.
What do you think? Do I need a psychiatric rehabilitation?
Cheers.
Larry.
Hi Larry,
My opinion is that is totally irrelevent because, any way, with EI laminations, the lenght of the winding is already (approx) 1/3 of the magnetic path lenght.
Just my opinion, but I beleive in 😱
Yves.
My opinion is that is totally irrelevent because, any way, with EI laminations, the lenght of the winding is already (approx) 1/3 of the magnetic path lenght.
Just my opinion, but I beleive in 😱
Yves.
tubetvr said:
i know i should know this, but was the original williamson a class A design?
RDH suggests that both primaries should be well coupled to each other for class B designs and that this (kinda) holds true for class AB designs as well but they stop there and don't mention pure class A desings.
dave
No, it was in AB1 but very close to A with high bias current.
I think there is a concern with close coupling with class B as mentioned in RDH, I have seen it elsewhere but for AB1 close to A as is used in many high quality amplifiers it shouldn't be a problem and isn't in for instance a Williamson amplifier. A more serious concern is probably to make the 2 sides equal in order to eliminate unbalance so as to minimise 2nd order distortion and eliminate the risk of saturation due to the unbalance.
Regards Hans
tubetvr said:
agreed, and it is the only way to assure matched capacitances from side to side. People seem to forget that above the self-resoant frequency of the primary inductance and winding capacitance, the load is capacitive in nature and any capacitive mismatches between halves will show up as different loads on the tube.
dave
Hello,
So? Yvesm says that it coul be but the influence is negligible. Shure it is, in a class A, where the primary is just one. But in a class B where the current flows alternatively in both section? I think Yvesm is right again, and there is no evidence of any strange fenomena. But... in a one section winding interleving five primary and four secundary there is a better coupling because of the full colum winding, but if we split the windings in two sections when current flows in the primary left section the flow is premanently on a one half of the central column. Geometrically thinking there could be a way to better couple windings still having a good balance... Could it be?
Cheers.
So? Yvesm says that it coul be but the influence is negligible. Shure it is, in a class A, where the primary is just one. But in a class B where the current flows alternatively in both section? I think Yvesm is right again, and there is no evidence of any strange fenomena. But... in a one section winding interleving five primary and four secundary there is a better coupling because of the full colum winding, but if we split the windings in two sections when current flows in the primary left section the flow is premanently on a one half of the central column. Geometrically thinking there could be a way to better couple windings still having a good balance... Could it be?
Cheers.
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