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Old 20th February 2007, 09:58 AM   #1
zogu is offline zogu  Serbia
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Default Toroidal SE transformers

Does anyone has experience with toroidal SE transformer. I heard that Plitron transformer are great and much better than anything else on the market. I tried to find some text about that, but there is nothing. There is a book available with explanation: "Modern High-End Valve Amplifiers: Based on Toroidal Output Transformers" by Menno van der Veen (all plitron audio transformer are based on his knowledge), maybe someone has it and is ready to share experience? I'm willing to buy this book, but it is not available in my country what means that it will cost me about 200$ because of our laws, too much. Toroidal transformers are ideal for diyers because there is no air gap which is impossible to build correct with amateur tools, and quality is always the same. So guys please help!!!
Regards, Zogu
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Old 20th February 2007, 10:13 AM   #2
ulibub is offline ulibub  France
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...as you said:

"Toroidal transformers are ideal for diyers because there is no air gap....."

so this excludes them absolutely for conventional SE output stages - only Parafeed with a conventional air-gapped plate choke would still be an alternative. Their comparably small cores don't tolerate any DC-magnetisation.

Toroidal OPTs are good fpr PP-amps. If you need a low prim./sec. ratio only, you might even try simple and cheap mains transformers, generously oversized in their power handling capabilitiy. I useed such transformers (115:30V, 160 VA) as OPTs in my Circlotron amp (about 60 W rms) - with very satisfying results.
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Old 20th February 2007, 10:28 AM   #3
zogu is offline zogu  Serbia
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Hi ulibub,
yes, i thought that too, but there is a way to eliminate D.C. Look at hxxp://www.plitron.com/audio_se.asp. This transformer works great, it is expensive but square signal on 50hz looks almost like square which is very rare, much better than Tango U-808, my friend has both and we tested. And this transformer is toroidal SE transformer!!! How they eliminate D.C.? This is maybe written in book which i mentioned in previous post, or maybe someone else nows that. Only way which looks logical to me is low primary D.C. impedance with huge core for big impedance, for example 50ohms and 70H, if i use ohm law 50ohms * 0.070a = 3,5v D.C and big impedance is always good for eliminating D.C. Maybe core can tolerate this amount of D.C. I'm not transformer expert and I now very little about it, that is why I'm looking for help and maybe is way to completely eliminate D.C. with wiring methods....
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Old 20th February 2007, 11:33 AM   #4
ulibub is offline ulibub  France
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Hi Zogu!

Ok, if you give up with toroidal's main advantages (saving space and weight, thus reducing stray capacities and inductances), but using large cores and air-gaps (which indeed should also be possible with toroidals - double C-cores are nothing else than with iron band toroidally wound cores in rectangular form stabilized with resin and cut into two halves), you also could realize any transformer you'd like in toroidal style.

To achieve some tolerance of pre-magnetization, a big amount of iron is the only thing that matters (if you dont want to use such things like DC-compensation windings....), the more if you don't have an air gap - the shape of the core being secondary. So certainly using for instance a 500 VA-core (without air-gap) to transfer finally 13 Watts of power certainly would allow for a fair amount of pre-magnetization.... Looking at the mechanical dimensions of these Plitron transformers I would assume something like that. But then I'm asking if I still have the advantages typical for a toroidal - or if a conventional xfmr then wouldn't be equally as good.

But that's just our advantage of being amateurs - trying everything imaginable!

I haven't used Plitron (van der Veen) toroidals so far, but I've heard only good things about them. I also have an older book of Menno van der Veen, and I certainly can say that I got the impression looking at the physical backgrounds he presents there that this man knows what he's talking about...

I also used a generously oversized toroidals in my amp to allow for some (always present) DC-imbalance of the output tubes.

All the best

Uli
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Old 20th February 2007, 11:35 AM   #5
Yvesm is offline Yvesm  France
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Hi zogu,

Perhaps Plitron simply cuts the toroïd to form a gap ?

Yves.
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Old 20th February 2007, 11:55 AM   #6
jlsem is offline jlsem  United States
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Quote:
Perhaps Plitron simply cuts the toroïd to form a gap ?
Yes, Plitron uses gapped tape wound cores. Their 3500 Ohm SE OPT has a primary inductance of 28 Henries.

John
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Old 20th February 2007, 12:35 PM   #7
zogu is offline zogu  Serbia
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Hi jlsem
I am pretty shure that there is no gap in plitron transformers. And there is no gap in datasheet of transformer, it is very important data.
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Old 20th February 2007, 12:45 PM   #8
jlsem is offline jlsem  United States
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Quote:
I am pretty shure that there is no gap in plitron transformers. And there is no gap in datasheet of transformer, it is very important data.
Their equivalent push-pull transformers have primary inductances of ~500 Henries. The primary inductance of the 5K SE OPT is only 28 Henries, which would indicate the core is gapped. I have seen the Plitron single-ended output transformers used in conventional SE amplifiers. They wouldn't work in those situations without a gap.

John
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Old 20th February 2007, 01:02 PM   #9
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I emailed Plitron about this several years ago. The reply stated that they use a "distributed gap" to allow for the DC in SE designs. At the time they also made toroidal chokes for power supply use the same way.
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Old 20th February 2007, 01:05 PM   #10
ulibub is offline ulibub  France
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Quote:
Originally posted by zogu
Hi jlsem
I am pretty shure that there is no gap in plitron transformers. And there is no gap in datasheet of transformer, it is very important data.
That's most probably a production secret, not of any importance at all to know for amp builders. Such a low prim. inductance, just with such a big xfmr at the given impedance can only be reached with an air gap - and quite a large one.

Or do they use some kind of ferritic core material?

physics is as it is....

Uli
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