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Old 6th February 2011, 03:33 PM   #1
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Default side by side bifilar windings in excess of 1000 volts

Having decided to embark on my first tube amp project a few questions came up. It seems that there is much information in favor of multifilar wound output transformers, however, I am having a hard time finding enameled or high tech finish copper wire which has an insulation rating above 1000 volts. It may even have to approach 2000 volts. Laying the primary and secondary layers side by side makes the full voltage potential difference only a coat of paint apart.
I could choose to wind one layer of primary and then secondary etc. Then there will be a huge amount of insulation in the stack.
Does anyone have any suggestions? Perhaps teflon fabric as insulation. It is very thin and has good dielectric properties.

The high impedance and voltage needed to drive the 813 tubes is the reason for constructing a multifilar stack. This math is already giving may a major headache. Though I am lucky to have all of the information one could ever need on this subject.

Thanks Tad
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Old 6th February 2011, 04:17 PM   #2
jcx is offline jcx  United States
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multifilar separate windings minimize leakage inductance at the expense of maximizing parasitc C, I believe that for any high turns ratio other than 1:1 turns ratio this will cut your bandwidth vs balancing the two types of parasitics with interleaving/sector winding techinques

Lenard Audio - Education - Valve Amps shows interleaving (not that I agree with everything on the site)

parallel bifilar is the 1st step in the bunch winding, Litz wire minimizing of skin effect eddy current loss reduction

if you still "need" KV wire-to-wire insulation then search for "triple insulation" magnet wire - 3 layers of differing insulation material (not to confuse with double/tripple/quad "build" - multiple coats of the same enamel/varnish)

alternating single layer windings with added insulating tape uses standard materials

Last edited by jcx; 6th February 2011 at 04:19 PM.
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Old 6th February 2011, 07:36 PM   #3
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Nowadays it is much easier to obtain teflon insulated winding wire, as used for switchmode transformers to gain safety compliance. The commonest types of insulation are single layer (used for an extra/supplemenary layer of insulation) and triple layer (for full rated compliance of primary to secondary mains). In the case of the OT primary, the single layer insulation type would be most applicable - if it provided an appropriate balance of parasitic capacitance and leakage inductance.

References are Furukawa FWX-E and Tefzel.

Ciao, Tim
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Old 6th February 2011, 07:45 PM   #4
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Quote:
Originally Posted by jcx View Post
multifilar separate windings minimize leakage inductance at the expense of maximizing parasitc C, I believe that for any high turns ratio other than 1:1 turns ratio this will cut your bandwidth vs balancing the two types of parasitics with interleaving/sector winding techinques
This is 100% true.
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Old 6th February 2011, 09:22 PM   #5
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It seems for my particular application side by side winding of these transformers may not provide the overall improvement I was trying to achieve. I shall begin by winding a rather simple unit and test just what the results are.
Wanting to make this amp ultralinear is going to require some more complex primary stacks than usual. A continuing learning process sometimes with prodigious amounts of magic smoke. The success is worth the effort.
Most go find some Teflon magnet wire -- cheap. It sounds like a good place to start even for conventional winding arrangements.

It would be nice if Champ amps could provide the transformer winding specs. He has done such a good job of capturing my interest with the information he has already provided. He no longer accepts email.

Thanks for the help. Tad
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Old 7th February 2011, 10:32 AM   #6
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hey-Hey!!!,
you speak of needing a high impedance for your 813 application and that leaves only a portion of the secondary available to the benefit of a bi-filar wind. Consider winding the two coils separated by a layer if insulation 'paper', same pitch, same number of turns instead of side-by-side in the same layer. You'll get most of the capacitive coupling benefit you seek as well as most of the reduction in leakage L.
cheers,
Douglas
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Old 7th February 2011, 10:55 AM   #7
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Originally Posted by Bandersnatch View Post
hey-Hey!!!,
you speak of needing a high impedance for your 813 application and that leaves only a portion of the secondary available to the benefit of a bi-filar wind. Consider winding the two coils separated by a layer if insulation 'paper', same pitch, same number of turns instead of side-by-side in the same layer. You'll get most of the capacitive coupling benefit you seek as well as most of the reduction in leakage L.
cheers,
Douglas
Go ahead, it won't work.
Because the winding ratio is not 1:1 (but let's say 35:1 for a high impedance application) the capacitive coupling is always working against you, no matter when it's done bifilarly or by layers.
"The same number of turns" per layer means rather thin wire for the secondary. Unless you wind a huge stack of primary / secondary layers with parallelled secondaries the DCR of the secondary will be high, bad for damping / output impedance. Winding the huge stack will lead to much capacitance, goodbye treble.
Trick is to find the right balance between capacitive coupling / leakage L to reach the acceptable bandwidth.
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Old 7th February 2011, 11:37 AM   #8
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Originally Posted by pieter t View Post
Go ahead, it won't work.
Because the winding ratio is not 1:1 (but let's say 35:1 for a high impedance application) the capacitive coupling is always working against you, no matter when it's done bifilarly or by layers.
"The same number of turns" per layer means rather thin wire for the secondary. Unless you wind a huge stack of primary / secondary layers with parallelled secondaries the DCR of the secondary will be high, bad for damping / output impedance. Winding the huge stack will lead to much capacitance, goodbye treble.
Trick is to find the right balance between capacitive coupling / leakage L to reach the acceptable bandwidth.
The capacitive coupling does *NOT* work against you always. Consider the first turns after the connection to B+ ( at AC ground ) and the turns of secondary starting from DC ground the coupling will work *FOR* you.

As to the same number of turns requiring thin secondary wire, that is again not required; keep the primary at the same pitch as the secondary...you'll have space between the primary wires axially on the bobbin.
cheers,
Douglas
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Old 7th February 2011, 12:02 PM   #9
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I think the idea may have been for multiple layers, but where each layer is a 1:1 filar mix of primary and secondary - the primary turns in each layer connected in series to the next layer - the secondary turns in each layer connected in parallel with the other layer secondaries. Either the primary, or the secondary turns, would use insulated wire (but not both - probably the primary). The diameter of the insulated primary would be the same as the enamelled secondary.
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Old 7th February 2011, 12:31 PM   #10
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[QUOTE=trobbins;2460436]I think the idea may have been for multiple layers, but where each layer is a 1:1 filar mix of primary and secondary - the primary turns in each layer connected in series to the next layer - the secondary turns in each layer connected in parallel with the other layer secondaries./QUOTE]

You will have excellent coupling between secondary ground and the whole primary. Result: no treble.
But try it and let us know the results; I've been all through this.
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