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Asymmetrical P-P OPT Primary

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The output primaries of the the Rowe R-4359a jukebox amp bought for iron (a simple common cathode/cathodyne/p-p pentode circuit) measure ~55 ohms DCR from B+ to one side and ~74 ohms on the other. The effective operating impedances haven't been measured. It was designed for 7868 tubes and rewired a couple days ago for 7591s.

Is the primary DC imbalance a result of 'economical' winding technique or was it common in jukeboxes to maintain a little 2nd HD sweetener? Manufacturers played with the secondary windings, including returning the speakers to an out-of-phase tap of the opposite channel to make up for the narrow driver spacing of the typical box. A distortion analysis though shows pretty good 2nd harmonic cancellation.
 
Transformer Symmetry

What matters the most in a transformer is the number of turns - this gives the voltage and impedance ratio. If one half of the primary is on the inside, nearest the core, and the other half is on the outside, the outside windings will have a longer length, hence higher resistance, even though they have the same number of turns. It is pretty common for push-pull transformers to have different resistances for each primary half. Some forms of interleaving the windings tend to equalize the resistance, but it is hard to get it exact.

Sometimes when I build an amp, I "build out" the low resistance half by adding an external resistor chosen to make the two halves have the same resistance. Then I can check for current balance by putting a voltmeter from plate to plate. If both the current and resistance is balanced, the DC voltage will be zero.
 
Thx John. A non-interleaved winding technique seemed the most likely cause but the large difference in resistance had me wondering. As did the measurements, nasty little piece of work this circuit. The manufacturer does in fact exactly what you describe, insert a small value resistor in the plate circuit of one side. Thx again!
 
hi RDF & JohnAtwood,

Many sorry to expose too much informations.

P1~B1 & P2~B2 was Asymmetrical P-P OPT Primary.

But it must PP in same two type of tube such as 6L6,KT66 PP etc.

the reason of this was making for Rich 2nd & 3rd harmonic.

Snce SE OPT was rich in 2nd & 3rd harmonic. If PP use Asymmetrical P-P OPT Primary can increase some of 2nd & 3rd harmonic too. BTW, It need more technical skillful to do this.

thz

thomas
 
I have a good supply of P-P OPT's that were made for guitar amps by Schumaker. The DC resistance of one half of the primary is 52 ohms and the other side is 60 ohms. I dissassembled one of these to find out why this is so. There is no interleaving. One primary half is wound first, closest to the core, then the secondary (common, 4, 8 and 16 ohms) is wound next, followed by the other half of the primary. As John pointed out the outer winding has the same number of turns as the inner winding, but the winding path is longer, using more wire, and having a higher DC resistance.

On paper these would suck for HiFi, but I have used them in dozens of HiFi amps including my 300Beast (P-P no feedback at all) with good results. They were rated for 80 watts in the guitar amp world, but are good for about 30 watts in HiFi. Several years ago I bought 200 of these for $16 each, a deal! They still show up on Ebay from time to time.

I have seen similar transformers where the outer winding was wound with a slightly thicker wire (I am always taking things apart). The DC resistance was more balanced, but the transformers didn't sound as good.
 
Hey-Hey!!!,
DCR differences between halves of the primary are no reason to exclude interleaving. It just shows that the sections are on average different distances( radially ) from the start of the bobbin.

First example that comes to mind are all the Dynaco outputs...:)
cheers,
Douglas
 
Yeah. Virtually every PP OT that I've measured had different DCR on the two halves, even some that are considered to be high quality. I strongly doubt that any of them were deliberately made asymmetrical. Only the very, very best of the best big money designs went to the trouble of balancing the DCR.

Most of these transformers were loading pentodes which have plate resistances in the tens of thousands of ohms. Ten or twenty ohms in the OT is irrelevant and completely swamped my variations between tubes anyway.

-- Dave
 
hi,

looks the atmosphere was not good. but I would like to say, IF OPT not had interleaving the capacitance will very large. Bad for HF. Another reason was most PP amp OPT design was P1--SG1--B1 & P2--SG2---B2 was use seperate Bobbin, same volumn to winding........why will had different DCR!!! pls considerate!!

there contain little design secret of the OPT. sorry to say I cannot talk more.

thx

thomas
 
Thanks all! It wasn't so much a difference as its magnitude, in the range of 35%, which had me wondering. Couple that to an OPT having two secondary windings with a total of eight taps in a stock circuit that measures :yuck: and I though it worth picking a brain or five before going forward. As Thomas suggests the extreme top end peaks or dives depending on which of the main secondary outputs is chosen, even into a resistive load with overall loop feedback.

For the curious I posted the schematic of the near-identical Rowe R-2620 on ImageShack. URL below, be forewarned it's a ~675 kB JPG. (Interesting way to load a 5U4GB with 80 uF, and it works!)

http://img158.imageshack.us/img158/7290/jbm200completeschematiccj9.jpg
 
tube-lover said:
IF OPT not had interleaving the capacitance will very large.

It's the other way around. More interleaving means more capacitance. More interleaving also means less leakage, which is the reason for using it. Transformer design is a balancing act between capacitance, leakage, and money. The last one weighs differently depending on the application.


Another reason was most PP amp OPT design was P1--SG1--B1 & P2--SG2---B2 was use seperate Bobbin, same volumn to winding........why will had different DCR!!!

The vintage transformers that we're talking about did not use dual or split bobbins, or any bobbins at all for that matter. They used a cardboard coil former with all of the windings built up one on top of another, layer by layer. Sectional windings (side by side) without a split bobbin was very expensive so used only in the most expensive transformers.

Plastic bobbins are more common today (though still not used by every manufacturer; some prefer the old school method of layer winding.) It is much easier to wind balanced sections as you describe in a split bobbin, though it is still rarely done in the *output* transformers that I see. Lundahl is one exception, there are probably others... (Power transformers are a different matter. I regularly see power transformers with the primary and secondary in separate bays of a split bobbin. But, when dual primary or secondary windings are employed they are still wound one on top of the other, so they do not have balanced DCR.) Perhaps the transformers in Hong Kong are traditionally different from those in the US ...

True dual bobbins are rare except on single C cores where one bobbin is placed on each side of the core. Double C cores are similar to EI type with a single bobbin. The output transformers that I have seen with C cores are all double C, not single C, though I certainly have not seen every output transformer ever made.

-- Dave
 
hi Dave,

let me share some opinions of OPT.

True dual bobbins are rare except on single C cores where one bobbin is placed on each side of the core. Double C cores are similar to EI type with a single bobbin. The output transformers that I have seen with C cores are all double C, not single C, though I certainly have not seen every output transformer ever made.

what is the meaning of true dual bobbins??? I don't understand.


The vintage transformers that we're talking about did not use dual or split bobbins, or any bobbins at all for that matter. They used a cardboard coil former with all of the windings built up one on top of another, layer by layer. Sectional windings (side by side) without a split bobbin was very expensive so used only in the most expensive transformers.

But most europe type PP OPT was use double Bobbins.

for real double c-core. EI type such as Brian sowter was use double bobbin.

In Power tansformers, most digital pwer supply of DAC was seperate into wo bobbin, seperate into pri & sec. Tis type of Trans will lower noise, so good for DAC.

It's the other way around. More interleaving means more capacitance. More interleaving also means less leakage, which is the reason for using it. Transformer design is a balancing act between capacitance, leakage, and money. The last one weighs differently depending on the application


yes, some was right, since to many interleaving will large capacitrance, but I would like to know, U means interleaving will use paper or any polyester film to layer or not????

If U did not use paper between interleave will very large capacitance. Two metal cannot close together otherwise capacitance will large. another reasons was the voltage different between pri & sec. In the safty reason, we must use interleaving to solve this problem. There will had another reason wasU can use winding method to lower capacitance But the best still use better material. as U said, balance for leakage, capacitance in OPT was important. But pls don;t remember that lower DCR, higher Inductance & phase shift was very important for OPT.


Lundahl single core was good in higher 'K'impendance OPT & said higher turn ratio transformers.

But if u use lower pri impendane trans still use double c-core will better better the path of the magnetic flex wil more balane, shorter distance.


sorry for my bad English , hope U understand what I mean.

thx


thomas
 
tube-lover said:
what is the meaning of true dual bobbins??? I don't understand.

We are using different terminology. When I said "dual bobbins" I meant two physically separate pieces. A single bobbin with two separate areas for winding I call a "split bobbin."

But most europe type PP OPT was use double Bobbins.

If you say so. When did they start doing that? In the US bobbins didn't start to become common until fairly recently, i.e. after SS took over the audio market. The particular transformer that started this thread was manufactured much before that, so almost certainly does not use a bobbin of any kind, dual, double, split, single, or otherwise.

It might be that our sense of vintage vs. contemporary is different. When I talk about what a manufacturer did I'm talking about 50 years ago. You might be talking about something much more recent.

For example, you say:
In Power tansformers, most digital pwer supply of DAC was seperate into wo bobbin

To me, the word "was" typically implies vintage, or at least 'old' technology that has been more or less replaced by something else. "Was" is past tense. I would probably use the word "are" here.

I don't know if you are confused by my language or not, but I understand that none of this changes any of the facts, it just highlights a potential problem in communication between us. Really, I'm not trying to criticize you in any way. I'm just trying to make clear the context of my previous posts.


yes, some was right, since to many interleaving will large capacitrance, but I would like to know, U means interleaving will use paper or any polyester film to layer or not????

I'm talking about interleaving with some kind of insulation between the windings.

Lundahl single core was good in higher 'K'impendance OPT & said higher turn ratio transformers.

But if u use lower pri impendane trans still use double c-core will better better the path of the magnetic flex wil more balane, shorter distance.

That's interesting. I'll remember that.


EC8010: it doesn't surprise me that Sowter uses dual chamber (split) bobbins. I'm sure there are other top quality units that have gone to those lengths. I doubt any of them have shown up in jukeboxes though. :D

-- Dave
 
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