OT interleaved or not

[benito_red]

good OutputTransformers have interleaved windings in order to reduce leakage inductance n parassitic capacitance.
In this way it is possible to extend the treble response, thats good for HiFi amps but not for guitar amps where a given amount of treble filtering due to the OT results in a sweeter tone.
Somebody told me that vintage OTs used silicon iron (instead of grain oriented) and the winding schema for PP OT was:
1ST half Primary winding | secondary winding | 2ND half Primary winding

Can somebody confirm that?

[trobbins]

No. Exemplary OT's were being made before 1950 - but like now, you get what you pay.

[Steve Conner]

The RDH4 has a huge amount of information on output transformer design. The winding scheme mentioned above is the lowest performance option listed there.

IIRC, the next option is to split the primary in three parts and the secondary in two.

I'd expect most guitar amp OTs to use one or other of these schemes.

[jpfamps]

good OutputTransformers have interleaved windings in order to reduce leakage inductance n parassitic capacitance.
In this way it is possible to extend the treble response, thats good for HiFi amps but not for guitar amps where a given amount of treble filtering due to the OT results in a sweeter tone.
Somebody told me that vintage OTs used silicon iron (instead of grain oriented) and the winding schema for PP OT was:
1ST half Primary winding | secondary winding | 2ND half Primary winding

Can somebody confirm that?

Well some transformers are even simpler than that, eg the Tweed Deluxe OT where all of the primary is would first, then all of the secondary. Champ transformers are similar.

From inspection, some reading and chatting to transformer winders, it seems that the most popular winding pattern is a 5 section transformer consisting of 1/4 primary, 1/2 secondary, 1/2 primary, 1/2 secondary, 1/4 primary. The two out layers make one half of of the primary and the centre section the other half. This neatly helps to balance out the resistances of each half of the secondary.

An example of this transformer winding pattern is the VOX AC30. Steve Giles site goes into the teardown and rewinding of a transformer here:

Vintage Pedal Workshop » AC30 Output Transformer

For reasons that elude me, but outlined in the RDH4, using a 1/2 section on each end of the winding pattern (1/4 is 1/2 of 1/2. Did that make sense?) gives you much more bang for buck in the leakage inductance stakes for a given number of sections.

Whether extending the treble response of the transformer by interleaving "improves" the tone of the amp will I suspect depend very much on the design of the amp and the tastes of the player, however an extended treble response will give you more phase margin when applying negative feedback.

[EFK]

Interesting subject. I talked a little about this with a guy named Phil at Heyboer a while ago when I was having some custom transformers made. We were discussing a bf bassman type transformer with a custom impedance ratio, and he asked if I wanted it interleaved to the same degree as the original that they tore down; iirc, he said it was interleaved in something like 7 or 8 stages but at the time these were intended more for hi-fi use and most people recreating them for resale today (not just that particular style but others who are heyboer resellers to spec) usually just go w/ the 3 layer type described by the OP here to save money. He also said that for guitar amp use, it was his opinion that a more highly interleaved OT was not really necessary and that in fact it might enhance some higher frequency stuff that guitarists might otherwise prefer to be subdued. Anyway I just thought this information might be germane to the topic. I ended up going the full nine yards myself, and I do have to say that *in my opinion* the transformers they built for me sounded noticeably better than other bassman types that I had auditioned for the amps in question, although as I mention the impedance was not the standard 4K-ish so it's not really an apples for apples comparison.

[J M Fahey]

Congratulations on making things the good way and not just settling for cosmetics.

[Steve Conner]

Thanks for the info, jpfamps! The 5-section pattern you described is exactly the one I was thinking of. I've seen single-ended OTs that aren't interleaved at all, but I didn't know the 5E3 one was like that.

I think the reason for using half-sections at the ends is that they are only coupled to the secondary on one side, so they have more leakage inductance than the inner sections. Therefore, moving wire out of them into the inner sections, which are sandwiched between secondaries, gives a lower overall leakage inductance.

(But if you moved all of the wire into the inner sections and did away with the half-sections altogether, presumably the leakage inductance would go up again. )

I seem to remember that quite a few guitar amps used hi-fi OTs. Didn't Sunn just buy transformers in from Dynaco? And there were the old Marshall amps with the RS "Universal" OTs.

[Gregg]

Most of the early guitar amps had poorly interleaved OTs if at all. Some of the more recent versions are non or poorly interleaved.
Concerning the treble response from my experience I can tell you that a two half primary and 3 section secondary can give you easily higher than 20kHz response the question is whether you need that much (same applies for the low frequencies).
It's a matter of taste and depends on the amp design and speakers used. Usually a 5 section OT is more than enough for guitar amp purposes.

[benito_red]

thanks to all of you for very interesting comments.

hello jpfamps
very nice to know about AC30 OT windings, however in VintagePepedalWorkshop site, they do not tell what kind of core material needs to be used, as grain oriented (M6) or not oriented (M19) will make a big difference, do somebody know
which core material for AC30 or other amps (ie 5E3 5F1) ?

[Steve Conner]

All grades of transformer iron are actually silicon steel. Grain-oriented silicon steel is just a premium grade with carefully controlled manufacture and special heat treatment.

It was invented in 1935, so it could have been used in any of these transformers. It's just a case of how much the designer wanted to spend.
Electrical sheet and method and apparatus for its manufacture and test

The output transformer design section of RDH4 briefly discusses special materials, but concludes that "a larger core of ordinary iron will do better". Unfortunately the writer doesn't specify what he meant by ordinary iron.

The number in the "M" designations is just core loss in watts per pound, so M6 is the lowest loss stuff.

Interestingly, the patent says nothing about GOSS having a preferred direction of magnetisation, but Wikipedia claims that it is 30% better in the direction of rolling.

[Gregg]

Actually M6 is the lowest grade grain oriented steel but it has lower losses compared to the highest grade electrical steel which is M15 I think.
The next grade M4 is twice as expensive as M6 but this is if you're into HiFi and stuff.
I have transformers made with M27 and they still sound OK if you use more iron.

[Steve Conner]

Interesting... maybe M6 is the best grade worth using in EI laminations, and the better grades only show a benefit in toroids and C cores?

According to Thyssen Krupp, their grain oriented steel has 3x the core losses when the field lines are at 90 degrees to the "grain", and 4x at 60 degrees.

[Gregg]

Check out these files from one of the manufacturers. There's a lot of data on both types of steel:

Electrical steel.pdf

Grain oriented steel.pdf

[trobbins]

The winding layer structure is aimed to keep the MMF magnitude as low as possible moving from inner to outermost layer - this is pretty well understood in switchmode transformer design - I can't find the example I've seen in the past, but this ref has a good description:
http://ecee.colorado.edu/copec/book/...Ch13slides.pdf

[Diablo]

Here's a short overview on electrical steels. There are two classes of fully processed electrical steel; Grain oriented (GOES) and Non Oriented (NOES). GOES contains around 3% silicon. NOES is produced with 2-3% silicon, and some other alloy additions depending on the grade. NOES has similar properties (not identical though) in all directions of the sheet. The core losses for any grade decrease as the sheet thickness decreases. It's also more difficult to make thinner steel, because of the brittle nature of high silicon steel.NOES is much less expensive than GOES because the processing in the mill requires only about 6 steps. GOES requires more than 23 steps in the processing. GOES is a highly textured steel, which means it has a lot of crystalline anisotropy. The grains are aligned so that rolling direction of the sheet is the easiest to magnetize by a large amount. Transformers can take advantage of the superior properties in the rolling direction of GOES by aligning the flux in the rolling direction. You can also use (and many manufacturers do use) NOES in transformers to lower manufacturing cost. GOES is available at very thin gauges, much thinner than NOES. GOES is available down to M3 grades. NOES is also used in electrical motors where the flux moves through every direction of the sheet, and you really wouldn't want a highly oriented steel - the motor torque would pulse with GOES as the shaft turned. Thinner laminations lower the core losses, but increase the manufacturing cost becaue the steel costs more and because the punching and handling costs increase for a given stack height. There's a relatively recent addition to the GOES. Besides the regular grain oriented (RGO), there is also a High-B grade. High-B has higher permeability than RGO, through special processing at the mill. The best GOES costs less than $5000/ton these days, while NOES is under $2000/ton, and cold rolled sheet steel is under $1000/ton. Prices do fluctuate by a considerable amount depending on the world economy.