Other sources claim that the orners of the tranny are most "magnetic active".

good idea mate, sorry i did not get back sooner

Here is Edcor's response to the question: does Edcor know in what direction (relative to the core) the magnetic field the strongest?

Actually this reply from Edcor makes perfect sense.

Consider the attached image. The transformer electrical coils are represented by the red and green circles (red=current into the page, green=current out of the page). The blue arrows are the contained magnetic flux (B) and the black boxes show the magnetic core cross section.

The job of the magnetic core is to contain the magnetic flux so that any items in the vicinity don't have an adverse impact on the field. If the inductor were wound like a solenoid (i.e. with open ends) then the magnetic field (H) would spread out at either end and anything of different permeability (µ) in the vicinity would affect the magnetic circuit by interrupting the H field and other magnetic fields would couple directly into the electric circuit. By closing the core (and hence the magnetic circuit), all the magnetic flux is contained and the leakage is very small. If the core were continuous, there would be virtually no flux leakage at all. This is the concept behind a toroidal core transformer. All the flux is entirely contained within the transformer core and hence there is no external field to induce 60Hz interference in anything in the immediate proximity. In addition, the transformer cores are made up of laminations of grain oriented steel specifically designed to ensure the flux actually stays in the magnetic circuit as shown in the figure.

Now in most output transformers (especially SE ones) there is a gap between the "E" section and the flat "keeper" bar. The purpose of this gap is to raise the magnetic flux saturation level of the core and hence allow it to handle a greater bias current without saturating. (Saturation causes very nasty audio distortion.) In these transformers the only place for flux leakage to occur is the fringing field at the edges of the gap. In the construction of the core, this gap is very narrow. As such the coupling between Bo and Bo' is very good. This means that the fringing field is very small. In addition, edges of the gap contained within the end bells have their fringing fields contained by the metal of the bell. Hence the only place that there can be any magnetic interference is at the edges of the gap showing on the side of the transformer between the end bells. The probability of getting any interference there is minimal.

In addition, the grain oriented steel used in modern transformers is orders of magnitude better then that used even twenty or thirty years ago. As such, magnetic flux containment is really excellent in modern transformers. The old "orient the transformers orthogonally" is a hold over from earlier times when most transformers were of questionable quality in terms of flux containment.

Today, flux coupling between transformers is almost non-existent. However, there still are sensitive parts of the circuit which should be protected. High gain driver tubes can be influenced by even the small gap fringing fields on modern transformers. This means that driver tubes (especially small signal stages used in phono preamps) should not be placed along the sides of transformers where the gap field may be exposed. Also, the magnetic field coming off a rectifier tube can be one to two orders of magnitude (10 to 20 dB) greater then that produced by modern transformers. A greater concern then transformer placement is therefore rectifier tube placement w.r.t gain stages.

I hope this helps to clear things up a little bit. In either layout the amp looks really good. The only suggestion I would make is maybe rotate that big Edcor in the front by 90 deg so that the end bell is facing the driver tube instead of the edge of the core. This keeps any fringing field which may be present away from the driver tube on the PCB.

I'm looking forward to seeing how this one comes out. Keep up the posts.

Thanks Matt for that excellent explanation!

That makes sense. What about the OPT? Better pointed at the power tubes rather than the RCA input?
Cheers

I would always chose to point the side of the output transformers at the output tubes rather then the input. Personally I prefer the layout which puts the second OPT on the far right, away from the power transformer.

My general rule in amp layout is protect the inputs and first gain stage at all costs. After the first gain stage, the signals are large enough in a power amp that any risk of interference (except from a closed ground loop) is minimal.

All good suggestions. I 'tend to put speaker binding post near AC and keep low voltage stuff far away. Inside power amps I very rarely use shielded wire - 'cause its crap. Good quality single strand OFC copper wire, hard twisted, works better at some many levels.

Trannies and chokes with bells looks good and is usually very quiet.

Layout 1 is also my preference. The difference from that photo will be that the power transformer will be rotated 90 degrees. What about the choke? Is it fine with that orientation?

Also, I will make the monoblocks a mirror image of each other.
Cheers

I'd keep the choke the way it is in photo 1. This way the fringing fields are in line and not directed at any part of the amp. The power transformer and choke will always be operating in phase anyway.

It may also help the WAF (transformer mountings don't look so random to untrained eyes) even though you previously said that wasn't a concern.