OT Primaries and Secondaries

[Littlewyan]

Slightly possibly odd question. I'm in the middle of a new build where I've drilled all the holes myself and I have orientated the OT so the secondaries come in near the front of the chassis and the primaries near the valve sockets. Due to the length of wire I had left (second hand OT), this seemed to make sense to me, but I've just noticed on other builds that people tend to always have the secondaries come in near the valve sockets.......Am I going to bring myself into a world of noise once this amp is finished?

[Stevem]

The primary wires need to be kept as short as possible and away from the high gain low signal level of the preamp section ( like the first 2 gain stages) so that the high level of AC signal ( high level low current ) on those primary wires does not get impressed on those high gain low signal level parts.

The secondary side is low level and high current so as long as your secondary wires and speaker jacks are 4 to 5 inches away from the input jack(s) itself you will be fine.

[pdf64]

I’ve always thought it perverse that the typical Marshall arrangement has the OT primaries coming into the chassis immediately adjacent to the tone stack controls. As Stevem describes, seems like there ought to be way less opportunity for unintended coupling if they‘re the other way around, which, like you suggest, is how I lay them out.
So for the OT arrangement I’d carry on as you are. For the board layout, I’d make sure that the red and black choke wires are well away from any sensitive circuits.

[Littlewyan]

Thanks guys. The speaker jacks are quite far from the Input Jacks and the Choke wires come into the chassis under the Phase Inverter circuit, so I think it'll be ok. I have been taking photos throughout the build process and will post them up here at some point! It's the most complicated amplifier I've ever attempted!

[TUBEDUDE]

If you use the headphone technique to position your OT you will get minimum hum. As far as signal induced from the wires, remember the field strength generated is dependent upon the current, not voltage. So the secondary connected wire generates the larger field.

[pdf64]

... As far as signal induced from the wires, remember the field strength generated is dependent upon the current, not voltage. So the secondary connected wire generates the larger field.

I can see how that would work in regard of electromagnetically induced coupling, but not sure of the relative significance of that compared to capacitive coupling from the plate wiring if the OT was rotated 180degrees. Can you shed any light on that?

[TUBEDUDE]

Magnetic hum is always there and not harmonically friendly with standard tuning (A=440). Electrostatic radiation from plate wires is mainly guitar signal. Unless in outright oscillation it may be less noticeable.
If most builders seem to position the OT one way....
If I understand, your secondary leads to the speaker are farthest from the low level circuitry, then that's best imho. You twist your plate leads right, it should be fine. If I understand your question it's, If the OT is swapped 180° what will be the capacitive coupling change be? As to that I must say i don't know. If your leads are twisted well, likely no difference, you could try it both ways. I always locate and orient the choke and OT for minimum magnetic coupling, as hum bugs me, and it's different positions for each set of transformers. I've never had issues with plate lead position. The Marshall you're building isn't super high gain so it should be fine.
What say those of you who have built one of these style Marshall?