OT location?

[daz]

If i have to punch my own chassis for the 6V6x4 amp I'm planning, what guidelines do i need to follow when locating the OT? I'm fairly familiar with it's requirements as far as the location in respect to the PT, but what about the rest of the amp......turret board, tubes, etc?

[J Martin]

Keeping it away from other inductors should be enough. If you are really worried about it, wire up the amp and connect the transformer without fastening to the chassis. Power up the amp and rotate the output transformer to find the point of least noise. I have built a lot of amps and have never had a problem with the transformers coupling if they were 8 - 10 inches apart.

[daz]

Ok, so i guess i can just put the OT on the opposite side of the chassis right under the turret board at the input circuitry then. What about the wires? Would it be a good idea to run them to their destinations on the outside of the chassis and drill the holes for them right at their destinations, or does that matter? I'd prefer to put them inside, so does it matter what other areas of the circuit they are close to on their route to the destinations?

[J Martin]

Keep the transformer wires away from the preamp circuitry. I use single screw wire holders to keep the primary wire bundle at a safe distance. Holders like the ones that secure line cords are available in many sizes.

[daz]

Thats why i asked because the only place to put it if i can't put it near the PT is right under the preamp. So theres no way to keep it away from it unless i route the wires like i mentioned before on the outside of the chassis, then bring them in thru a hole right at their destination. I'd have no problem if the chassis were huge, but i want to fit this into a small combo about the size of a JCM 1-12" combo or smaller.

[Paul P]

I think you'd be better off having your OT close to your PT rather than
having it close to your inputs. The usual Fender inline design is input at
one end and transformers at the other. The transformers can be close if
your orient them properly :

http://www.dynakitparts.com/store/im...s/zoom/51C.gif

That's a stereo amp, so there are two OTs. They're almost touching.

Paul P

[Steve Conner]

I put the OT at the input end of the chassis on both amps I've built, right next to the input jacks. I like this method because it gets it away from the PT, and also equalizes the weight of the amp making it easier to carry with the handle. It worked fine for me even with extremely high gain.

If you want to do this, what you need is a grounded metal screen between the OT wires and the input circuitry. Capacitive coupling is what causes the problem, so it just takes grounded metal to stop it dead.

Putting the wires on the outside of the chassis like you suggested should accomplish this just fine, though keep them away from your preamp tubes, unless you're using tube shields. Or screen the wires. I used screens made of aluminium sheet and copperclad PCB stock on my builds.

[tubeswell]

route the wires ... on the outside of the chassis, then bring them in thru a hole right at their destination.

that's how I built my first amp and how I'm building my current amp. I use rubber grommets mounted through the hole in the chassis to stop the wires fraying.

Got no problems with noise on my first amp. The PT and the OT are about 12" apart on opposite sides of the chassis - OT on the input side

[Gregg]

I put the OT at the input end of the chassis

If you want to do this, what you need is a grounded metal screen between the OT wires and the input circuitry.

That's my recipe for it too. A metal shield on the first preamp tube also helps.

[daz]

It's also what peavey did (wires on outside of chassis) with the classic 30 which is what gave me the idea. I think i may go that route because it would also make for less clutter, as i can bring the wires in right at thier destination. Thanks all.
By the way, is it good practice to twist the primary wires? If so, all 3 or just the 2 plate wires?

[Synaptic Amps]

It's good practice to twist all AC carrying wires. This includes the filament and primary lines on your power transformer. I don't usually twist either the primary or secondary leads on my OT.

[daz]

Thanks, i'll leave em straight then.

[tubeswell]

It's good practice to twist all AC carrying wires. This includes the filament and primary lines on your power transformer. I don't usually twist either the primary or secondary leads on my OT.

Wouldn't twisting the OT Plate Primaries (not the CT) help to cancel 'stray' AC hum field from them as well? (This was what I was intending to do with my 5G9 BTW)

[Synaptic Amps]

I suppose the practicality and potential benefit of twisting your OT leads might depend on the style of amp. I should have mentioned that in my case I'm only ever dealing with single ended amps because that has an influence on why I don't twist the OT leads. The primary on a SE OT may have a measurable AC component but it's mostly DC. Also there's the simple fact that the two leads are going to very different places in the amp - one to the output tube plate, the other to either the standby switch or the power supply node. The best thing that can be done is to keep these leads close to the chassis and as far away from any wires that might be susceptible to interference.

I don't figure any harm could come from twisting your primary lines in a push-pull amp. At least it's feasible since the leads are terminating in roughly the same place on adjacent tubes. However, since the efficacy of the twisted pair relies on having two anti-phase AC currents cancel out each other's induced electromagnetic fields there will be no benefit in a class B amp. Only in push-pull class A would any measurable benefit from the twisted pair be anticipated. However, even this savings would be negligable compared to the benefit of careful lead routing which I believe is the most important concern for OT primary and secondary leads. Personally I think it's easier to get the wires to sit along the chassis and run them where you want them when they're next to each other with maybe a little glue to hold them down together. It would be so much easier if OTs would come with cloth-covered solid conductor leads in the first place.

[Synaptic Amps]

As far as the original question goes about transformer placement I've always wondered if there might be a cool way to visualize the actual magnetic fields eminating from a running amp. I have a friend who's got an obsession with ferro-fluid (you know, the magnetic liquid) and I wonder if it might have any usefulness for this task. It would be better probably in zero gravity so you could get the fluid surrounding the transformers on all sides. Short of a shuttle mission I'll just have to stick to thought experiments I guess...

If you remember VSEPR theory from chemisty class back in the day (the theory that explains how electrons fill the orbitals around the nuclei of atoms in a manner that fills the lowest energy spaces first) I've always imagined transformers radiating their EM fields a lot like p orbitals - shaped like two balloons shooting out in opposite directions. Just as three p orbitals can occupy the same space around a nucleus by aligning themselves in the X, Y, and Z planes so you can have 3 transformers share virtually the same space on a chassis with minimal interaction. As such, I'd rather keep all the transformers on one side of the chassis as far away from the sensitive input circuitry as possible.