OT wires - keep secondary or primary wires short ?

[Tilman]

Hi,

if you have the choice on an amps layout for OT positioning, which set of wires would you like to keep as short as possible ?

The primary wires to the output stage tube anodes or
the secondary wires to the speaker jacks ?

Both set of wires will stray some electromagnetic fields into other circuits and might cause oscillations or other unwanted noise.
I would think the primary wires are worse because tension is higher and impedance is higher that in the secondary circuit.
If you look into a JTM45, the OT is positioned in a way that the primary wires are long and the secondary wires are short. The other way around.

What do you think ?

Regards!
Tilman

[Steve Conner]

Yes, if you have a choice the primary wires should be kept short.

But they can be as long as you like if you run them outside the chassis, or enclose them in a grounded metal shield.

[Tilman]

Hi Steve,

is your statement based on experience, theory or both ?
Thank you!
Tilman

[jpfamps]

Hi,

if you have the choice on an amps layout for OT positioning, which set of wires would you like to keep as short as possible ?

The primary wires to the output stage tube anodes or
the secondary wires to the speaker jacks ?

Both set of wires will stray some electromagnetic fields into other circuits and might cause oscillations or other unwanted noise.
I would think the primary wires are worse because tension is higher and impedance is higher that in the secondary circuit.
If you look into a JTM45, the OT is positioned in a way that the primary wires are long and the secondary wires are short. The other way around.

What do you think ?

Regards!
Tilman

Yes, you want to keep the primary wires as short as possible. The primary wires carry the largest voltage signals in the amp making capacitive coupling to other parts of the amp's circuit more likely.

I've had problems with this in amps.

Vintage amps aren't always the best examples of lead dress.....

[Tilman]

I have had problems with the primaries too. That is why I keep them as short as possible. Lately some experienced builders told me the secondaries are more critical.
That is why I asked for some international experience and expertise :-).
Thanks! Tilman

[guitarmike2107]

Primaries, I have had an amp where if you moved the primary wires 5mm out of place then the thing would squeal at full settings on the controls.

On the other side I have many a time run secondary wires right down at the pre amp stages without a problem.

[Steve Conner]

Both theory and experience. I've built two amps where the OT was mounted next to the first preamp tube. (Reasoning behind this: The PT needs to be far from the first preamp stage, and the OT needs to be far from the PT.)

I used a sheet metal shield to make sure that the preamp couldn't "see" the OT primary wires, as far as capacitive coupling was concerned, and the amps have performed fine.

I had one problem with feedback from the secondaries. But in this case, the NFB wire from the speaker output was actually sitting right on my solid-state EQ PCB. When the EQ was turned to full boost, the amp started to squeal, and moving the wire cured it.

[R.G.]

One bit of complication: the need to keep the primaries short because of the very high signal voltages and possible capacitive coupling conflicts with the need to keep the OT, PT, and choke both as far apart as possible and oriented in orthogonal directions because of magnetic coupling, and that conflicts with the need to keep the voltages on the primaries of those transformers and the magnetic leakages away from preamp and reverb recovery tubes, and that conflicts with the need to make a compact chassis.

If there was only one issue to be optimized, it would be easy.

[Steve Conner]

Yes, and putting the OT next to the beginning of the preamp is my personal solution to that optimization problem. I've got it to work even in a 5 gain stage grinder, but YMMV

[GregS]

My first thought was the secondaries would be more of a problem for inducing noise into other parts of the circuit because of the higher currents in them. That's also my understanding of why the heater wires are potentially a problem for this as well. The voltage in them is low at 6.3V but the current in them is significant. It seems I've got this the wrong way around?

[Steve Conner]

High voltages induce noise by capacitive coupling, high currents do so by inductive coupling. The relative severities depend on the impedance of the circuit it's coupling to. Tube circuits have lots of high impedance nodes, so they tend to suffer a lot worse from capacitive coupling.

As opposed to, say, a moving-coil phono preamp which would pick up hum by inductive coupling. This is worse, because any grounded conductor will stop capacitive coupling, but you need thick plates of steel to shield against the other stuff.

[loudthud]

On heater wiring (and OT secondary), the twisted conductor with an equal current flowing in the opposite direction cancels a large part of the magnetic field. And the same can be said for the electrostatic field from an OT primary if you twist it with the plate lead from the other tube in a push pull amp.

[NorCalTuna]

There's a few things that are done in practice. Some ampeg amps, like the vt-22, have the OT opposite the PT but run the wires in the chassis in a trough. This is problematic for obvious reasons, besides the sourcing and space consumption of the trough. Later marshalls IIRC are opposite, like ampegs, but have the lead dress run through the chassis neatly and away from the sensitive input areas. Mesa boogie has an interesting approach with the MK3 and 2 as well IIRC, in that the preamp runs behind the faceplate, the PI in the middle and the power tubes along the back behind the input, with the OT underneath all that and the leads going back. The eq and all that bs sit to the right, with the power supply. The music man hybrids have it in the center on the back with the tubes to the right. The modern fender "hot rod" amps have it basically in the center of the chassis, about six inches away from the PT. Just walking around feeling up amps here in my office...

It's not uncommon to also run the leads outside the chassis-especially on a non-commercial, personal amp- and give them good dress- shrink, spiral, what have you- to avoid abrasion and such. I have had tremendous luck doing this on "deluxe" style amps, ie, tubes adjacent to transformers, with the leads ducking back into the chassis over on the PT side with a grommet and ties to make sure it can't be pulled at all. The chassis (steel may be better in this regard, and others) shields the circuit better than a typical shielded cable ever could. Also, it helps to hook up a Osc or meter to the secondaries, mount, protect and power up the PT and leave the OT primaries unhooked and move it around to find induced voltage. That makes the speaker hum minimal if present at all, as well as a fun learning experience. You can also hook up a speaker or headphones : ) If i'm not mistaken, the the speaker wires carry more current than the OT primary and seems to cause more "wire dress" gravity than the primary, at least in my world. Hence, the neg feedback wire is also a usual suspect.

Greg, I think you're probably right, but heater hum doesn't have to be a problem, even in ishy dressed amps. Typically elevating the heater center tap makes this virtually go away. Merlin makes a good point in his book that the nine pin socket was cleverly made to allow very clean entry and exit of the heater wires from the side, not the top, which is typical.

[jrfrond]

Keep the primary leads short, twist 'em up good and tight like you would with heater leads (I twist ALL AC leads anyway), and keep them close to the chassis. You don't have to worry about the B+ lead (usually red), just the plate leads. However, ALWAYS strive for good lead dress.