The Electrical Layout can be as important as the physical one:
Here is a typical hookup:
Wire is rated by current-carrying capability. A lower gauge wire means a thicker one, and it is able to handle higher current.
The power tubes are placed first in the chain. This is not as most
suppose, that the signal-tubes with their lower current needs are
protected from the large current draw. That is not relevant. The
average current and voltage will not fluctuate significantly, regardless
of the order in the line.
What is being controlled here is current through the feed lines.
The more current in a wire, the more hum it can induce in other components. The high-current lines must be kept as short as possible,
while the lower-current lines (for the signal tubes) can be longer and
live more amiably with other components, since they produce less hum.
This usually means that the power-tubes are located closest to the power
transformer, which in turn is kept away from the input section
generally, along with its current-carrying lines.
Final notes on heater lines:
The insulation restraints can generally be lower on heater-lines,
because of the low voltages (5, 6.3, 12.6). However, some runs may be
held at some D.C. (direct current) voltage above ground, to prevent
leakage of current to/from the cathode.
Thus a whole heater line might be floated at +100 volts or higher, to prevent both leakage and arcing on large signal swings.
Also, an amp might have two different isolated heater-lines, floating at
different voltages for different tubes in different parts of a circuit
(such as a SSRP or Mu-follower).
Thus one should never assume that a heater line is "only 6 or 12 volts".
It might be carrying 200 or even 500 volts D.C., and could give a lethal shock!
Thus 600volt insulation should be used for heater-lines if needed.
Other than that, the thinnest possible insulation should be used,
so that twisting can be tight as possible.
Nor should they be directly grounded to the chassis, unless it is known
that they can rest at 0-volts D.C. without degradation of the sound
quality or damage to tubes and/or power supplies.
If one suspects that hum on a line is generated by grounding problems
from the heater-circuit, one should first try grounding it through a
High Voltage small value Capacitor such as .1 uF 1Kv rating.
The infamous RCA / Taylor Filament theory FAIL!
In the early days, when hum was not so pressing an issue,
RCA recommended the following layout,
to alleviate current and voltage imbalances when many power tubes were hooked in series.
The idea was to run two hypothetical 'rails', connecting power to opposing ends of each.
The voltage drop caused by current imbalances in the various sections of
hookup wire was balanced by the voltage drop at the other end on the
other 'rail' (wire), ensuring that all tubes got the same average
voltage, even if they floated up and down a little in their absolute
voltage position (relative to each other) during each cycle.
Presumably the runs between each tube would be twisted as usual.
This however resulted in NO HUM Cancellation whatever!
Lets see why:
Even though the wires are twisted together, they cannot cancel,
because the current is flowing in the same direction in both wires,
at any one time.
Instead, doubling up the current just doubles the hum!
The only way this circuit could work as intended,
would be to switch to constant current D.C. voltages:
Now since there IS no hum, no cancellation is needed.
But of course no twist is needed either,
and one might as well run two physical rails.