The yellow wires are typically 5 volt rectifier heater wires. The red ones are high voltage AC from the power transformer. The rectifier produces high voltage DC output (aka B+ or HT - High Tension) from the high voltage AC produced in the power transformer (PT).
There are step up and step down windings in your PT.
There is generally another low voltage output available, that is the 6.3 volt AC circuit for your heater filaments in your power tubes and preamp tubes. These wires are typically the green ones.