You may think you know it all about main transformers? Well Perhaps read this still! When using a FILAMENT Choke, you are going to use more iron, right? Well this is wrong!

Here is mistake #1 when people design a transformer. A mains transformer is always specified for a resistive load. if you load it purely with a very big capacitor as first element, you have to take a LARGER transformer. So you are going to pull for instance 150 Watt from the transformer, and ALL windings are connected to a rectifier bridge, and then come big capacitors. This is THE classical circuit. Most transformer manufacturers, Lundahl included, say for a capacitor loaded transformer, you must reduce the output capacity of the transformer with 1/3. This is completely normal for all transformers, and of course also for the Lundahls. There are no such transformers on the planet where this makes no difference! reasons is, the charge impulses into the capacitor will saturate the transformer core. Above a certain limit, mechanical hum will occur. So to avoid this, the mains transformer is advised to choose larger as you need. And still, even with the obligated 1/3 oversizing, you may be heading for some residual hum, because it is simply not the best way.

The ideal way to overcome this, is use CHOKE LOADED circuits. This is so for the high voltage, and also for the low voltage. If you consequently stick to this, you do not have to oversize the mains transformer. transformer, and low mechanical hum is an additional reward. So you see, you do gain iron weight by using the Filament Chokes, but you loose that by no needing to oversize the mains transformer.

Then, in the end what stays is the lower hum of a filament choke, compared to electronic circuits.

Free wiring introduces inductance which can cause small or larger oscillations in the RF range. Reason is, series regulator IC's by default are instable devices. They can do high current, and have very high gain inside. An partial oscillation will be unnoticed often, specially for filament supplies. The result is, the RMS output voltage is higher than indicated in (any normal) voltmeter since they cannot measure RF voltage. So you can have 5V on a voltmeter, and 6Volt RMS without being aware. You would not be the first one damaging expensive tubes this way. Furthermore such a high frequency RF signal will generate audible noise when it gets into amplifier circuits. Everybody thinks that will only happen to others, until you connect a scope to the series regulator, and get the surprise of your life. Specially the Low Drop regulators are very sensitive for that, since these have even higher gain inside. Another issue with those is, you can get a short oscillation when connecting long voltmeter wires to the IC connections.

The above IS a potential problem, and the #1 reason for broken filaments is the use of stabilizer IC's. With a choke solution, this risk is eliminated.