What are HT Smoothing Caps? Dropper Resistors?

[repeatclicks1]

Sorry, some of these terms are confusing to me, but I hear them a lot on this forum. What are they and what do they look like?

Thanks,

-Adam

[Station X]

HT Smoothing Capacitors is a term often used to describe reservoir and smoothing capacitors. In a typical receiver the AC mains input voltage will be rectified to give a DC voltage as required by the valves. However this DC will have a very "humpy" waveform which will give rise to "mains hum" which we are all familiar with.

So the rectified AC is first fed to a reservoir capacitor. Think of this as a cistern which is being filled by a tap which is being rapidly turned off then on. Once the cistern contains some water a tap connected to it will give a smooth flow.

To make the waveform even smoother the output from the reservoir capacitor is fed to a choke and then to a smoothing capacitor from which the HT Voltage is taken.

A dropper resistor is used in an AC/DC set. Even though all the valve's filaments are conected in series the sum of their voltages will still be lower than the mains voltage, so a dropper resistor is connected in series which them to reduce the voltage to the correct value.

In vintage sets smoothing capacitors are big shiny aluminium cylinders. Dropper resistors are generally green or grey. They are mounted above the chassis as they give off a lot of heat and have several tags used to select the correct mains voltage.

[paulsherwin]

Originally Posted by Station X

To make the waveform even smoother the output from the reservoir capacitor is fed to a choke and then to a smoothing capacitor from which the HT Voltage is taken.

A choke looks very like a transformer, but it only has one winding. In many sets it is replaced by a resistor to save costs.

Paul