[Amps] FW: Power factor correction on tube amps

[Jim Brown]

On 3/7/2013 3:07 PM, KM5VI wrote:
> I thought that conventional power supplies (transformer-bridge-filter) were
> relatively clean on reflected harmonics?

Nope!  Power line harmonics are the result of current flowing in 
relatively short pulses at the positive and negative peaks of the 60 Hz 
cycle to charge the input filter capacitor, which is used in both linear 
and switch-mode power supplies.

It's quite simple -- capacitor input filter => strong harmonic current 
on the mains power line. These are harmonics of the mains power 
frequency, either 50 or 60 Hz, depending on where you live.

What switch mode power supplies add is RF noise -- they start with 
rectified DC to a capacitor input filter, then use that DC to generate 
square waves at a much higher frequency (typically 10-20 kHz), then 
rectify and filter that to feed the equipment.  The RF noise consists of 
harmonics of the square wave, modulated by random noise added to 
"dither" (frequency modulate) the square wave. Dithering is a sort of 
"cheat" to get around EMC regulations (FCC, and EU) -- instead of a 
strong carrier at each harmonic, the noise is spread over a wider 
bandwidth. The result is "humps" of noise that slowly drift up and down 
the band with a weak carrier in the middle of the hump.

73, Jim K9YC

73, Jim K9YC