On Wed, 10 Oct 2018 15:02:53 -0400, Ralph Mowery wrote:

When restoring the old tube equipment (tube receivers ) does it make any
difference if the old paper capacitors are replaced with the ceramic
disk or should another type be used ?

I know for RF one needs to use capacitors that are rated for rf but in
the audio stages does it really matter what kind of capacitor is used as
long as it is the correct value ?


Reason for asking is that I bought an old receiver that someone has
replaced many of the capacitors. The book says pape capacitors for some
but they have been replaced by the ceramic disk type.

This topic takes me back. One of my first "real" radio repairs was back in High School electronics class on another student's old Philco cathedral radio. Not one of the fancy, classic cathedrals but a lower end one, maybe a 5 tuber. Still, a cool old radio.

The radio was nearly dead, no audio, just a minor hum from the speaker. I used the shop's signal tracer and I could get audio at the IF stage but no audio at the volume control. Long story short, the coupling cap had opened up. The first suitable cap I could find was a disk ceramic and I subbed that for the opened up cap in the bathtub. It worked!

The other guy was real happy about the repair. I thought the audio wasn't great but I didn't know much about old radios except old radios on TV or in the Movies always sounded crappy. But now I figure the ceramic coupling cap may have worsened the sound.

As I've since learned, there's two kinds of ceramic caps. The first kind are very stable but are bulky for their capacitance and voltage rating. They are normally used in such things as tuned circuits.

The second type of ceramic cap is compact but they show high changes in capacitance with temperature, frequency and applied voltage. They are fine for bypass and coupling as long as the cap's limitations are taken into consideration. I'm sure the ceramic cap I used was the second type. The changes in capacitance with voltage can cause audible audio distortion which won't happen with other types of caps. Here's an article which goes into the non linerarities of various types of caps without delving into audiophile metaphysics:

http://diyaudioprojects.com/mirror/m...h102/caps.html

But if you're OK with the sound of your radio, stay happy. Most of this audiophile stuff is lost on the 99,9% of us who lack the superpower to tell the difference between the characteristic sound of a paper in oil cap and a polyester cap.