[R-390] Ceramic .01 bypss buy

[James A. (Andy) Moorer]

The short answer is that it depends on what they are used for.

If they are just for power supply bypass, then it won't make much difference 
what dialectric they are. Of course, low-inductance caps (like ceramics or 
high-frequency polystyrene) need to be used in the RF sections, but for IF 
and AF, just about anything will work and nobody will know the difference. 
All you really want there is reliability.

For interstage coupling, it is a different matter. Different dialectrics can 
give the capacitor nonlinear behavior. This is why engineers really like 
mica and polystyrene and a few others for high-frequency coupling. Ceramics 
with NPO dialectric also seem to be pretty linear (I haven't done a 
definitive test - this is anecdotal).

Mind you, in the entire RF section, the signals are pretty small, so one 
could argue that no amount of nonlinearity in the caps could make a 
difference. In the IF and AF sections, the signals can get to be several 
volts with hundreds of volts of DC bias, so nonlinearities can make a 
difference, albeit a small one.

The only place I have actualy heard a difference in capacitors is in 
interstage coupling of audiophile amplifiers. With a good set of speakers or 
headphones, you can discern a slight difference among capacitors.

Enjoy!

James A. (Andy) Moorer
www.jamminpower.com

----- Original Message ----- 
>
> Silly question, but: What possible effects could there be in replacing a
> plastic-wax-foil type capacitor (BBOD) with a ceramic?
>
> Certainly elsewhere in the 390A (esp. RF deck) there are plenty of 0.005
> ceramic bypass caps.
>