[Boatanchors] Ceramic disk caps

[Glen Zook]

I have found that in some radios manufactured in the 1930s that various stages cease to operate when the old paper type bypass capacitors are replaced with modern ones.  To get the stages operating correctly I have found that I have to add a resistor between 470K and 1 meg across the capacitor.  It seems that the original design relied on the leakage through the older style capacitors which the new capacitors do not have.  Thus, the need for a shunt resistor.

Not every radio nor every stage.  However, it can definitely be a headache to find the problem.  Therefore, whenever I run into such a problem I try a resistor in parallel and, so far, that has "cured" the problem.

Glen, K9STH

Website:  http://k9sth.com


--- On Tue, 3/3/09, Carl <km1h at jeremy.mv.com> wrote:

Discs give far superior RF bypassing and I use them just about 100% there. Exceptions are some 30's radios house radios using the 78/6D6 era or earlier glass tubes where a little shunt inductance helps stabilize RF and IF amps at times. There is a workaround using ferrite beads and small value carbon resistors to do the job.
 
Some say that ceramic caps can cause distortion when used in audio coupling but I havent been able to notice any at the low levels of single ended stages. To play it safe I use film caps with PP stages. There is a series of scholarly papers on the subject out of the UK that claims all caps are prone to distortion and some ceramics are fine.