UK Vintage Radio Repair and Restoration Discussion Forum

Quote:

Originally Posted by G6Tanuki (Post 1193725)

I call them 'chocolate' capacitors becaue to me they look like classic squares of a chocolate-bar.

There are a couple of them in my Pye PCR. One of them is 'that capacitor' between the EBC33 anode and the 6V6 grid.

Are these 'replace-on-sight' like Micamolds, or can they be trusted?

It will almost certainly be a paper capacitor with encapsulation that's similar to or the same as the maker used for mica. The old adage that 'if it looks like a duck, walks like a duck and quacks like a duck - it's a duck' doesn't apply to capacitors that look like mica caps will be mica caps. The clue isn't their encapsulation, but is in their capacitance value and the application.

If it's low value, is in a tuned circuit, has retained its value, and passes a 'Megger' test, it's almost certainly mica. If it's a higher value - say above 500pF and is used for coupling purposes (as in your case) or in a bypass role, it's highly likely that it's paper. The two concerns are whether the capacitor is still fairly close to its stated value, and if it shows signs of leakage. If the desire is to leave it in place rather than to replace on sight, I'd check that it's retained its capacitance, and poke 250 - 500V across it with an insulation tester to see if there's any sign of leakage.

This topic cropped up recently in another thread here:

https://www.vintage-radio.net/forum/...10#post1189410

As the same encapsulation in the 1940s was used for both mica and paper capacitors it’s understandable that many will assume that if it looks like a mica capacitor, it probably is, especially when it carries the name ‘Micamold’ but that isn’t necessarily so. Higher values were mostly paper and have proved no more reliable over time than any other paper caps. The company simply use the same encapsulation for both mica and paper.

Low value capacitance caps in tuned circuits (say up to 500pF) probably are mica, which have generally proved to be reliable decades later. They were invariably close tolerance and hold their value. Often (Dubilier for example) marked them ‘Mica’. Over 500pF, if used as by-pass or coupling capacitors of higher than .001mfd, even if they look like mica, they’re highly likely to be paper.

It’s a topic that often crops up on the US Antique Radio forum:

https://www.antiqueradios.com/forums...c.php?t=112157

'Micamold' is the name of the Micamold Radio Corporation' - not the structure of the caps.

The war made it more likely than not that moulded paper rather than mica caps would be used in military equipment and the makers used that fact as a selling point. Page 78, ‘Electronics Industry’ August 1944 carried a full-page advert by the Micromold Radio Corporation entitled ‘The Facts About Molded Paper Capacitors’ in which they outlined the properties of moulded paper versus Mica capacitors.

The advert emphasised the military use of the caps, with pictures of tanks, planes and soldiers.

‘Wartime equipment has definitely proven the ability of Micamold molded capacitors to function satisfactorily in by-pass, coupling and filter applications above .001mfd. As strategic mica is scarce Micamold Molded Paper Capacitors not only filled the breach but materially assist in maintaining the flow of equipment to the Armed Services’.

I guess that the reason for the advert was to reassure those in industry who had doubts (well-founded as it turns out), that moulded paper caps aren’t as reliable as mica ones, the underlying theme being 'the armed forces are using our moulded paper capacitors to help win the war'.

Source:

https://americanradiohistory.com/Arc...es-1944-08.pdf

Hope that's of interest and use.