[Ron Ramirez]

Yup, the original capacitors in Model 20 filter cans are all paper.

If you're referring to the schematics on my website, I added those + and - symbols in case someone wanted to use electrolytics to replace the caps inside those cans. The symbols are also there to help beginners see which side has positive voltage and which side is negative.

Arran is right, it is better to use yellow mylar (polyester) to replace these caps - there is plenty of room inside the can, and they shouldn't go bad nearly as fast as a replacement electrolytic will.

I have to disagree with you regarding Mershons. Philco's first radio to use a Mershon was model 21 - it used one as the input filter. The other filters are paper. This was in early 1931. Then came the 70 and 90, which each used two Mershons. The 70 and 90 went into production in the spring of 1931, and were introduced to the public in June of that year. Model 112 was the last Philco to use all-paper filters (1931). The 112 was a slight revision of the 111 that came out in January 1931 as Philco's first superheterodyne radio. The 112, with its push-pull 45 output tubes, continued to use a large can filled with paper caps until its circuitry was slightly changed in the latter part of 1931 to use two 47 output tubes; at this time, two Mershons were also added to it.

Most of Philco's 1932 and early 1933 line continued to use copper Mershon electrolytics, although a few soon appeared that used aluminum cased electrolytics, such as the 53C cigar box pee-wee, and models 89 and 19.

By 1933-34 Philco had switched completely to aluminum cased electrolytics and no longer used the copper-cased Mershon electrolytics.

The copper Mershons are now highly collectible. When polished up and installed in a clean chassis, they look great in a 70 or a 90, or any Philco of the period that used them, for that matter.