Archive for November 10th, 2016

Silver-Mica Capacitor Assortment

With RF projects looming on the horizon, now seemed like a good time to restock the silver-mica capacitor supply:

Silver-mica capacitor - assortment

Silver-mica capacitor – assortment

That’s 150-ish little brown envelopes, found on eBay in the lowest-entropy state I can imagine, with about 11 pounds of caps delivered for a bit under $5/pound.

The envelopes bear date stamps from the mid- to late-60s:

Silver-mica capacitor - 188 pF 0.5 pct - envelope

Silver-mica capacitor – 188 pF 0.5 pct – envelope

I think these came directly from the Electro-Motive Mfg Co production line or QC lab, because some of the envelopes have notes about “WE”, “Bell Labs”, and suchlike. They seem to be special-production items, not the usual caps from your usual distributor.

The values and tolerances are weird beyond belief:

Silver-mica capacitor - 6160 pF 0.5 pct - on envelope

Silver-mica capacitor – 6160 pF 0.5 pct – on envelope

If you’re taking notes, 6160 pF lies halfway between the 6120 and 6190 values in the E192 series.

And, yes, that’s a cap with ½% tolerance (forgive the bright-red color imbalance):

Silver-mica capacitor - 6160 pF 0.5 pct - detail

Silver-mica capacitor – 6160 pF 0.5 pct – detail

Most of the caps are 1%, which is kinda-sorta typical for silver-mica. Then you find something unbelievable:

Silver-mica capacitor - 22.8 pF 0.1 pct

Silver-mica capacitor – 22.8 pF 0.1 pct

Stipulated, I’ve lived a sheltered existence. Have you ever seen a 0.1% tolerance cap? The assortment has more of those, scattered throughout the range.

Regrettably, the entire decade from just over 300 pF to just under 3000 pF has gone missing: somewhere out there, someone has another box from the room that housed this collection. So it goes; given the plethora of values, I can always make series-parallel combinations to get what’s needed.

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