• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Learned something (the hard way) about mica capacitors

It turns out if you pull a mica capacitor out of a drawer and it says "470", there is a 50% chance that it is 470pF, and a 50% chance it is 47pF!

I was puzzling over why the second channel of the 5B/254M amp I just put together was oscillating when I biased it over 30mA.

Looked all over, could not see any differences.

I looked over the mica caps... they were the same (two said "100" and two said "470". Hmmm, but they were slightly different...

Turns out one said "470 +/-5%"and one said "470J" ("J" means 5% too). But the one that was labelled 470J turned out to be 47pF!

DOH!

Pete