Precision vs plain capacitors???

[init4fun]

Hi Tubejunke ,

Great for you to have gotten a C3 ! I'm happy to hear you got one since I know you had been searching for one for a while . Tell me , in your question you mention "precision VS plain capacitors" and I wanted to expand on this a bit . If the precision aspect is referring to actual value only there is one more piece of the pie missing . There is the rating "temperature coeficient" to consider as this is actually far more important than raw capacitance and here's why ;

A cap who has a perfectly defined value is put into our bridge circuit and the indicator pointer is aligned properly . All is great and in use the dial / eye tube / cap under test indicate properly the value of the cap being tested .

BUT ! the cap with the perfectly defined value has a poor temp coeficient and in the cabinet being heated by the tubes the capacitance begins to drift a bit . Now , after it has sat running for a while the same cap we tested when we first powered it up* will test with a different reading . I only put this out there as a side discussion on the fourth major cap classification (the first three being rating , working volts , and environmental temperature expectations) since most usually mention the first two , some get into the third when dealing with hot tubes in tight cabinets , and it's usually in the realm of highly accurate measuring equipment that the fourth charactistic comes into play ....

* PS , it's generally considered good practice to warm up any tube containing test equipment for a period of time to stabilize temperatures (and thus component values leading to more accurate readings) . I usually let mine run 10 or 15 minutes before use , this seems time enough for them to reasonably stabilize .

Happy cap testing