Puff caps in Monitor Board?

[petemoore59]

I think I need to get the ~30pf cap inbetween  the comensation pins of the NTE938 [round'dohh' cross ref for the LM308] in the Rat!!!
 I tried a couple caps .0047uf [being the smallest on hand] and this just about ~halved the output...You may be able to tell I'm also fishing for info on the actual function of compensation pins of these type opamps.
 Any way I have no way to measure capacitance, and those teeny [look like a drip], orange [all over] caps that left are on one of my paritally scavenged CRT monitor's PCB....anyone have any idea what voltage rating or range of capacitence they dwell around...very inspecific [?] I know...I'm just looking for whether it's werth it to start pulling and trying them as pf types...like 30pf for the RAT's Comopensated OA.
  I just got back from the NTE store, twisted the legs of the round (938 I ordered] to fit in the square IC pin socket, to go in the socket I laid out for the LM308 [in perf]...whew...'gdsoabdgit..oh gotit' and I figure this is as good a time as any to ask if those little caps have found a use for anything on your bench.[?] Thanks, sorry for the 'funny French'   Pete!!!

[Gary]

Pete,

Go down to your local Radio Shack and buy one of the little blister packs of ceramic caps.  You'll find an assortment of caps in there, usually with quite a few in the range you need.  I think the pack costs about $2.

[WGTP]

Are you the Pete that was working on the Muff Fuzz?  If so, how did the resistor across the diodes work?  If not, sorry to bother you.

[R.G.]

The 30pF compensation cap is there to make the opamp stable - literally keep it from oscillating all on its own. What it actually does is cut the high frequency response in one place instead of several other places, making for a clean drop off of response before the miscelleous other phase-changing imperfections can change negative (and stabilizing) feedback into positive (and de-stabilizing) feedback.

For the 308, 30pF will compensate the opamp so it's stable at unity gain, which is the hardest case. Higher gains require less compensation. Books can be - and have been - written on how compensation works.

0.0047uF is about 100 times too big, and I don't doubt that it cut output dramatically.

Best simple choice for compensation caps is to get the right one. Order yourself some 30pF caps from Mouser, Radio Shack, Farnell, etc.

R.G.