Re: Q122/222 Jfet J300 subs for 2215A scope ?


Ed Breya
 

Many JFET types should work fine there, but the trick is to get the right actual Idss so the DC bias conditions will be normal. Look at the Idss range specs for any candidate, versus the original J300. You can also measure the Idss of your "good" sample, and use that as reference for selecting others.

Which topology is used here? As I recall, some 2200 models may have used a totem-pole JFET follower, while most use an opamp-stabilized one. The opamp one is probably more forgiving of part variations. If this is a totem-pole, then you may have better luck replacing both upper and lower Qs with the same type. Even if they are quite different from the J300, they will be closer to each other in Idss, and should fall in a reasonable bias range.

I don't think the 2N4392 family will work - their Idss ranges are fairly high, since they're more intended for signal switching. I vaguely recall that the 2N5485/5486 family may be about right - I think these were used a lot in totem and differential matched pairs under 151-1041-XX.

Ed

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