Well, J74's are getting harder to find -- the J270 is a lower gm, higher noise device -- anyone out there wish to comment?
I used the J271 years ago in my preamps as source floowers feeding the cathodes of the voltage amps. Lots of LF noise; it was fun to watch my woofer cones wander around. The 2N5114 series have worse noise specs, but have in practice been reasonably quiet for me.
SY said:
I will measure the 1/f noise -- as soon as I get a battery pack for the HP3581. Can only do this down to 10Hz, however and I have to set up the brass cage. I wonder whether the noise of the devices was causing the transformer to saturate -- I would guess that it would have to be many orders of magnitude higher than spec'd in the data sheet to have an effect on a tube amp.
The J270/271's are specified at 50nV/rt Hz by Fairchild, 20nV by Vishay -- not in the same ballpark as the J74/K170 --
It's worth hooking up your digital scope and an amplifier to get an envelope, see what the 10 Hz cutoff has you missing. I just kind of watched the output of my preamp (two cascaded J271-6KN8 gain stages and a follower) wander up and down on my analog scope. I tried two batches, both did the same thing. Maybe the newer devices are better- mine were from 1980 or so.
Interfet have some very interesting Jfets including a p channel which is not too bad. Have a look at IFP44.
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