Originally Posted by zwack
I don't think there will be any issue with them. If anything, there may be more bias current capability that there was with the PN4392. The original Diamond Buffer for the Millett Hybrid - designed by Steinchen, used the 2n5486. In the original configuration, there wasn't enough current leverage in the buffer to allow biases much above the turn-on voltage for the output transistors. That's because the IDSS of the 2n5486 is only about 20ma (whereas the turn-on voltage for many of the MiniMAX output transistors is between 20-30ma.). You could run the buffer up a bit higher, but the Steinchen DB used doubled-up output transistors in place of single output transistors with heat sinks. This meant that you got higher overall Class A bias with lower current set in the JFet loop. Indeed, the overall limit set by Steinchen was 30ma.
The MiniMAX used single output transistors with heat sinks and so we were seeking lot higher biases. We seek at least 50ma or more with the MiniMAX. We switched to the J301 during prototyping so that we could increase the bias, but later on decided to double the current mirror in the buffer, instead. This was done by decreasing RB4 and RB5 to 100R. Since RB6 and RB7 are 220R, this effectively increased the bias by 2.2 to 1.
Once we had done that, the PN4392 was the most readily available and dependable JFET at the time and paralleled the diamond buffer in the Tangent PPAV2. We've used the PN4392 ever since, until they quit making them. However, there's no reason the J310 would not work. Its IDSS is 60ma max, which means it should work for the MiniMAX even if RB4 and RB5 were upsized to 220R. At 100R, the J310 should allow biases of 120ma (definitely too much for only 1" tall heat sinks).
P.S. I may have that history a bit wrong in terms of the DB resistors - we may have upsized RB6 and RB7 to 220R instead of decreasing RB4 and RB5 to 100R. I can't remember whether we started out at 220 or 100. It makes no difference, though - it's the 2.2 to 1 ratio that increases the current over what's in the JFet loop.
Edited by tomb - 6/28/13 at 4:57am