Parallel input stage idea - 1/2 12AX7 ll LND150 ?

[stephenl]

I was thinking of trying a parallel 12AX7 ll LND150 for an input stage, but I don't have a lot of experience using FETs. I'd like to try the conventional arrangement of common Anode and Cathode resistors...but I'm guessing the LND150 might not like the operating point?.

My understanding is that the LND150 likes to be biased around .4v - .7v

Thanks in advance for any education on this.

[Gaz]

All you need to know:

Design a Depletion-Mode FET Preamp: Curve Method, Part 1

[Dave H]

I was thinking of trying a parallel 12AX7 ll LND150 for an input stage, but I don't have a lot of experience using FETs. I'd like to try the conventional arrangement of common Anode and Cathode resistors...but I'm guessing the LND150 might not like the operating point?.

Looks more like an unconventional arrangement to me I always just use a parallel 12AX7.

The standard 12AX7 circuit (100k plate, 1k5 cathode resistors) biases at 200V, 1mA from a 300V supply i.e bias voltage is -1.5V. Laying a ruler on the LND150 output graph shows that to bias it at the same point requires about -0.35V to -0.4V i.e a 350R to 400R source resistor (best guestimate from the non linear Vgs scale)

You could use separate bias resistors and not connect the cathode to the source. Set up the triode and LND150 as above each with its own 100k plate/drain resistor then adjust the source resistor value until the drain is at the same voltage as the plate, now link the plate to the drain. I'd bypass the cathode resistor with 22u but leave the source resistor un-bypassed. LND150 has higher transconductance than a 12AX7.