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Looking to finish a Benson build and need some assistance. I have 2N5485's and just want to double check that they will work if installed according to the silkscreen on the board. Any feedback would be greatly appreciated.
I've created SOB ltspice model. Let's assume J201 with Vp 0.76V and 2N5485 with Vp 2V.
Then we check build document - set Q1 trimmer to 2k and Q2/Q3 drain voltage to 4V.
There's Q1 10k drain resistor + 2k (trimmer) so we have 12k total.
With J201 as a Q1 we have 5.6V on a drain. With 2N5485 there's only 0.77V...
Let's move to Q2/Q3. To get 4V on drain with J201 we need around 18k. But with 2N5485 we won't ble able to set 4V even with trimmer set to zero. In order to set 4V with 2N5485 we need only 3k6 (remember - there's already fixed 10k). So you'd have to put 1k resistors (or lower value) or even jumper them (btw - with 50k trimmers resistors are superfluous). Back to Q1. To set the same voltage as with J201 - 5.6V we have to lower overall drain resistance to 2k4 (was 12k).
But voltage is not the only factor here. So let's inject a signal into the circuit. 660Hz, 1V p/p.
And now some simulation results. Blue plot - J201, green plot 2N5485, all controls maxed.
In my opinion - if you replace J201 with 2N5485 it's not SOB anymore.