Re: 6DJ8 substitute idea - reverse engineering

Tom Lee

A FET is in saturation (the happy region for amplification) if VDS exceeds (VGS - VT).

The standard fetron connection is a pseudocascode, where the upper (cascoding) device has its gate connected to the source terminal of the bottom transistor. That connection makes the bottom transistor have a VDS equal to VGS. That automatically guarantees operation of the bottom transistor in saturation if the transistors are reasonably matched depletion-mode devices, without the need for a separate positive bias. The fetron really is designed to drop into the same socket as the vacuum tube it replaces, without requiring rewiring.

-- Cheers,

Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 6/23/2021 13:48, Dave Wise wrote:
You need positive bias anyway, unless the bottom device in the cascode can operate with just a volt or two across it.

Dave Wise

From: [] On Behalf Of bhome1959 via
Sent: Wednesday, June 23, 2021 12:18 PM
Subject: Re: [TekScopes] 6DJ8 substitute idea - reverse engineering

Hi Bert,
You would need a depletion type mosfet. DN2540 would work, or LND150 for lower currents. A normal mosfet is an enhancement device and would need a positive bias on the gate w.r.t. the source.


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