Wal Pro II E preamp - U406 dual JFET

Discussion in 'Pickups & Electronics [BG]' started by gandalf906, Oct 16, 2020.

  1. gandalf906

    gandalf906 Commercial User

    Oct 7, 2020

    A few years agoI would like to make a preamp based on this diagram:


    This is an old Wal bass preamplifier.

    At first I wanted to build this preamp using modern parts. Instead of the double U406 transistor I chose two J201 as a similar replacement. It was a mistake... When I used J201, the circuit was unstable, immediately after switching on the battery, the operating point was shifting and the preamplifier did not carry the signal... Then I realized that I needed paired J201 transistors.

    I bought ~100pcs J201 transistors, I measured each of them and chose a few pairs with almost identical parameters. I swapped pairs of transistors in my preamp and... WOW! It works! Unfortunately, after some time it turned out that this preamplifier was also unstable and the operating point was also shifting. After an hour of operation, the preamplifier stopped working. I was able to turn off the battery, turn it on and it worked again an hour. I analyzed the chematic, took measurements and compared it with simulations. I figured this preamp can only work with the original dual transistors, but where to get them?!

    I gave up the idea of building this preamplifier. It doesn't make sense, I waste a lot of hours finding out I'm doing something that can't work because there are no original parts...

    But now a miracle has happened! I bought a few pieces of NOS U406 on sale in the original Siliconix packaging! The fun starts all over again! I will build a new preamplifier based on original parts. I'll make it as a stompbox. I'll let you know how it turned out, keep your fingers crossed!

  2. RonC72


    Apr 18, 2007
    Weert, Netherlands
    Cool!! Did you manage it?

    Around 2006 I bought an old Wal ProIIe bass for only €850,-. It was this cheap becuase it was in a very bad condition.
    The neck pickup had a very low output, lecetronics were bad and it was also in need of a new fretboard.
    I opened both PU's and compaired them, 2 of the 4 magnets in the neck PU where lying in opposite direction so the magnets cancelled eachother. after turning the magnets the PU was working again.
    The electrocics where a bigger problem, bot U406 Jfet's were broken... It took me some months to find them, but i found a company that supplied them so I orderd them plus 2 spare ones just in case!
    Later i replaced the pots, i couldn't find the original Radiom pots but Paul Herman of Wal supplied me good replacement pots. I had the fretboard done by a local luthier.
    The bass now is in perfect condition again!
  3. Dennis05


    Mar 5, 2021
    Hi, I've go the same problem as gandalf906
    I would like to build that Wal preamp but am having trouble finding the obsolete U406 FETs. I did find that Mouser supply them, but I thought a minimum order of 140 @ £5.93 was a bit of overkill for a project on a whim!
    You mentioned that you found a company that supplied the U406s. Would you be willing to share who that was?
    I'd love to give it a go as I'm making myself a bass body out of a lovely single piece of iroko and I think the tone of the timber would be a great partner to those cool retro Wal active electronics.
  4. gandalf906

    gandalf906 Commercial User

    Oct 7, 2020

    I haven't finished this project yet, I have a lot of other work and very little time. But I'm on the right track and believe that soon I'll be ready to design and order a finished PCB.

    Right, I saw the mouser has these transistors too, but the price and minimum order quantity are actually killer.
    I bought my U406 on ebay from a small company in Bosnia and Herzegovina, they come from old warehouses and it looks like I bought all that were on offer.

    When I'm ready and everything works fine, I'll share some photos of my work.

    Bushmaster and RonC72 like this.
  5. RonC72


    Apr 18, 2007
    Weert, Netherlands
    Hi Dennis,

    I ordered them via Farnell US (around 2007)but i don't think they sell them anymore..
    Here's a company that sells them too but I don't know the minimum order..
    DMOS Switches, JFET Amplifiers, BIFET Amplifiers, MOSFETS
    Here you can find them in an IC housing: Search Results for “u406”
    And check the datasheets for the right specs:
    U406 Datasheet pdf - Dual N-Channel JFET Switch - Calogic

    Good luck!
  6. A9X

    A9X Inactive

    Dec 27, 2003
    Not sure LS will sell you 1.
    It's an old not particularly good analog design anyway.
    Bushmaster likes this.
  7. gandalf906

    gandalf906 Commercial User

    Oct 7, 2020
    I have contacted Calogic, and as a business owner, I can only order 10 U406s once as paid test samples. Next orders need to be in line with their MOQ of 500pcs.
    If my prototype works fine I will consider ordering more U406. ;)
  8. Kinkh


    Dec 11, 2011
    Found this thread by accident and while it is old, I might have some useful info for OP and anybody interested.
    I have built a working version o fthe attack part of this circuit using J201 JFETS (MMBFJ201 to be exact).

    In the original schematic Q2 is configured as a common source amplifier (Q2B) with a current source as an active load (Q2B). To that a negative feedback consisting of a bridged T network is added to get the desired frequency response and gain. With these values, the output DC operating point is set up to around +V/2. This config can work in this way only because the JFETs are matched and have exactly the same characteristics.

    For this to work with J201s, I have reconfigured this to be a mu-amp configuration. Which means more or less the same circuit (current source as a load), but the Gate of Q2A needs te be biased to +V/2 by two resistors, and AC coupled to the Drain of Q2B. Just like in a regular mu-amp. The feedback network is the same. For better performance I added another source follower at the output.
    See mu-amp examle:

    As for Q1A and Q1B, they are regular common source amplifiers with a negative feedback network added to get the desired frequency response. They don't need matched JFETs, but to work with J201s, the DC operating point would have to be calculated for the original U406, and then the component values would need to be adjusted to give operating point with J201s.
    Alternatively a mu-amp can be used for each of them, in which case no extra calculation is needed - just replace the amplifier and keep the feedback network...

    If anybody is interested, I can post schematics for this. In case the explanation is not clear enough :)
    rankot likes this.
  9. rankot


    Jan 8, 2022
    If you can post J201 version schematics, please do. Thanks!
  10. Kinkh


    Dec 11, 2011
    Here's a quick schematic form LTpice. To bypass or turn on the effect, install a switch on R2 like in the original.
    To adjust quiescent current, change the value of the 3.3k resistors. Larger value decreases current, but also decreases maximum voltage swing (not noticeable though unless you have very large input signals).


    For a less noisy solution, try 2SK208 or 2SK209 JFETs.
    rankot and Passinwind like this.