We have upped the game!  The WA-251 standard upgrade is discontinued and now moved up to the Ultimate Upgrade.  In addition to all that is mentioned below, we will now be replacing 100% of all semiconductor components inside the mic with the highest grade tantalum and polystyrene capacitors we can find, the board is completely reworked from end to end, all internal wiring in the mic is pulled an replaced with Belden's silver teflon wire, and the power supply is given a slight redesign and discrete modification, removing the IC package fixed regulator (heater voltage rail) and replacing with a new US-made fully discrete regulator custom-made for Signal Art, delivering a rock-solid and ultra clean +6.3v to the tube heaters no matter what operating conditions or tubes used or where you move to, and ensuring a much longer life and higher performance from the tubes.  A longstanding issue with slight overheating of the filaments and some tubes not lasting as long has finally be resolved for our customers, and we're very excited about this newest improvement.  This will be incorporated into the mod on all orders after 6/1/22.  
 
Just to be clear, this is a modification whereby you send in your WA-251 and we fully upgrade it and return it to you.  You are responsible for sending us the (more or less) working microphone; this is not a new microphone sale or a repair service.  We need the microphone, PSU, and 7 pin cable only.  The rest is optional.
 
This is probably our most thorough upgrade modification service as of yet, and we have spent several months ensuring that we got this right and brought it up to the highest level that is practical for this design.  Please see the blog entry relating to this mod for much more thorough and in depth explanations on this.
 
In short, we replace the tube with the quietest, most neutral tube that can be put in this design (and also the tube that this mic was originally spec'd to have to begin with).  We meticulously clean and re-flow both the power supply and microphone PCB.  The correct value output capacitor for this design is installed, and is a special NOS part from our personal stash of new old stock components.  All electrolytic capacitors from end to end (mic and PSU) are pulled and replaced with high performance, high temperature, extra long life versions.  The generic polystyrene capacitor (in mic PCB) is pulled and replaced with a more appropriate value NOS military spec polystyrene, and a touch of special adhesive applied to eliminate microphonics.  The capsule is re-installed and re-terminated, with good terminations and shorter wire lengths.  The high-z section and its resistors are meticulously cleaned with anhydrous alcohol to remove flux residue, oils, and other contaminants, and the component orientation and placement is adjusted slightly to reduce the amount of PCB exposure to the initial high-z components.  The power supply is rebuilt, all wire lengths shortened and tidied up, the rotary switch is sprayed and rebuilt as needed.  The 7 pin cable is tested and rebuilt if needed (sometimes they are not assembled great, sometimes they are, and we check both ends).  The entire mic PCB is thoroughly cleaned and component leads trimmed, solder points are re-flowed.  
 
Our goal here is two-fold.  Firstly, we want to re-voice the microphone to be slightly closer to the sound of an actual 251, which it unfortunately falls way off the mark from.  It is not open or extended enough in the top end (largely due to tube choice, a very messily assembled high-z section, and board-mounted components that should have been floated and wired point-to-point by hand), and a somewhat lumpy/muddy, bass-heavy bottom end (mostly due to incorrect output capacitor values and other component values which overshoot and malign the low end response).  Our secondary goal is to turn this into a microphone that can last you for a lifetime of use (with proper care); by curing it of its hurridly done Chinese assembly and generic, lightweight electrolytics that don't have a long lifespan.  I initially did not plan to offer this mod because I knew the amount of work that would be involved; but after quite a bit of R&D, I can safely say we can achieve this and bring the mic to its absolute fullest potential and actually make a proud addition to your mic locker.