The WA-47 was the first mic mod I developed after moving back to Baton Rouge in mid or late 2017 (although I worked on the Sterling ST55 mod prior to that while living in TX), so it is something that has been weighing heavily on my mind lately after I was recently asked whether anything could be done to 'take the microphone even further'. That question, and the current scarcity of my favorite tube for the WA-47 put me on sort of a quest; so, after quite some time deliberating on this subject, I think I've come up with a way to get 2 birds with one stone.
My favorite tube for this microphone upgrade, by far, is the JAN Sylvania or JAN GE NOS 5751 (green military screen printing). The two sound exactly the same, and were interchangeably purchased by the Army under the same stock number back in the day; however, I have found (the hard way) that after this many decades behind them, the JAN Sylvanias have a better reliability rate now than the JAN GEs. But I have used whichever I could get my hands on, as long as they have been tested by a reputable vendor and then successfully cleaned and burned in by me.
Long story short, these tubes have become so hard to come by that I have almost given up several times. I have been burned by less than reputable sellers so many times and thrown away so many NOS 5751 tubes that it would bring a grownup to tears... probably thrown away about $1000 USD at this point on them. This microphone is finicky about tubes. It doesn't like certain tubes and it eats certain tubes for lunch. The company (Warm) have had the exact same problem I have had over the course of this product's life, and have already moved from JJ to Tung-Sol and Sovtek 5751's in the hopes of mitigating this issue. Unfortunately, I do not favor those particular tubes for sonic reasons. I also know there are many 12AX7s and 7025's one can use that will last a long time and not get too warm (the circuit IS a 12AX7 circuit after all), but alas I still am certain just as I was when I developed this product for the US market originally, that a 5751 has a certain three dimensionality that a 12AX7 in this design simply does not. I have found basically two 5751's that the microphone 'likes' (and which I like also), and on which I don't get many, if any, bad tube returns: the JAN Sylvania 5751 (or a really exceptional specimen JAN GE 5751), and the new production TAD Germany 5751, Highgrade or Premium Selected version. The TAD tube sounds DAMN good in this microphone. It does almost exactly what the Sylvania does, and I have a hunch that they modeled it after that part (most TAD tubes are based on a golden reference of some sort). It has perhaps only a tiny tiny bit more top end or presence but we are talking like 1%, and the important notes such as the warmth, fullness, roundness, less shrill or strident nature of the JJ part, etc., are still strong.
So moving forward, our normal WA-47 modification is going to feature the TAD Germany 5751 Highgrade, ordered straight from the factory. As always, the pins will be sprayed with contact enhancer, a Sandy Levy silicon tube damper ring will be added, and the tube (and mic) will be allowed to burn in for 8-12 hours. I think it is the only way to make these mods sustainable with the quantity we are doing, and I think the end result is still just about as strong.
However...... for those who have asked me for an ultimate modification for the WA-47, allow me to geek out for a bit, if I may.
We have one source left for reliable JAN Sylvania 5751 valves, a highly reputable source in Germany who does test all of these tubes for us (and of course, we test them again here). Their stock levels are sufficient that we can keep offering this for the time being, and have already gotten in a couple of them and are very pleased. However, they do come at a premium and rightfully so. Since I cannot afford to continue to offer these at the price of the normal WA-47 mod, we are going to offer this only as part of an upcoming ULTIMATE WA-47 mod, which I will describe below.
One thing I learned from taking these things completely apart and studying them is that this mic has its sort of design rev history baked into it with at this point a number of unnecessary 'bridges to nowhere' and over-complexity in places. That, and its not always the most elegantly wired and assembled to begin with. The mic sort of has 2 circuit ground wires on either end of the top deck (not great for ground loop avoidance) and a ground wire for the output transformer which comes all the way back down from the top deck, unnecessarily, and even the transformer deck is unnecessarily complex with thick traces running around from side to side and the vestiges of through-hole locations for the long abandoned RC filter from revisions past, etc. What we will do on the Ultimate WA-47 mod is essentially strip out all existing factory wiring (there are at least 6 long leads that run up from top to bottom) and replace it with a far more streamlined wiring scheme using the highest grade Belden silver-teflon wire. One ground wire is eliminated entirely, and there will be one dedicated circuit ground, one dedicated chassis ground (added), and the transformer will be grounded to the circuit ground on its journey up the ladder, so to speak, rather than ridiculously running it back down from the other side. The transformer deck will be completely taken off the existing (discarded) factory wiring and wired up directly to its transformer pins on the underside, as directly as possible and ignoring the traces of the transformer deck.
Here's where it will get even more geeky. The top deck (high z section and tube amplifier stage) will be taken out, COMPLETELY stripped of components and de-soldered, then cleaned and scrubbed in anhydrous alcohol until it is absolutely clean and bare. All of the cheap and fragile radial resistors will be tossed out, along with all the generic high watt metal film resistors, and any generic caps, replacing the high-value resistors with precision name brand parts and the higher watt/lower value board mounted metal film resistors with all 1/2 and 1 watt new or NOS carbon composition or new stock carbon film resistors. Mostly carbon composition 1 watt parts wherever possible. We will put in the audio grade, high tolerance WIMA film capacitors in place of the industrial grade, and use a higher quality multi-layer ceramic capacitor in place of theirs for the bypassing. The 3 generic electrolytics (as in the standard mod) will be replaced with 2 Nichicons and one mil spec tantalum. The high-z deck will be fully re-populated down to the polystyrene capacitor and low capacitance jumper wire and teflon insulators, and then wired back up using silver teflon wire. The tube will be replaced with a JAN Sylvania or JAN GE 5751 with tube damper ring, tube pins cleaned and de-scaled here in our shop, and the microphone burned in for a full 24 hours.
This mod is not for everyone. Most people probably do not need this mod. Many people would likely not really be able to even appreciate the differences fully. But people have asked me over the years to dive deeper back into this product, to look at how it was designed and see what could be improved, and this is what I have learned and come up with. On the Ultimate WA-47 mod, we will still be doing 100% of the things we already do such as an end-to-end upgrade and cleanup; but in addition we will be doing the audiophile level rebuild I've described. I would say this mod is for the real enthusiast who intends to keep this microphone for MANY years as a centerpiece of their recording studio and make records with. The increased cost of this mod mostly reflects the cost of importing in one of these remaining certified NOS JAN Sylvania 5751's and some of the extra parts, and really doesn't even include the additional labor. It's just something we wanted to offer for those who might wish it.