Klaus, ok, I will give some information and hope, that with my limited English-knowledge I can express it so that it is clear and nobody feels offended.
I have heard several tests in Germany made with one U47- they listened to it (with the original VF14) and then changed the tube to the VF14 solid state replacement, and listened again, and nobody could tell the difference. To me such tests are not accurate enough, as I think very few people really can make the difference about the performance of a musician and subtle changes in tones close enough. Normally they just choose the better take as the better sounding.
My test is always with two short-body U47's that sound extremely close; in fact I often use them as stereo mics for orchestra or grand piano. I set up both mics with the original VF14, and a singer sings into both at the same time. One is slightly better; I will then install the replacement tube in that one. The other U47 remains unchanged. The test is blind for the listeners, so we just have channel A and channel B, not knowing which is which.
If I remember Andreas correctly, his goal was, to make this solid-state replacment part such, that everything should remain the same, and he could not measure any differences (between VF14 and his solid-state replacement). That obviously does not mean that his sounds the same; only a good ear can tell.
So how did it sound? As the poster before, I can also say that the way Andreas managed to get that timbre right is impressive and the U47 with his solid-state replacement part sounded really great! Still I was able to hear the difference to the "real" U47 (VF14) immediately, even before I switched to the control U47 (VF14). Yes, the test was blind, just the mic I heard first was clearly the solid-state one, that was clear even without hearing the other.
What seems clear to me: The U47 "magic" is somehow to present a voice (or other instruments if they fit for the mic) absolutely beautifull; it has a lot of warmth (but that has nothing to do with the frequency response in my opinion, it still sounds warm if you filter away typically "warm" frequencies). A voice with a U47 must sound big and lush- the high frequencies are there but the "s" sounds in the voice never hurt and never sound closer than the rest of the voice. It is "pergamentig" sounding (vellum-bound might be the english word). A voice recorded close to the mic does not stick out of the speakers (like they would typically with a solid state mic). In German, I use the word "Vollbad" to describe the large sound of a U47 (Full-Bath-Sound?). It almost sounds larger than mono (although obviously it is mono).
Those are the things that to me are magic with a real U47 (in good condition) and, as a Jazz sound engineer are so important to me. Those things are not there with (Grosser's solid state) replacement part.
That said, let me clearly say that the replacement part is really good: what you get is still a great mic that even has some benefits and might even work better in several situations. You could record slighly louder signals and with less distortion, if I remember correctly; also, there is less noise. Also a voice sounds a little bit closer but still nice and big and that could be great for pop/rock. Many of my pop-engineer-friends often prefer the U67 or 77 or 87 to a 47 exactly for that reason - it sits better in the mix for what they need than the warm, big U47 sound. With Andreas' replacement part the U47 could become the perfect mic for them as it goes a little bit in that direction, but only a little bit - you still have the nice timbre of the U47.
The question is what do you use it for, and, if your original VF14 breaks, what is it worth to you to fix it? I have four U47, and if one VF14 breaks I would most likely get Andreas' solid-state replacement, and then just see and learn for what I would use it. As a Jazz engineer, I would however not replace the VF14 in all of my U47's, I would at minimum want to have 2 with VF14's and if they break I would be ready to spend the cash for (replacing them).
And that is, (if I remember Andreas correctly) also what he is saying: He tried to offer a solution as good as he could that would work and still provide a great mic but for a price that most engineers will find fair and realistic. If somebody needs all aspects of the "magic" (stupid word, but it expresses that we do not understand every part in electronics that relates to sound), then he must get a VF14 (logically to me, and in no way disappointing).
For sure, Andreas' solid state replacment is close enough sounding that it will work for a lot of applications and for a lot of engineers. It is probably the best replacment that you could get (but I have to say that I don't know all of them).
I hope this post is helpfull. Nice Christmas,
Daniel
www.ideeundklang.com
