The EF800 tube circuit U47's have arrived

At long last, we have finally made the EF800 tube circuit version of the U47 and U48 available for purchase. This will simply be a drop-down menu option when you place an order for the microphone.

I have been asked for years to make this version available and hesitated for mostly silly reasons (because it's the version most other people make, and also I felt the dual-tube version remained closer to the original schematics, particularly with regard to the PSU requirements) but it finally makes sense to make this mic available. Our original dual-tube design that we have made by hand for almost a decade now is still available, and we have no plans to discontinue it; but the availability of good 408a Western Electric pentodes is starting to show signs of drying up, and the quality is going down (they are more used and aged tubes at this point rather than NOS, requiring more cleaning, descaling, burning-in, and very careful screening. We now have to buy about 12 to 16 tubes just to find a perfect pair to go in one microphone and a second pair to serve as a complimentary spare. The cost of these tubes has also gone up substantially in recent years, and I now have to devote more of my time to the acquisition and screening process.

I don't intend to stop offering this version until circumstances absolutely force my hand to do so, but I can certainly see the writing on the wall and the need to have a viable backup option. Enter the EF800 version. I have made or serviced a couple of these types of 47 mics over the years on request; and while good, I always favored our dual tube circuit. I needed to spend the time to really come up with our own take on it, and I finally had time earlier this year to embark on this saga. We found several areas of improvement over some of the implementations we've seen before, and found some ways to get the self-noise down lower than even our other mic builds and mods (barely, but audibly!), through extremely overkill engineering, ultra-high-quality parts, super clean work, super good quality boards and board designs, and a ridiculously overkill power supply with extremely good filtering and massively over-spec'd components. We commissioned both a newly custom wound and shielded US-made BV-08 transformer as well as a custom-wound shielded toroidal PSU transformer from US-based high end power transformer designers Avel Lindberg. We are using ultra long life and ruggedized PSU capacitors, high end resistors (PRP thin film in much of the mic circuit) and even one of the trim pots in the PSU is custom made, and almost everything in the PSU is over-spec'd to be at least twice as good as it needs to be. Of course much silver Teflon and silicon wire is used, and the custom 7 pin cable is still handmade. I think it is the pinnacle of my OCD up to this point.

One difference that will be immediately apparent to users is that the EF800 version does not ‘get hot’ at the base of the microphone.  While the dual-tube version deliberately uses a drop-down heatsink-mounted resistor at the base of the microphone, using the bottom bell to effectively radiate out heat just as the original mic was designed to do; the EF800 version uses fully discrete and separate voltage rails to provide heater voltage and B+ voltage, so there are no ‘power supply elements’ located within the microphone as there is with the dual-tube version.  I understand that for some people, that has been a concern; and so I am pleased now to offer this version of the mic which ‘runs cool’ (at the microphone, anyway, the PSU still has some warm elements as they both do).  I used to always retort that if the microphone base ‘doesn’t get warm, then it's not a 47’.  Well, it looks like I’ll be eating my words this time. :)

The Telefunken EF800 pentode valve is a fantastic tube from the golden age of vacuum tubes. This is the tube that will ship stock with each of these microphones as long as they remain available. We will ship an RFT (Germany) gold pin EF800 tube as the backup/spare as a courtesy. They are less expensive but nonetheless still reliable workhorses.

So what are the sonic differences between the two versions? Frequency response wise, there is no real difference; but there is certainly a perceived difference. I find the dual-tube version to be more 'diffuse' and girthy, with the EF800 version seeming more focused and up-front. I have to stress anytime we get into these descriptions that not only are we getting into a very subjective area but even more importantly we are always (what I like to call) 'dancing around in the 1% area'. Its always important to stress that even if I write an essay on sonic differences, the 'differences' I'm describing are differences of less than about 1% of the sound. Unless you have the privilege of owning both versions and setting them side by side, you are probably not going to pick up on these differences. With that in mind, I like to ask customers to instead think about which version they want to take on the responsibility of maintaining. Committing to the dual-tube version means needing a backup set of 408a pentodes that are pre-screened to be the cream of the crop. Finding a single EF800 tube is comparatively a lot easier, and their failure rate is far lower. I estimate it at much less than 30% based on the stock I've purchased thus far, meaning that if you buy one working EF800 tube it will probably be good enough, particularly if it is a quality name brand EF800 like Telefunken.

I am glad to finally get past this milestone so that I can be more certain that I'll be able to offer the SAE 47 and 48 for many more years to come, with the flexibility of two different driver options. I will have more professional photos up soon; but here are some from the first 2 units that were built. One was a mechanical headbasket switch and one was the relay version. 

Onwards and upwards… and the next stop is the M49!