U48 Premium Reference, Historic Switch Edition

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The so-called 'dual tube' method for reintroducing the classic '47 circuit into production was first suggested by the late Oliver Archut of AMI transformers, whom we worked with and still support to this day by using his amazing vintage transformer designs in our products.  It is an honor to do our small bit to keep his legacy alive.  

 

There are a number of ways to go about making the classic '47 circuit, all of which have their positives and negatives.  We had to look at availability of tubes to ensure a long-running product as well as the age of the tubes in question.  While the original VF14 is long unavailable; its is also important to note that even if it were, it is long past its theorized operational lifespan.  Whether in use or kept in a box, vacuum loss and oxidation eventually comes to all, and even functioning original tubes are not operating optimally.  They may sound great, but they are not dynamically and sonically what they once were.  The reason this is important is because many of the most coveted tubes chosen to make a '47' are very close cousins of the VF14 that were made around the exact same time.  Steel tubes such as the EF12 and EF14 are probably just as past their operational lifespan as their rarer sibling.  One of the better options out there is the Telefunken EF800, a glass pentode that was made to fairly high standards and manufactured at least up through the mid 60's.  It's aged a bit better and a bit less than its ancestor; and several manufacturers have used it in their '47 designs with good results.  The problem we had was in securing a steady source for these tubes.  One here, one there, from different production runs and stored in different circumstances, makes it hard to maintain a consistent product.  Also, these tubes are nearly as old as their predecessors, so we felt that the EF800 is merely 'buying a little more time' before the inevitable.  

 

The 408a (6028) was manufactured consistently up until at least the early to mid 1980's by Western Electric, Northern Electric, Ericksson, and JAN GE.  We have invested in procuring a large stock (close to 1000 pieces) of our favorite variant of this tube (Western Electric 408a) and have spent quite a bit of time cleaning, testing, and re-activating them.  At least one manufacturer who sells possibly the most expensive '47' replica on the market today has come across the 408a as a valid substitute, though modified and encapsulated into a VF14 compatible plastic housing for retro-compatibility to the original valve.  We feel this approach is not seeing the forest for the trees.  The time of the VF14 has come and gone, and backwards compatibility to that tube socket is pointless.  Of every option available out there, only a matched pair of working 408a's operating in parallel presents approximately the same plate resistance and gain factor as the original VF14, and runs nicely on the same B+ voltage and heater voltage (+35v) that the classic '47 was typically operating on.  This simply means a great deal to us, because it allows us to maintain the same power supply design and the same output transformer design as the original '47 without changing really much of anything.  It's hard to quantify exactly where the magic lies in the classic '47 design; but for us, the less we have to change, the better.  The original '47 power supply is a strange beast... mostly passive with one main high voltage rail, divided right at the mic to provide both rail and heater voltage by a large thermally coupled high-watt resistor... this is why the classic '47's got warm to the touch and often hung upside down to help disperse heat more effectively from the bottom bell... and ours is the same in every way.  Many manufacturers have sought to modernize or improve this design; we just seek to keep it as close as we can to what it was, and to build it with care.

I get asked a lot about the differences between the capsules. I'm going to try to 'encapsulate' my thoughts down into just a brief summary. Please note that it is somewhat subjective, and always we are 'splitting hairs' and talking about the last few percent of difference.

 

So, the stock OPR is warm but neutral. It is tuned lower than many capsules, to historic specs. it has a wider frequency response than some capsules, particuarly low end reach. It gets down into the rumble/subsonic region; but it is not by any means muddy or bassy, dark or bright. It's quite neutral, but just casts a wide net. I think for general music application, to include vocals, this capsule wins out by a hair, and also if you are trying to stay close to what the first K47-fitted U47's behaved like.

 

The Heiserman HK47 is a modern approach to the K47 design. New mechanical design (dual backplates). It delivers a more mid-forward, polished and 'finished' sound. Mids and upper bass seem more enhanced and brough forward. Extremely musical. Allows vocals to punch through a dense mix with clarity, without requiring huge amounts of gain. If vocals are going to be the primary duty of the mic, particularly pop, rock, or modern country, the HK47 capsule may the winner.

 

The Heiserman HM7 is not enormously different from the HK47; but has not quite as much mid emphasis and not quite as much top end, a slightly warmer, darker, and smoother capsule with a little bit more emphasis on lower end of the voice and a little bit of top end taper. The earliest U47 models as well as its predecessor, the CMV-3, shipped with an M7 variant; so if you are after a more vintage tone, then this capsule would be the winner. It's a matter of taste; but many people prefer the sound of a 47 with an M7 above any other capsule.

 

The imported custom K47 capsule is something I offer to allow people to get a great microphone at a significant discount, and still retain about 90% of the overal sonic quality. It is absolutely the best Asian K47 capsule made. There is no other capsule made in China that I know of that I would even consider. offering to my customers. Is it as good as a handcrafted capsule? No, its not; but it isn't bad. It has a natural and somwhat hi-fi sound with sufficient low and high end without being overly bright, sibilant, muddy, present, or harsh. Slightly warm, slightly scooped, and slightly 'hyped' sound; but overall fairly well balanced and pleasing. Also very versatile. If you are budget conscious or plan to upgrade the capsule later and still want an outstanding microphone; this is the winner.

Both the dual-tube method and EF800 pentode method are 'tried and true' proven circuit attempts to reproduce the electrical conditions necessary to mate a K47/M7 type capsule to a BV8 type output transformer and give a close sonic approximation to the original U47.  Both have their advantages and disadvantages.  In my opinion, (and really, more of a fact) the dual-tube method is the closest one can possibly come to reproducing the original U47 circuit design and power supply design of the original mic.  It allows all of the same component values, and the same single-rail power supply design.  It requires the least alterations to Georg Neumann's original circuit and therefore is the most purist approach. 

The EF800 pentode tube does require at least a dual-rail and more modernized power supply design  and other changes to the circuit to accommodate it; but those changes are minimal and this is a really good tube with a possibly closer familial relationship to  the original VF14 steel tube.  It is probably the tube of choice for the largest number of '47' reproductions out there (Heiserman, Wunder, Golden Age, Stam, and others) and thus is a very time-proven design. While the EF800 and 408a pentode tubes are both still available in the used market, the EF800 circuit only requires one tube and generally it is much easier to get a good single tube than two perfect 408a's (which can actually be quite challenging).  So that is certainly a point to consider.