VF14 Background and History
When I started servicing and restoring Neumann tube mics in 1978, one of the things I learned quite early was the background why Neumann replaced the VF14 with the Nuvistor at the end of the U47 production and then finally ended the U47 series completely: They simply could not get any more VF14 which were suitable to be used in microphones!
Because in an U47 microphone the VF14 is used outside the manufacturer's recommendations and specifications, only few within the series were suitable for this use. As a result Neumann had to carefully pick the suitable ones from the mass: They had an agreement with Telefunken between 1950and 1957, that all production VF14 tubes were sent first to the Neumann test lab in Berlin.
All VF14 were checked for noise, plate current, distortion and frequency in a especially designed test setup (which simulated the circuit in a U47) and the suitable ones were kept and stamped with the famous white "M" (for 'Mikrofon'). All other VF14 were sent back to Telefunken, where they were packed and sealed for the "normal" radio market (to be used in domestic radio receivers).
Keeping this in mind, it becomes obvious that it is nearly impossible today to find even one single "NOS" VF14, that is within these Neumann specs. Although there are several VF14 around (and sometimes for sale for ridiculous prices), it is nearly impossible to get the full sonic performance and the sound quality today with a standard VF14, because all "NOS" VF14 you find today are Neumann rejects, unless you find a VF14M. This also explains why usually an U47 with a 55 year old and sometimes noisy (but "M" stamped) VF14 often sounds better than with an expensive "NOS-in-sealed-box"-VF14 from eBay in it...
So in other words, when you find a NOS VF14 for sale today, it is nearly 100% certain that this particular tube was rejected originally by Neumann in the 50s, because it did not fill the requirements to be used in microphones (unless it is marked 'M')!
So why did Neumann use the VF14 in the first place?
According to Telefunken the VF14 was made between 1946 and 1957. Exactly 27,548 VF14 were made and most of them used for German "univoltage" AC/DC radio receivers ("Allstrom Empfaenger"), because they only needed a single supply voltage and so they could be used either on AC or on the 105V/DC which was the provisional grid supply standard in parts of Germany after WW2. This was also a huge problem for the new broadcast stations after the war, so the Berliner Rundfunk (Radio Berlin) asked Georg Neumann 1948 to supply them microphones which could be used without a power supply directly from the 105V/DC mains. (Technical background: with a DC supply you can't use transformers, so it was extremely difficult to generate the two normally necessary voltages - a low voltage/high current for filament and a high voltage/low current for the plate).
Neumann solved this by using the VF14, which was simply the 'univoltage' type in the steeltube family, so the U47 could be used either with a very simple AC power supply, or directly from 105V DC .
There is a whole family of similar tubes in the Telefunken "steeltube-family", the EF14, UF14, EF11, EF12, just to name a few. There is a general opinion the EF14 must be a good replacement for the VF14 (maybe because it has a "14" in the name). Although the EF14 is certainly related to the VF14 as well, it is not a suitable replacement, because it is an AC- heated tube which will become noisy or "numb" quickly when DC-heated (as in microphones). Also it is known to be highly microphonic and it requires an immense 450mA filament current, which would heat up the complete microphone to a degree that you can barely touch it. With EF14 in a U47 we measured temperatures up to 82deg/C (180deg/F) within the mic. Materials like polyester - as used for capsule diaphragms - are chemically stabilised only up to typically 55 deg/C (135deg/F) .
Apart from this it is nearly impossible to find an EF14 that fulfils the technical specifications Neumann had for their selected VF14M. If you are very lucky, you might be able to select one as good as a "normal" VF14, but never as good as a VF14M! For this reason most circuits in U47 (or so called 'clones'), using an EF14 have some additional components to "enhance" the sonic characteristic to get it a bit closer to the famous U47 sound.
Performance in steel tubes spread a lot, so to select and find high quality tubes suitable for microphone use you need a very large quantity of the same type.
The EF12 was the top-of the-range steel-tube for German audio equipment in the 40's and early 50's, (similar to the later EF86, EF806 and EF804S) and was developed for use in preamps and microphones. It is a very close relative to the VF14 and is also designed for AC/DC heating and small signal. The main difference to the VF14 is that it uses a separate heater supply voltage, but it is only heated with 100mA, so it does not melt the capsule diaphragm and lasts much longer. It was used in the Neumann CMV3, CMV5, V30 and RFT CM5171 microphones. Far more EF12 than VF14 were manufactured and there is still supply available to find some which can be selected to meet the critical quality criteria for microphones. With our selected EF12 no extra components are needed to keep the U47's authentic sound.
We select the tubes for our U47w using exactly the same selection criteria as Neumann did in the 50's.
VF14...? there are about 100 different stories out there, here are my findings, based on historic internal Telefunken documents, talks with former developing engineers of Telefunkens tube factories, RFT engineers and the IRT employees.
It is not true that the VF14 is a WWII surplus tube, a VF14 was not made before 1944. The first time it shows up in Telefunken data as well as price sheets in 1946 catalogue (issued around November 1945). There is a reference in the manufacturing report of the VF14 to a LVIII made for the German Air Force in 1944, but this tube never showed up in any reference data and I only could find one specimen in over 20 years of searching.
It is not true that a UF14 is the 100mA version of the VF or EF 14. The UF14 is a 2 Watt universal pentode made only for a short time 1947 to 1954. It shares the same cathode dimension and pin layout but that's all. 1st, 2nd and 3rd grid are different as well as the smaller plate construction
It is not true that only Telefunken made the VF14 alone, RFT Erfurt made approx. 15k VF14 in an glass version with a Zink coating (like the pre war tubes). Only found two specimen.
Here are some facts,
About 27,000 VF14 were made in three production runs in the Telefunken factory in Berlin, they stopped the entire steel tube production in March 1958 due to the high production cost.
Only 5% met the desired M specs, the rest was unusable and used for a Telefunken made AC/DC radio receiver (only short time made in 1948) with one VCH11 for the radio part and the VF14 as and 2 Watt output driver (these are the VF14 you may find today as NOS tubes)
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