In this article I would like to show you how to take care about the first link in the classic recording chain: The tube condenser microphone. Perhaps you are the proud owner of such a jewel?
Your customers look at it in awe and admiration while you tell them it has been in good use for many years, always providing an indescribably beautiful sound… and then you have to spoil it by admitting it just recently started to crackle or sizzle from time to time. These microphones, even if they are old, can be fully restored with some luck and a little effort. "Some luck" meaning that it is hopefully not the tube built into the microphone showing the defect: Unfortunately, tubes cannot be repaired. "Little effort" meaning cleaning, replacing common components and fixing loose contacts. If you dare to treat your loved microphone to a much-needed service, I would like to show you step by step where the faults can occur and how they can be fixed with relatively simple means.
First Diagnosis: Start at the Beginning
Restoring tube microphones means moving on two grounds: The first is the microphone itself and the second is its corresponding power supply. In my experience, investigating unwanted noise should always start at the very beginning: The capsule. The capsule is of course the most sensitive part of a microphone, since it is very similar to our eardrum in terms of sensitivity and risk of damage. It is also directly exposed to the environment, like dust, humidity and differences in temperature.
Listening Is Better Than Measuring
To make a preliminary diagnosis, listen carefully while switching the microphone on. A tube mic needs about 30 - 60 seconds until the tube has reached its operating temperature. You will notice some noise immediately after switching on. This is caused by the beginning of the electron emission of the heating cathode, which is completely normal. When the tube has reached its intended temperature, it should become quiet and only the room ambience should be heard, as the microphone is now working. If there is some crackling or light sizzling, you can breathe gently on the diaphragm. If the signal then starts to sizzle violently in response, after which the sizzling disappears again within a few seconds, you can assume that the capsule surface has caught some dirt.
Vocal microphones are particularly affected because they are often exposed to aerosols from the air we breathe. Let’s say the membrane has caught some dust which sticks to its surface. When the exhaled moisture evaporates, these dirt particles start to contract and cause the sizzling noise. Listen again when turning the microphone off: It temporarily "says goodbye" with a soft, diminishing, crunchy noise. Now the time has come to free the capsule from its grille in order to clean the membrane surface.
You need a steady hand, because one wrong move can immediately destroy the membrane, which is only a few µm thick.
Never use any solvents or Q-Tips which are much too quick at hand for cleaning! See the images below for the sad results of such attempts. At this point, only a soft badger-hair brush (available in artist supplies) and distilled water (from the pharmacy or supermarket) should be used. The membrane can literally be washed this way. With double membrane capsules, of course, both sides should be sufficiently cleaned. For drying, place the entire capsule structure on a heater for several hours. The rising warm air (40 - 50°C/100 - 120°F) allows all residual moisture to evaporate even from the tiny gaps.
Next Step: Measuring Needed
After a careful reassembly and another test, the microphone should be quiet again. However, if constant sizzling is still audible, regardless of the "breathing test", there is still hope that the problem is caused by an aged, dried out capacitor and not by a damaged tube. In this case, it makes sense to examine the voltages of the power supply with an oscilloscope. A 10:1 probe and a good isolating capacitor should be used for the connection, because DC voltage must be prevented during the measurement. Sizzling can be detected in form of short voltage drops.
Especially with older power supply units from Georg Neumann, it is possible that the so called "Stabilyt Cells" used for stabilization have leaked. These cells (an original Neumann invention) serve to stabilize the tube heating voltage of approx. 4 volts. These are NiCd batteries which naturally decompose over longer periods of not being in use.
Replacing these cells is no longer necessary today, because an electronic stabilization can be easily set up with quite well-known longitudinal controllers such as the 7805 or similar. A small circuit board finds enough space inside the microphone body. The components do not need mentionable cooling as the heating current of the AC701 is only 100 mA. However, it is also possible that the voltage regulator tube for stabilizing the anode voltage is not working properly anymore. This is a vacuum tube which, similar to a Zener diode, limits the voltage to 150V. These VR tubes are still available in larger quantities today.
I think I don't need to emphasize that a prophylactic replacement of the numerous electrolytic capacitors will be of great benefit for the power supply. A further source of problems can be oxidized connectors in between the microphone and the power supply. Be very careful when moving the connectors and listening at the same time: This kind of cracking can hurt your ears even at normal listening levels. Cleaning the connectors can be done by using pipe cleaners soaked in Ballistol (a famous weapon oil brand. I hate guns, but this stuff is excellent when it comes to cleaning contacts or pots).
If the microphone now works without sizzling or cracking, but still produces more noise than comparable models, you can finally start to suspect the tube being the culprit. If you are lucky, you are dealing with a microphone which has a dedicated tube socket. For Microtech Gefell, this could be an EF86 or an EC92. Neumann, Berlin also built such a microphone: The U67. Tube microphones by Schoeps were usually equipped with an EF94.
Tube Measurement Circuitry
The measurement of such tubes is usually done with special circuitry. I built my own specimen according to laboratory reports from Telefunken documents for my own purposes. Supplying this circuit with power is exclusively based on batteries to avoid any negative influences coming from mains.
For microphones with the AC701 “Pencil Tube”, replacement is becoming more and more difficult. Finding this tube often needs a good portion of luck, and sometimes you will only be able to find a used one. Unfortunately, there is no interchangeable replacement tube available.
When replacing this kind of tube, you need a calm hand and good tweezers. The AC701 is a socketless miniature tube that has been specially designed for use in microphones. It has a particularly low noise performance, a reliable sound and a very low distortion factor. This tube is soldered directly into the circuit (inside the microphone) without a socket. Since this has to be done in a very confined space, care must be taken that the connecting wires are not unnecessarily bent (if they break off directly at the glass body, the tube is lost) and that the soldering heat is sufficiently dissipated with tweezers. This is actually very similar to soldering instructions for germanium transistors.
A Classic with Problems
The most difficult tube to obtain is the VF14 for the classic U47. This tube was built shortly after the war for simple straight-line receivers and antenna amplifiers. These devices were usually operated without a transformer. The heating was done by series resistors, because transformers were very expensive. In addition, there were direct current networks still in place in the south of Germany as well as transformerless devices, so-called "Allströmer", which could be operated with all types of voltage. Therefore, this tube has a relatively high heating voltage of 55 volts and a low current of only 50 mA.
In the U47, the VF14 was heated with approx. 35 volts via a series resistor together with the anode voltage supply of 105 volts. There are comparison types for this tube which are directly compatible in terms of their system, but their heating values differ considerably. The EF14 has a current consumption of 470 mA at a heating voltage of 6.3 volts, the UF14 requires 25 volts at a current of 100 mA for heating. You actually have to convert the microphone if you want to use one of these substitutes, including a change in the microphone itself as well as in its power supply.
Fortunately, there is an unused wire in the original connection cable of the U47, so that the modification can be done while keeping the original plugs.
When the VF14 was slowly running out of stock towards the end of the 60s, the option of converting the microphone to nuvistor operation came up. The specific nuvistor for this is called 13CW4, and its heating current of 60 mA is close to that of the VF14. The necessary modifications on the power supply and microphone are quite small.
Nuvistors are full-fledged electron tubes in a special metal and ceramic housing the size of a germanium transistor. They were built for operation under the roughest mechanical conditions. With a lot of luck and/or money, however, a VF14 can still be found here or there. Even more lucky is the one whose find can really be used for microphone operation in the end. Remember my last article in which I described that not every NOS tube really corresponds to the actual "new data"…
In this U47, a used but very good VF14 was chosen to do the job after the original tube had failed operation after many years. The obvious advantage: The microphone and its power supply did not have to be modified.
Preview
This episode was about the restoration of the first link in the recording chain: The microphone. More than once I had to try my luck when it came to obtaining a rare tube… sometimes a modification was necessary. But if you have looked at tube tables in search of a suitable replacement, you will have found that several types are very similar – either in terms of pin assignment or technical data. But which tube can really act as a substitute for another one? The next episode will give you information about what to pay attention to when replacing a tube and what kinds of compromise can be made.
This article is presented with kind permission of its original publisher, the amazing Studio Magazin, enriching Pro Audio since 1978! The author, Uli Apel, is an incredibly versatile and experienced engineer as well as one of the most qualified experts in vintage broadcasting and audio technology around these days.