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Author Topic: Microphone Tubes: How to spot fakes, select and test real ones  (Read 31268 times)


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Originally Posted: Thu, 14 October 2004

During the course of  my work I go through hundreds of tubes for tube mics every year.
Initially, I set up proper supply voltages and test the tubes for functionality and noise. I will leave them on inside the mic they are supposed to be used in for at least 7 days, often 14 days, straight, without turning the power supply off.
I then re-adjust supply voltages, if necessary, listen for background noise and make comparative noise notes, in dB, and also compare the specific tube's noise to other, already tested, low-noise tubes I use as reference.
I then excite the tube with a blunt instrument or tap the mic body with my knuckle, to evaluate the tube's microphonics or possible filament weakness.

Then I listen to the tube's ability to process complex musical information: What is the sonic character of the tube? Is it harsh, or soft in the highs, or tubby in the lows, or overall aggressive? Sweet and fast, and emotionally attractive?
I then try to select the right tube characteristic for a specific microphone or application, to complement, rather than neutralize, a specific mic characteristic or personality.
I am sorry to report that I have not found a less time consuming method to evaluate a tube for the use in a high quality condenser mic. If you bypass any of the above steps of tube testing, you may end up sorry, or poorer.
As you can imagine, this type of testing is quite time consuming. But, unlike tube testing for mic pres, audio processors, or power amp input stages, the information provided by professional tube testing equipment is useless by itself to predict performance in a tube mic, due to the extremely high input impedance of a condenser mic (in excess of 10 thousand million ohms, or 10gigΩ.)
------------------------------------------------------------ -------------------
My findings regarding brands, etc.:
1. A tube's usefulness in super high impedance circuits (larger than 10 thousand million ohms), like those for impedance conversion in microphones, is largely unpredictable, and cannot be pre-tested, except installing the tube in a microphone, leaving the mic on for several days, then listening. You can never deduct by brand, series, or price you paid whether an EF86 or other tube will perform satisfactory in a circuit for which no testing apparatus exists anymore.

Never trust a seller's "test results" citing specs like transconductance, gain, etc. "2.8mA, 1% matched" means nothing in predicting a tube's suitability for a circuit that does not amplify but works as impedance converter. There is currently no testing device to predict a microphone tube's noise level or its microphonics, other than plugging the tube into a tube microphone, keeping it running for a few days straight, pinging it to test for microphonics, and listening for discharge sputtering or steady-state noise floor.
2. In general, from experience, N.O.S. (New Old Stock) Western European tubes (like Amperex, Valvo, Mullard, Telefunken, Miniwatt, etc.) are superior in performance and sound to currently-made (and cheaply constructed) Russian, Chinese, or Eastern European tubes, and almost always last longer.
I find currently-made tubes are so uniformly unreliable, cheaply made and/or bad sounding that I will not even test any Sovtec, Svetlana, or similar types anymore.

Aside of the high failure rate, current production tubes, without exception, sound noticeably inferior in high quality condenser mics. (Oliver Archut and other tube experts have the technical knowledge to explain why a cheaply constructed tube cannot process audio in a satisfactory manner.)
3. By and large, Neumann-preselected Telefunken EF86 (the long-last version EF806 is identical in build with the EF86 but has been tested more thoroughly), AC 701/AC701k (the 'k' version's low microphonics were eventually achieved with all AC701 tubes so Telefunken stopped putting the 'k' on the tube), and VF14-M are very good and often last a (human) lifetime. These pre-selected tubes, which are exceedingly rare now, are identified by Neumann packaging and by an additional set of numbers on the tube itself. Neumann only used German-made Telefunken, in the case of EF86 identified by the diamond-shaped protrusion in the tube's glass bottom, where the pins are. Other than those Neumann-selected Teles, my information in paragraph 1 applies.
4. To complicate matters (or to make it even more compelling to follow my advice in paragraph 1!) many tubes and tube cartons are stamped with information that is incorrect, or misleading:

For example, a Telefunken-labeled EF86 may be made by Valvo, Hamburg, or Tungsram, Hungary. An Amperex, Holland tube may have been made by Mullard in England, Valvo in Hamburg, and so on.
Plus, often tubes were made at some place other than what the country of origin on the tube box or tube's imprint may indicate.

So, the only way to identify a tube is by learning and then comparing its specific construction: If you look carefully at the plate, heater, cathode and other construction elements in a tube, visible through the glass envelope, you will find subtle differences of how the wires are shaped, routed, and connected, how big and what color the plates are, and so on.
After gaining some knowledge of tube construction, you may find yourself note even looking at the information on the tube anymore, but you will simply identify the tube's origin and manufacturer by its physical layout.
natpub wrote on Thu, 14 October 2004 19:03

Considering the expense of some of these tubes, what kinds of precautions might one take when, say, purchasing through eBay? What considerations should one be sure of before purchasing such an item?

Assuming that at best about one in two miniature glass tubes, like EF86/6267, 6072, etc. will perform in an acceptable manner in a high impedance circuit (read: tube mic), one can do two things:
1. Pay the seller's (realistic) asking price but insist on a no-hassle money-back guarantee
2. Incorporate the expected failure rate in the purchase price.

As an example: when I buy AC701 tubes from an unknown source, I may pay only one third of the going market price, knowing that, on average, only ca. one third of these tubes will be usable in a microphone.
The alternative would be to buy three tubes at a price which is low enough to reflect these circumstances.
Mark Lemaire wrote on Mon, 18 October 2004 01:07

Klaus- How do you manage to pay 1/3 of the going market price for AC701 tubes?

I manage that two ways:
I usually buy large quantities of these tubes, and make it clear to the seller that my terms are perfectly reasonable, (and not just for me), given the nature of AC 701s. No honest seller wants word spread that he is selling duds or fakes without recourse for the buyer.

I cannot stress enough that a seller's (fantasy?-) prices for tubes can only be sustained with enough ignorant or fearful buyers-
ignorant of the odds of failure, and fearfull that someone else will grab the tube first, so it will be twice as expensive tomorrow.


I have a mic that uses a 6072 tube. I recently found a source for RCA 6072-A. Would the "A" be compatable ?

All RCA 6072s I have seen to date had the 'A' suffix. They are fully interchangeable, as long as they are quiet, which is rare with this model tube. That's why AKG used ONLY GE Five Star 6072 for all its mics (C12, C24, EL AM251, C28A, etc.), not even the Joint Army-Navy-designated 6072 (JAN 6072), which was deemed inferior in longevity and selection.

I did notice some microphonics with the 6072. Is there a substitute replacement tube number that would be better?

Hard to tell without you telling me what mic it's supposed to go into.

If the tube is to go into any of the AKGs which originally came with the 6072A, or their currently manufactured clones, then stick with it- the sound of these mics is partially derived from this tube type.

Klaus Heyne
German Masterworks®
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