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Author Topic: Microphone Tubes: How to spot fakes, select and test real ones  (Read 17490 times)

Offline Klaus Heyne

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Microphone Tubes: How to spot fakes, select and test real ones
« on: October 14, 2004, 05:32:17 pm »
In my work I go through hundreds of  tubes for tube mics every year.  
InitialIy, I set up proper supply voltages and test the tubes statically: 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 them off.

I then re-adjust supply voltages, if necessary, listen for background noise and make comparative noise notes, in dB, and also compare teh specific tube's noise to other, known low-noise performers 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 in operation.

Then I listen to the tube's ability to process complex musical information: What is the character of the tube? Is it harsh, or soft in the highs, or tubby in the lows, or overall aggressive, sweet, etc.?

I then try to select the right tube characteristic for a specific microphone or application, to complement, rather than neutralize, a specific 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 cut out any of the above steps of testing, you end up sorry.

As you can imagine, this type of testing is enormously time consuming. But unlike with tube testing for mic pres, audio processors, or power amp input stages, the information provided by readouts from a professional tube testing apparatus is useless by itself to predict (musical) performance in a tube mic.

    ------------------------------------------------------------ -------------------

My findings regarding brands, etc.:

1. A tube's usefulness in super high impedance circuits, like those found in condenser mic impedance conversion, is unpredictable: You can never deduct by brand, series or price you paid whether an EF86 or other tube will perform satisfactory in a microphone.

2. In general and from experience, N.O.S. (New Old Stock) tubes are superior in performance and sound to currently made (and cheaply constructed) Russian or Eastern European tubes.
As a matter of fact, tcurrently made tubes are so uniformly unreliable and/or bad sounding that I will not even test any Sovtec, Svetlana or similar 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 EF86, AC 701, and VF14-M are very good and last a lifetime. These tubes, which are very rare now, are identified by Neumann packaging and by an additional set of numbers on the tube itself. Neumann only used German-made Telefunken, 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 chapter 1 is valid.

4. To complicate matters (or to make it even more compelling to follow my advice!) many tubes and tube cartons are stamped with information that is not correct:

A Telefunken-stamped 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 comparing and memorizing its specific construction:  If you look carefully at the plate, heater, cathode and other construction elements in a tube, visible through the glass, you will find subtle differences of how the wires are shaped and routed, how big and what color the plates are, and so on.
After a while, you don't even look at the information on the tube anymore, but simply identify the tube's origin and manufacturer by its innards.

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?

Wishful thinking often gets the best of us, and we end up losing money on E-Bay tube 'deals':

If one assumes, that at best about one in two miniature glass body tubes, like EF86, 6267, 6072, etc. will perform in an acceptable manner, 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, I pay one third of the going market price, knowing that only about 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 to spread that he is selling duds without recourse by the buyer.

I cannot stress enough that a seller's (fantasy?-) prices for tubes can only be sustained with plenty of ignorant or fearful buyers-
Ignorant of the technical realities, like odds of failure, as mentioned, 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.


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.

Because of the ever increasing possibility that you are being offered a fake VF14, I have added links to the following, very informative, information how to spot a fake VF14:
  http://recforums.prosoundweb.com/index.php/mv/msg/22206/0/0/ 318/
Klaus Heyne
German Masterworks

Offline Klaus Heyne

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Re: Microphone Tubes: How to Test, Identify and Select
« Reply #1 on: January 14, 2007, 02:46:21 am »
Here is a reply to a recent inquiry regarding tube life in mics.
Some of it may already appear above, then some of it may be of additional value to this topic:

Tube life in low-power applications like in microphone impedance converters
is still largely uncharted territory: No one would or even could test hundreds or thousands of same-model and manufacturer NOS tubes, to see what happens with certain operational practices, like constant switching, continuous operation, etc.

The reason is two-fold: the tubes have gotten too rare and expensive for such an experiment now, and impedance converting application is such a rare exception for most triodes and penthodes that most manufacturers in the past never bothered to optimize, let alone, test tubes for that specific use.

It is also worth mentioning in that context that, say, an EF86 that measures and works great in an amplification circuit may fail a super-high-impedance application like in a mic without any way to measure it, unless installing it in a mic and measuring the (discharge) noise.

What we know, though, is that lack of ramp up time, i.e. current surges and other over- or quick voltage/current stress, will shorten any tube's life. And we know that excess heat is not good either for the longevity of a tube.

However, aside of the VF14 issue I touched on earlier, most tubes in mics get (barely) hand warm at most, or at least never get too hot to the touch. And NG-type power supplies (clones included) will ramp up very slowly, due to the hefty dropping resistors used.

So that is why you may find some old fogeys who have never turned off their U47 in twenty years, and still have a working VF14 after that time (never mind the collateral damage I mentioned.)

My experience so far (and this is not scientific, merely from many years of working with tube mics) has led me to recommend to turn off any tube mic when it's not in use for several hours.

But then I cannot be certain that, say, an AC701 will last any longer through any extra precautions we can come up with beyond keeping heat, surges and mechanical shock away.

I also tend to slightly underheat the filaments, by, say, 2% to 3%. to extend filament life without under heating it so much that I get contaminant buildup on the wire, which is as deadly as overheating to an impedance converter tube.
Klaus Heyne
German Masterworks

Offline christianlund

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Re: Microphone Tubes: How to Test, Identify and Select
« Reply #2 on: April 22, 2007, 03:15:37 pm »
I am using a Rode Classic II mic (JAN 6072 tube), mainly for vocals.

I am experiencing some exaggeration (distortion?)of 'ch' and 'sh' sounds (2-4 khz) that I cant fix properly by de-essing.

Could it be a bad tube?

Also, if I buy a new 6072 tube - does it have to be a JAN?

Thank you for your help!

Offline Klaus Heyne

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Re: Microphone Tubes: How to Test, Identify and Select
« Reply #3 on: April 22, 2007, 06:48:09 pm »
If the 6072 works fine in all other regards in your mic, it is highly unlikely that your sibilance problem stems from the tube. You can always put in another one, and compare, though.

If you buy a new 6072 tube, you will have to select it for mic use, regardless of its brand and imprint. By and large, (J)oint-(A)rmy-(N)avy-stamped tubes are already at the top of the class, but, again, in mics all bets are off until you actually burn one in in your mic for 48 hrs straight, then listen and compare.
Klaus Heyne
German Masterworks