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[plurye]

Hi Klaus et al,

I’m new to this forum—very cool! I have two questions:

1. I’m pretty sure that the tube in my UM 92.1S is going—it’s getting very harsh in the mid-upper register when it’s pushed at all. I found an online tube supplier who offered me the following options (from low to high price, all NOS, tested and guaranteed):

• Mullard or Amperex, UK or Holland made 6267
• Telefunken 6267 Germany, diamond mark
• Mullard 10M series UK made
• Genalex 1960s UK made mesh plates, low noise

Any opinions as to which is the best option? Not really worried about price—this is my only “real" mic and I want it to be as high-quality as possible.

2. How easy is it to switch out the tube in this mic? I don’t solder, but I’m certainly handy enough to open up my Macs, switch out memory, hard drives, etc.

Thanks in advance for answering,
Peter

[radardoug]

Those tubes wont have been tested for use in a microphone, which is a particularly demanding application.
So the only answer is, buy them all, and try them all.

[plurye]

Supposedly they have been tested, and the website that listed them is one that specializes in tubes for microphones. The owner of that website gave sonic characteristics for each tube. Since I know Klaus has a lot of expertise with the UM 92.1 S, my question is what he or anyone else who has actually tried one or more of these tubes thinks of them, and which one he or anyone else who's tried them would recommend.

Thanks,
Peter

[klaus]

I am not aware of any tube dealers who possess the exceedingly rare, expensive, ultra-high impedance tube testing equipment which Schoeps and Telefunken/Neumann built specifically for selecting tubes for microphone use in the 1950s.

There is simply no need for a tube dealer to invest many thousands of dollars into such machinery. So their usual assurance "measured xyz on tube tester ABC" is useless information for a tube which, in this case, does little or no amplification, but primarily functions as impedance converter This puts entirely different emphasis on a tube's performance characteristics, especially its noise and microphonics.

Please first read the primer on EF86 tubes here: http://repforums.prosoundweb.com/index.php/topic,1141.0.html

In addition, from experience, I can report that buying original diamond bottom Telefunken EF86 is usually a good thing. But there are timbral differences: the chrome plates, to my ears, still sound the best. I also like Dutch Amperex, and their British cousins labeled with all kinds of names,but all made by Amperex.  As much as I like the initial sound of Mullards, I never found them to be lasting very long. I  have never used the GEC EF86, but they look awfully close to the Mullards.

Which brings me to this piece of advice:

Do not buy any of these tubes without a money-back warranty. Simply too of these, even the premium NOS, fail the five day test (continuous operation). Do this: calibrate your supply for each tube under test to 210VD/6.3VDC max. Leave the mic on for five days straight, THEN listen! If the tube has no frying sounds, does not resonate when you ping the housing tube, or does not develop a high noise floor by then, you did as much as you could to pre-select.

[plurye]

Well, I have a money-back guarantee, anyway! :-) Thanks very much for this info—I'll definitely do the five-day test. I went ahead and purchased the tube described as Z729 / 6267/ EF86 Genalex Gold Lion series, with GEC, Marconi or Emitron label, Great Britain made." And both you and radardoug were right—it wasn't tested in a microphone. I don't know how to calibrate the supply as you suggest (or at all, for that matter), but I'll be bringing this tube to a mic specialist to install. He should be able to calibrate the power supply as described, yes? Or is there a simple way for me to do this?

One more question: How do I "ping the housing tube"? Do you mean just tap the outside of the mic? (Sorry if it's a dumb question).

Thanks,
Peter

[klaus]

You ping the tube by using your knuckle and tapping the housing tube pretty hard while listeing with headphones. Pinging will trigger microphonics of the tube, if present.

Do not test the new tube unless the supply voltages are checked and adjusted as soon as the tube is installed. I would not recommend a do-it-yourself power supply voltage adjustment where several hundred volts are present.

[plurye]

OK, thanks. And again, pardon my ignorance—if I have my tech person adjust the supply voltages when he installs the tube, can I then shut the power supply off, take it back to my studio, and start the five-day test at that point?

[klaus]

Yes, you may.

[Piedpiper]

Are these power supplies typically regulated and if not, how much voltage variance from location to location is acceptable. I had a tube amp that was blowing very rare and expensive output tubes because my voltage was unusually high, up around 125 VAC. Once I figured out what was going on I changed the resistor values for optimal voltages.

[mwurfl]

On a related note here, I was just offered this tube in response to my request to a tube seller for a NOS Tele EF806S.  It's been offered as NOS, but no box.  This seller is one who has been around for at least 6-8 years that I know of, and is always highly regarded in forum postings, although I have never yet done business with him.

This tube looks suspicious to me in several ways, so I'd like some help here, as much as is feasible, in determining if it's genuine.

Apologies to Klaus if this is deemed inappropriate for the forum, but my thinking is that the more we can all help each other to spot and avoid fakes, the better.

Thanks,

Mark Wurfl

[klaus]

Check for the Telefunken diamond embossed in the center of the tube base, accompanied by a one or two digit number, also embossed into the glass. If that is missing, I would pass.

Besides, as you probably know, you still need to do the five day test before completing the purchase, as there are no tube testers for microphone-grade tubes.

[plurye]

BTW (as the person who started this thread) my story has a happy ending—I got the aforementioned Genalex tube. Found a terrific "old-school" tech in NYC if anyone's interested—he also swapped out the transformer (for a UM351) and two of the capacitors—now it's smokin'! (Not literally.) Did the five-day test. No microphonics or hiss, no harshness, and noticeably more level. All of this was not cheap, but it paid off.

Peter

[mwurfl]

Congrats, Peter, on the happy ending!

And yes Klaus, I do know about multiple-day testing, and have done that with the few EF86s I've tested so far.  I only have the picture of this tube, so can't verify the diamond mark on the bottom, unfortunately.

The main things that arouse my suspicion with it, just based on the photo, are:

1) The top looks rather "bulbous," in the same way that the Chinese tubes I've seen are;
2) The molded/welded glass bottom looks too crooked to have truly passed Tele's QC;
3) For all things I've seen made in (West) Germany during the years these tubes were made, and not just tubes, they would all say they were made in "West Germany," and not just "Made in Germany" as this one does.

Since I've never seen a real Tele EF806S in-the-flesh, I can't be certain of any of these things; they're just my impressions.  But I read about so many fakes out there these days that I'm extra-suspicious, I guess.

Thanks again,

Mark W

[klaus]

I will look at some of my EF806S to confirm the "West" Germany. In the meantime, have the seller snap a picture of the bottom of the tube, to verify the diamond.