American octal steel tubes in LDC microphones?

[Gus]

There where threads on the church microphone at this forum in the mid 2000's IIRC.

 

[dmp]

When I look at the Church mic design, it looks like the designer C12'd a U47. Switched to 6072 tube. Took away the 'fixed' bias using the heater current to just capacitor bypass the resistor. And found a small cheap transformer in the Triad JO-23, wired for 5.7:1! That low ratio may choke off the tube, as typically a 10:1 is used with the 6072. And I particularly like the characteristic the 'fixed' bias gives a mic, so going away from that is a downgrade, IMO.
I've never heard one, maybe it's really cool, but from my experience with mic builds, I'd guess it is a big downgrade from a stock U47.

Anyway, this was a really interesting thread for creative mic builds using octal tubes, not just clones of things already done. Let's get it back on track.

 

[Recording Engineer]

If no one has any other thoughts for the E810F in a mic, how about (2) RCA 5693 in place of the (2) EF12 in an RFT CM-7151 circuit along with a Cinemag CM-S217D transformer; that I’ve had all the parts for years and still haven’t gotten to?

 

[rock soderstrom]

how about (2) RCA 5693 in place of the (2) EF12 in an RFT CM-7151 circuit along with a Cinemag CM-S217D transformer; that I’ve had all the parts for years and still haven’t gotten to?

I have no idea whether this makes sense or not. I have seen this microphone once in real life, it has an imposing appearance.

The power supply is built in, which is rather suboptimal from my point of view. Of course, you don't have to repeat that in a DIY build. I took a look at the circuit diagram. AC Heater with humdinger only for V1! I think this is the first microphone I have seen with AC heater. Again, you don't have to repeat that for a your build, of course.

I had a look at the Cinemag transformer, man I love their datasheets, they should all do it like that.

How do you plan to wire it, 5:1?

About the original circuit. What are the possible advantages of the two amplifier stages? I think the original idea was that you could go straight into the PA without a dedicated mic pre amp. Does that make sense in the modern studio?

PS: I love the 5693 tube (a 6SJ7 SQ variant), they look really dangerous. I think they can do a good job of replacing the EF12k.

Schematic and datasheet attached:

 

[k brown]

I don't think I'd be able make myself plug a microphone into an AC outlet!

Every nerve would be screaming "wrong!" "wrong"!

 

[rock soderstrom]

I don't think I'd be able make myself plug a microphone into an AC outlet!

Every nerve would be screaming "wrong!" "wrong"!

I agree.

The hum problem should not be underestimated either. I think that is why this concept has not become established. I assume that most CM-7151s that are still active have been modified with external PSU for modern studio use.

For instance, here is one modified bei Andreas Grosser and reskined by S. Thiersch.

https://www.madooma.com/shop/neumann/RFT_FWL_CM7151_M7_M9_Vintage_Roehren_Mikrofon_Bottle-Mic.html

 

[Recording Engineer]

Yes, I’d definitely would not put the psu in the body, like the original. Yes, I’d use the CM-S217D as 5:1. Basically, it’d be a somewhat updated Americanized version!

I did have an original thought for 10-years… It was beautiful! Great on vocals, strings, and guitars. Low-headroom and would distort as a room mic on drums. Line-level output. Yes, it definitely hummed, but not really a problem unless it was a single-instrument recording with more than 50% of the sound being from the 7151. I loved using it as the mid mic for M/S. The side mic would pick up the acoustical hum from it though.The original Neumann M7 capsule was reskin by Thiersch. Sold it, along with pretty much everything 9-years ago when my studio closed.

 

[rock soderstrom]

Great photos, thank you so much for this!

I'm just asking myself how this kind of mic body shape, the bottle, is evaluated from today's perspective? Is the free-standing capsule acoustically rather an advantage or a disadvantage? One thing is for sure, the lower part of the body offers enough space for tube experiments of any kind.

 

[Mr. Moscode]

The Church mic uses only 1/2 of the tube. They could have paralleled the 2 halves for lower noise and more current to drive the xfmer. Or used a 1/2 as a follower. Probably that thinking wasn't in the air.

However it probably cost a lot less than a stock Telefunken.

 

[emrr]

People who have used a Church generally find them excellent. I don't think they were available for general purchase.

https://tapeop.com/interviews/107/church-mic-bonus/

Stanley Church was Chief Sound Engineer for the famed Metro-Goldwyn-Mayer Sound Department in the '50s. They bought a then-new Telefunken-badged Neumann U 47 to test and found that it was not good for soundstage applications; particularly dialog recording. The mic body generated too much noise, the electronics output was too hot for their equipment (which had been designed for low-output ribbon mics), and the mic pickup patterns weren't ideal. Plus, no one at MGM wanted to deal with obscure German tubes and transformers. However, they loved the sound qualities of the mic, as its presence peak and sensitivity were very useful for distant mic'ing.

Church ordered a bunch (less than 100 but more than 12, based on the number of known Church mics) of U 47 capsules from Neumann and set out to build his own mic body and electronics, utilizing a 6072 vacuum tube with a high-voltage circuit (though there are at least three circuit variations known). He focused on using off the shelf, U.S.- available parts for the electronics, in keeping with the Hollywood tradition of using non-exotic gear that was easily and quickly replaceable if it failed during a production day (when potentially millions of dollars of talent were on set).

Church's "new" mic was written up in the SMPTE Journal's 1956 "Year In Progress" roundup (indicating that it made its debut in 1955). The story told to me by my father, C. Robert Fine, and Bob Eberenz, his longtime associate, was that Church built the mics on his kitchen table, and built fewer than 100 of them.

 

[klausheyne]

I used these measurements as a guide:

http://www.moxtone.com/mU47_U47_part3.html

The author's conclusion in the link, that the EF14 is a tube very similar to the VF14 (and therefore potentially a lower-cost alternative) is deficient in several aspects- a fact that is instantly audible when comparing by listening to both tubes in a (properly prepped) original U47. The EF14 sounds dynamically challenged- anemic and glassy, with accented high mids- among other reasons- frequency deviations due to a less than ideal impedance match with the GN8/BV8.

Even when supplying these two tubes using the same ratio of filament under-heating, that still leaves a six-fold gap in actual heater supply voltage.

But there are two much larger reasons why no other metal jacketed tube, octagonal or not, will come close in performance to the VF14, no matter how much one adapts the circuitry around a substitute tube:

The filament arrangement (I posted a comparison picture showing two opened-up Telefunken octagonals a while back on GS), and most relevant, the cathode doping of the VF14 was never duplicated by any other tube.

 

[rock soderstrom]

The author's conclusion in the link, that the EF14 is a tube very similar to the VF14 (and therefore potentially a lower-cost alternative) is deficient in several aspects- a fact that is instantly audible when comparing by listening to both tubes in a (properly prepped) original U47. The EF14 sounds dynamically challenged- anemic and glassy, with accented high mids- among other reasons- frequency deviations due to a less than ideal impedance match with the GN8/BV8.

Even when supplying these two tubes using the same ratio of filament under-heating, that still leaves a six-fold gap in actual heater supply voltage.

But there are two much larger reasons why no other metal jacketed tube, octagonal or not, will come close in performance to the VF14, no matter how much one adapts the circuitry around a substitute tube:

The filament arrangement (I posted a comparison picture showing two opened-up Telefunken octagonals a while back on GS), and most relevant, the cathode doping of the VF14 was never duplicated by any other tube.

This is actually about American octal tubes, but I'd like to take this opportunity to ask you which solution is closest to the VF14?

Sure, a real U47 has a VF14, but how close are the known replacement options in terms of sound? (even if that means making changes to the circuit or OT).

 

[kingkorg]

The author's conclusion in the link, that the EF14 is a tube very similar to the VF14 (and therefore potentially a lower-cost alternative) is deficient in several aspects- a fact that is instantly audible when comparing by listening to both tubes in a (properly prepped) original U47. The EF14 sounds dynamically challenged- anemic and glassy, with accented high mids- among other reasons- frequency deviations due to a less than ideal impedance match with the GN8/BV8.

Even when supplying these two tubes using the same ratio of filament under-heating, that still leaves a six-fold gap in actual heater supply voltage.

But there are two much larger reasons why no other metal jacketed tube, octagonal or not, will come close in performance to the VF14, no matter how much one adapts the circuitry around a substitute tube:

The filament arrangement (I posted a comparison picture showing two opened-up Telefunken octagonals a while back on GS), and most relevant, the cathode doping of the VF14 was never duplicated by any other tube.

That guy made the best possible empirical analysis of these tubes, provided measurements, and detailed explanation for everyone to decide.

I guess you used once again your ears to come to this conclusion. Dynamically challenged is a term you probably came up with recently, and how is anyone in the world supposed to interpret that? You challenge this kind of work with replies like this? Anemic and glassy? Tinnitus? There is absolutely nothing you describe present in the EF14 frequency wise. For once post any kind of material, measurement, ANYTHING to back up your claims. You of all people have quite a nice stash of VF14 tubes.

Please stop this toxic disinformation dissemination, it is 2023, 21. century, this kind of stuff doesn't roll any more.

 

[klausheyne]

I agree that, if you believe that listening to a microphone, then analyzing what you hear is overrated and therefore listening should not be the relevant end point of any technical analysis and improvement of audio devices, then, by all means ignore my post and don't let it upset you.

Complex harmonics and their effect on an instrument's timbre is one of the fundamentals of sound analysis. I just don't think that static, sine wave frequency measurements of a tube will give enough (any?) information about its dynamic behavior or response to these complex waveforms encountered in music.

When designing audio gear, I think we should not ignore our ears' ability to differentiate between even minute variations in audio processing. That ability far exceeds current measurement technology, I believe.

 

[kingkorg]

I agree that, if you believe that listening to a microphone, then analyzing what you hear is overrated and should not be the relevant end point of any technical analysis and discussion about audio devices, then, by all means ignore my post and don't let it upset you.

I just don't think that static frequency measurements of a tube will give enough (any?) information about its dynamic behavior or response to complex waveforms, as encountered in music.

Your biased claims throughout the years have damaged this field more than anything ever. Either lie, or delusion they come from your head. You are financially invested in the subject, and have never provided any informantion that would help anyone get closer to the goal. Contrary only discouragement, for all of us lacking golden ears or the original unobtainium you among others stash.

When i said it's 21 century, we have internet, original documents, historical data, contacts, tools that can measure and show any aspect of audio, and debunk any kind of audio foolery many have used to make bucks, feed their vanity and ego.

Who said anything about static frequency response? A milion ways to test a mic dynamically and shootout components. Easy confirmation via null tests. But i am sure you are one of those believing there's something hidden even if filles null.

 

[klausheyne]

This is actually about American octal tubes, but I'd like to take this opportunity to ask you which solution is closest to the VF14?

Sure, a real U47 has a VF14, but how close are the known replacement options in terms of sound? (even if that means making changes to the circuit or OT).

I tested all of the Telefunken Octagonals, and to my surprise found that I keep coming back to the replacement Neumann had designated, even if teh low end is lacking a bit: the 13CW4.

 

[kingkorg]

I tested all of the Telefunken Octagonals, and to my surprise found that I keep coming back to the replacement Neumann had designated, even if teh low end is lacking a bit: the 13CW4.

No, 13CW4 doesn't lack low end, however it does lack microphonic nature of the VF14. Which in purely technical sense makes it better. Maybe you can post the audio of your performed test?

 

[kingkorg]

I tested all of the Telefunken Octagonals, and to my surprise found that I keep coming back to the replacement Neumann had designated, even if teh low end is lacking a bit: the 13CW4.

You seem to change your opinion depending on situation.
Quote:

"I can confidently address only one of your questions: The (un)suitability of the 13CW4 Nuvistor in the U47.

Three reasons, plain and simple:

- Impedance mismatch between tube and transformer primary (resulting in a horrible low-end phase shift which expresses itself in a cut-off, honky low end with repercussions reaching all the way into the all important mid band region of the U47)

- Propensity for RF-interference (hence the need for another butcher knife into the heart of the U47- a rather large choke coil between capsule and tube grid)

- The afore mentioned microphonics of most of even the selected Nuvistors. Longevity or low noise also were not strong points of these monsters."

Original post by Klaus Heyne can be found on his forum:

https://repforums.prosoundweb.com/index.php?topic=2414.0

 

[rock soderstrom]

I tested all of the Telefunken Octagonals, and to my surprise found that I keep coming back to the replacement Neumann had designated, even if teh low end is lacking a bit: the 13CW4.

Thanks for your feedback.

 

[JohnRoberts]

come on guys don't make me lock this thread too...

JR