Microphonics in VF14 and U47

[Drea]

Just to avoid any misunderstandings. The U47 was not an "Allstrom" device but used a tube that was built for it. The reasons from my point of view, as already mentioned were the simple "one voltage" power supply and probably also availability immediately after the war.

My thought is the U47 could work on
alternating current AC and DC

 

[rock soderstrom]

My thought is the U47 could work on
alternating current AC and DC

Well, with a modification of the PSU it will work with AC and DC. But it was not intended for a dual use. U47 is not an "Allstrom" device. These do not have a mains transformer, i.e. no galvanic isolation, but only a diode. That is the main difference.

https://de.wikipedia.org/wiki/Allstromgerät
Edit:

Because the question of omitting the mains transformer comes up from time to time, also here in the forum. I strongly advise against such experiments, especially for tube microphones, but also for other devices such as preamps. It is life-threatening and forbidden, at least in Europe. There are serious reasons why this is no longer done today. Work safely, live longer!

 

[tomas.borgstrom]

What total anode resistance value do you suggest for the 408a tube? Just another 100k in parallel with the existing resistor?

I am currently experimenting with the 6.3V siblings (5654/6ak5w) of the 408a.

In the U47 circuit with 105V and the filter with 30K and then 1uF to ground I settled for about 40K instead of 100K for the second. I did some measurements of the frequency response but I mostly used my ears (and we all know how wrong they can be).

I also add one resistor before the 29R between cathode and ground but the value of this varies with the tube brand.

 

[rock soderstrom]

I also add one resistor before the 29R between cathode and ground but the value of this varies with the tube brand.

Thanks for feedback!

Really manufacturer-dependent the additional resistor? Around 100 Ohm?

Do you underheat the 408a?

 

[terry setter]

I am currently experimenting with the 6.3V siblings (5654/6ak5w) of the 408a.

You'll find that the 6AK5 has consistently higher 2nd harmonic distortion than many other tubes. Some folks consider that to be euphonic and love it. Others feel differently. I've tested a passle of them and found that a reasonable percentage of them were low in noise and microphonics. The Russian Oktava 2500 mics use the Russian version of that tube and I think it is a very respectable sounding microphone (the actual Oktava ones, that is, not the Chinese knock-offs).

 

[rock soderstrom]

You'll find that the 6AK5 has consistently higher 2nd harmonic distortion than many other tubes. Some folks consider that to be euphonic and love it.

I totally agree! I think that's why I love to build tube microphones.

I'm a big fan of the whole 5654/6AK5W/E95F family, they sound really good to my ears! (completely independent of the VF14, I have no way to compare).

I now have a proud NOS collection. Telefunken, RCA, GE, Phillips, Mullard...the nice thing is you get them for extremely little money. I pay between 3 to 6€ in original box per piece. Incredible, really a lot of tube for the money.

My favorite tube family so far for microphones. I have built up some circuits with it, the always sounds really good to me.The size, price, availability and the low microphonics and noise have convinced me, the whole package is right.

The Russian relatives I still have to check!

 

[tomas.borgstrom]

Thanks for feedback!

Really manufacturer-dependent the additional resistor? Around 100 Ohm?

Do you underheat the 408a?

Generally a bit lower, perhaps around 30R. I haven't found any difference when changing the voltage to the heater.

What type of transformer do you use with E95F?

 

[rock soderstrom]

What type of transformer do you use with E95F?

10:1 Beyer bean and a bigger Siemens at the moment. I am currently looking at what UTM has to offer.

This one might fit well

https://utmindustry.com/utm0549/
They have other similar ones on offer. Less step down factor could also work.
Does anyone have experience with the UTM mic transformers?

 

[JW]

Thomas Borgstrom,

Is the anode resistance adjustment you're describing referring to the dual 408a's or single 408a? I finally found a couple tubes (I think) that sound pretty good and aren't too noisy for my dual 408 mic. Phillips miniwatts. Wondering about experimenting further.

 

[tomas.borgstrom]

Thomas Borgstrom,

Is the anode resistance adjustment you're describing referring to the dual 408a's or single 408a? I finally found a couple tubes (I think) that sound pretty good and aren't too noisy for my dual 408 mic. Phillips miniwatts. Wondering about experimenting further.

A single 408a. I only use that tube to make a replacement for VF14. I recommend building clones with a separate filament voltage for flexibility regarding tube choice.

 

[gyraf]

For reference,

Audio&Design Recording, Feb. 1980, pp. 93:

"..However, Gotham indicates that no VF14 has ever failed.."

https://worldradiohistory.com/hd2/I...X/IDX/80s/Recording-1980-02-OCR-Page-0091.pdf
/Jakob E.

 

[rock soderstrom]

For reference,

Audio&Design Recording, Feb. 1980, pp. 93:



https://worldradiohistory.com/hd2/I...X/IDX/80s/Recording-1980-02-OCR-Page-0091.pdf
/Jakob E.

Well, this statement was made in 1980, which is also a few years ago now! Vf14 are certainly very durable tubes but its long lifetime is also related to the starving heater and the small anode current/voltage in the U47.

At some point, however, even the toughest tubes are at the end - like everything else.

 

[bockaudio]

Good point, I think you are right. It seems that the MSC2 was exclusively designed for mic use, too. This is also stated in the data sheet.

I've edited the corresponding post.

As well Phillips EC1000: EC 1000, Tube EC1000; Röhre EC 1000 ID18565, Triode, vacuum | Radiomuseum.org

 

[rock soderstrom]

As well Phillips EC1000: EC 1000, Tube EC1000; Röhre EC 1000 ID18565, Triode, vacuum | Radiomuseum.org

This is debatable, as it says in the datasheet and also at Radiomuseum.org. "Special Quality Subminiature triode intended for HF measurement probes." That was the primary reason for development, unlike the AC701 and the MSC2.

What can't be denied is that this tube is very well suited for this job, not only because of the top pin grid, but also very promising in other respects. See the attached datasheet.

I think Neumann used them in the past, didn't they? Unfortunately, the tube is somewhat rare and correspondingly more expensive. Does anyone here have experience with this type?

 

[mrbutterskin]

I have a handful of EC1000s and have built an M49 type mic with it. It's an excellent tube with very low grid current (less than 100pA in that circuit) and relatively high transconductance even at low voltages/currents.
I've heard that it has a tendency to go noisy sooner than other tubes, so we'll see. I only have a few dozen hours on mine.
Neumann used it on early versions of the M149 mic. I heard they underheated it with 5V but can't corroborate that. That might account for the low lifetime though, if the cathode is more prone to poisoning from the underheating. I found no benefit to underheating so I'm running it at its' spec'd 6.3V

 

[bockaudio]

"This is debatable, as it says in the datasheet and also at Radiomuseum.org. "Special Quality Subminiature triode intended for HF measurement probes." That was the primary reason for development, unlike the AC701 and the MSC2."
Another word for "measurement probe" that would use a low noise active high Z element is "Electrometer", the other similar application to LF condenser mic.

 

[rock soderstrom]

"This is debatable, as it says in the datasheet and also at Radiomuseum.org. "Special Quality Subminiature triode intended for HF measurement probes." That was the primary reason for development, unlike the AC701 and the MSC2."
Another word for "measurement probe" that would use a low noise active high Z element is "Electrometer", the other similar application to LF condenser mic.

I know what you mean, but "similar" is not designed for a primary microphone use, which is mentioned in the datasheets for the AC701 and the Hiller tube.

Read the datasheets and judge for yourself, I don't want to follow this rather sematic discussion any further.

Based on the data sheet, the EC1000 tube is a very capable tube, that's all that matters to me, beyond the exclusivity of a tube designed for the microphone application.

What I'm more interested in is whether or not there is any truth to the rumour that it tends to get noisy after a while. I would be interested in your experience...

 

[Tubetec]

I knew Id seen this tube before somewhere and commented on it
Its used in a Tek CF probe ,

 

[PrivatCollector]

Just stumbled on this post, compliments! I really appreciate the efforts, valuable information!
Cheers
Roland

 

[Gus]

Microphonics in tube microphones make me think about maybe designing in some positive feedback in a microphone circuit

FWIW I read something years ago about the trainweck tube guitar amps circuits having the solid core? wires adjusted/dressed so the amps were on the verge of feeding back and this was part of the sound. I have no idea if this is true but it makes me think about a 70s strat I installed pickups in for a friend and the feedback from the large class 2 or 3 ceramic tone cap at high volume. We tried different caps(films etc.) and the ceramic sounded the best and caused feedback at lower volumes.