DIY Tube Mic project

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Just bought a second-hand SC440 USB for 35€... I don'y care about the connector 'cause I'm gonna fix the cable inside
the mic.
Must now order the NTE10/3 at Banzai > I've got the rest of components needed...
Let's go on this excinting first Tube Mic project !
:)

One more point : I supply the front diaphragm with +60v and get the signal from the backplate, isn't it ?
 
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Let's go on this excinting first Tube Mic project !
:)

A kind of final correction. The E-352 CRD is nice, but really hard to get.

So I changed the anode load to a pair of PNP & pair of resistors as options. Yes, I know - 4 parts instead of 1 part, but the parts are as generic as it gets, so it should be much easier to get parts. As for E-352, if you got 'em smoke 'em...

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Tube current is ~ 3.5mA, a lot higher than original SELA T12 (~0.3mA), as is the anode voltage at 60V compared to around 30V . So we can handle more signal and drive more difficult loads. Compared to the original HD is also reduced.

Gain with the NTE10/3 wired as 10:1 (20k : 200R) will be around 1.5dB, so in effect this microphone has no gain. It will be "quieter" than many current microphones. Frequency response will be largely down to the capsule, the NTE10/3 reportedly is flat enough at 20Hz & 20kHz to not be an issue.

With K67 (copy) Capsule at 60V Bias we expect appx. -30dB or 31.6mV @ 94dB input which will give arounds -63dB H2.

For 316mV (-10dB / 114dB SPL) input we get around -43dB H2 with H3 ~-80dB and H4 -100dB.

For 1V (0dB / 124dB SPL) input we get around -35dB H2 and are at the edge of clipping.

I think 124dB is good enough for many close miked natural instruments and vocals. Trained Opera singers at full steam don't really get over 120dB @ 6" distance and that's bad for their hearing!

If absolutely essential, a capacitive pad can be added to handle more SPL.

For low frequencies the transformer saturation will limit maximum SPL for a given HD. I would expect to have few issues with 0dB/120Hz, at 60Hz we will already see significant distortion and at 30Hz we really only can handle -10dB or so before exceeding 1% THD from transformer (usually odd harmonics).

So not the best choice for percussion and large drums. Get an AKG D12 (contemporary/Jazz) or D112 (Rock to heavy metal, punk etc) for the kickdrum and try a Sennheiser 421 on the snare, SM57 on tom's! Not every instrument sound best with an LDCM, especially if the drummer channels the Animal from the Muppet Show!

The +B (HT - High tension) should be around 90V and well filtered. With 1K/1,000uF it will be extreme filtering, nerarly 60dB killed at 120Hz and the DC-DC convertor recommended run above 20khz.

Around 90V produces 60V bias for the capsule. Bias can be adjusted changing the 2.2MOhm resistor R8, according to ohms law.

For the tube heater 5.5V...6.3V are recommended. At 5.5V the tube have a little less gain and transconductance and a little higher anode impedance, but aging will be less pronounced and noise lower. Perhaps more heater voltage will sound a little more alive. Experiment.

Simulated SNR is 64dB unweighted 20-20k, with A-weighting we likely get a bit more, how much, good question.

So that would be my recommendation to aspiring mike-builders that want to try their hand at "building a toobz mike, glassic style, like".

Thor
 
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Well...
I ordered E-352 at Ali Express : we'll see what we get !

Hum... internet tells me that 2N5551 is a NPN... ???

You said : For the tube heater 5.5V...6.3V are recommended. At 5.5V the tube have a little less gain and transconductance and a little higher anode impedance, but aging will be less pronounced and noise lower. Perhaps more heater voltage will sound a little more alive. Experiment.
Yes I will experiment : I will use my 12v SMPS so I have some room

For HT : I planned RC 620Ω + 1000uF...
 

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Hi
I've found a right aluminium enclosure for the tube mic PSU. It was first dedicated for a Mac power supply but no more need so I'm glad to to find it a new utility.
Not too much space but it (just) fits. Here a view with the Tr 2*18v & the SMPS 12v and the PSU board to see how to place elements. It will be fine (y)
Working progress...
 

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PSU is done and has been under "burning test" for several hours without any problem : no overheat nor smoke
As I previously annonced :
- HV is provide by a 2*18v doubled and regulated by LM317 (with 3,8mA load)
- Heater by a SMPS 12v droped down to 6v LM317 (with 180mA load)
Seems to be fine :)

* 6AK5 recieved in perfect mint condition (NOS from ebay) and missing components oredered.
* t.bone SC440 USB bought (second-hand for 30€) > delivery next thursday
to be continued...
 

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Seems to be fine :)

* 6AK5 recieved in perfect mint condition (NOS from ebay) and missing components oredered.
Well done @Emmathom, build is going well. Im counting on your success.

@Tubetec :"A pencil mic version might be do-able with a submini and a suitable donor body/capsule."

I was just thinking that. I just need to work a way to fix the capsule to body. 22MM Ø.;)
M
 

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Out of topic : I'm not gonna open a new thread just to say that I'm about to finish a Quad AOP for a customer (4 channels with OPA134 gain + NE5532 symetry) that I just tested (1 channel on 4 for now) : no hum nor noise / 14dB de gain by OEP Tr In + 37dB gain by OAP circuit. Just the minimum controls : a gain pot. + a 48v switch. No pad, no phase inverter, no filter... The simplest the better (for me).
Great sounding with transparency (what my customer asked for)


Now get back to the Tube mic project.
I'm gonna redo the HV part of the PSU because LM317 was not a great choice : I'm gonna use a Mosfet driven by a jfet.
The heater PSU stays as it is.
Mic body + Neutrik NTE10/3 supposed to be delivered this week.
To be continued...
 

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Hi there !
Micro t.bone SC440 USB received and immediately disassembled.
It's got an electret capsule of no interest at all. Original circuit is removed as well as the USB connector.
There's enough space to install the tube and the few components. I think the t.bone logo should be wiped off with some "nail polish remover" from my wify.

I will now cut the printed circuit board to the appropriate size and attach the tube support (as soon as I receive it).
Picts soon.

The PSU is functional and delivers 6.3v•180mA and 91v@4mA (still ok after several hours of burning test)
To be continued...
 
Wait, what?

What's the 91 V for if it's an electret? I'd expect electrets to need max. 10 V. Interesting PSU for diy. Is it clean?
 
The electret caps. isn't used for this project but a K67 and caps. voltage is reduced to 60v by a divider

About the PSU : well for some time now I use SMPS for heater. Small, no heat, 1,3A at min. current. 1,3A holds 2 tubes (300mA) but not 4 (because of the "current call" of cold heater).
And since I had a 2*18v in stock, I decided to combine the 2 on the same board.
The 2*18v AC when doubled and rectified (thru 2*560uF) gives me 130v under a 4mA load juste after the bridge.
 

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Ok.
I "cut" 2 cards: a main card and a tube socket card.
It barely fits but it fits correctly.
It's not "clean & beautiful" but for a first run it'll do the job...

the main card is moved back from the chassis using 2 small 1.5mm plastic spacers so the tube slides around perfectly.
the tube socket card is attached to the main card tby 4 pieces of rigid wire soldered and I can say it is well maintained
I'm now waiting for the tube socket to be delivered...
 

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So that would be my recommendation to aspiring mike-builders that want to try their hand at "building a toobz mike, glassic style, like".
Many thanks for your ideas, infos and the design! I think I will also build the modified SELA T12. Simply because I'm interested in how the sound and the technical characteristics will change as a result. As I have already built several T12s, I find this evolution exciting. :cool: Due to the low complexity of the original circuit, it is easy to understand what modifications of the components and circuit really change.

I would therefore be pleased if you could say a sentence or two about the mods and the tricks mentioned. I want to understand what I am building or at least be able to understand what your motivation was for the respective changes.

First, of course, replacing the anode resistor with the "Current Regulator Diode" (or its equivalent circuit). What are the benefits?

I already have an idea, but I would still be interested in your perspective.
Extra trick, connect the transformer primary to the cathode, not ground.
You didn't use this in the “final” circuit, but I've seen something similar before. Why do you do this, what are the advantages of this trick?
Further trick, use the heater voltage to bias the Cathode, perhaps via a divider.
The same here. This is also known from some other microphone circuits. But what is the concrete advantage of biasing the tube in this way? It increases the demands on the quality of the heater voltage, but this can be easily managed if there are advantages elsewhere.
With the suggested 90V HT the tube will draw around 3.5mA and total gain with the Transformer accounted for will be in the region of -3dB, so it's not a very "LOUD" microphone.
In my experience, tubes sound somewhat better when a little more current flows, but tubes in microphone headamps are very often operated with rather low currents (<1mA), as one would probably like to avoid current noise at this delicate point. Are you not afraid that the more than 3.5x current flow will be detrimental to the noise floor of the circuit? I'm curious to see what difference this makes in reality.



PS: If you are wondering why your mods/design generates so little resonance here, that's easy to explain. Important keywords are missing in the thread title. Try Neumann, Neumann, Neumann and AKG and invent a completely implausible background story with maximum WWII and "Braunbuch" references, then there will be more interest here.😂
 
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First, of course, replacing the anode resistor with the "Current Regulator Diode" (or its equivalent circuit). What are the benefits?

With 100k & 60V +B the tubes anode current is ephermal (~250uA / 0.25mA), anode impedance is high (> 20kOhm), overload is early (only 35V across the tube) and the tube operates in a very curved part of the plate curves.

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A CRD is actually a J-Fet (plus resistor in some cases) and it tends to have a near infinite or even negative impedance.

1717409799497.png

The E352 is a CRD in a classic wire ended glass case rated at 100V maximum Voltage and 3500uA (3.5mA) now in production with Semitech and normally stocked by Mouser.

1717409847320.png

The E352 with 30V across it actually passes around 3mA and has around 30kOhm NEGATIVE impedance, meaning as the voltage across the E352 increases the current drops (and visa versa) acting as positive feedback to plate, improving linearity and gain over a pure CCS.

That said, the effect is quite mild but present, so ideally an E352 CRD is used and not a BJT CCS.

With 1.2V bias & 3mA the EF95/6AK5 et al biases at around 60V and has a relatively low anode impedance (~5kOhm) a gain (Mu) of around 27 with 240R cathode resistor giving an effective output Impedance at the Anode of appx. 12.5kOhm.

60V across the 6AK5 and 30V across the CRD require 90V +B.

Our maximum signal swing on the grid will be 2V PP (~0.7V RMS) with appx. 55V PP available at the anode and 5.5V PP (~2V RMS) maximum output, much larger than the original design, despite the original using a 6:1 stepdown instead of 10:1.

There is no reliable DCR rata on NTE10/3, let's presume 1K on the "10" side, so the output impedance becomes around 150...200 Ohm and the gain from Capsule to Output around a factor of 2.7, load dependent.

The 240R cathode resistor is not sufficient to bias the tube, this means that initially the tube will draw grid current through the Microphone Capsule's capacitance, which will store the charge from the grid current until grid current stops, at around -0.5V (0.72V across the cathode resistor).

It is possible to short the 240R resistor out and/or lower it for lower impedance, without some resistance there is a possibility that bias will not start up correctly and the 6AK5 ends up in saturation acting as diode.

You didn't use this in the “final” circuit, but I've seen something similar before. Why do you do this, what are the advantages of this trick?

It creates a bass boost, which may be desirable or not. Without reliable detained data on the NTE 10/3 I felt it better to avoid this.

thor.zmt said:
Further trick, use the heater voltage to bias the Cathode, perhaps via a divider.
The same here. This is also known from some other microphone circuits. But what is the concrete advantage of biasing the tube in this way? It increases the demands on the quality of the heater voltage, but this can be easily managed if there are advantages elsewhere.

We want 1.2V bias and have (say) 180mA heater current.

This means a 6.8R resistor will bias the tube correctly without introducing degeneration.

It means we make our heater voltage 6.3+1.2V = 7.5V and make sure it's very quiet.

In my experience, tubes sound somewhat better when a little more current flows, but tubes in microphone headamps are very often operated with rather low currents (<1mA), as one would probably like to avoid current noise at this delicate point. Are you not afraid that the more than 3.5x current flow will be detrimental to the noise floor of the circuit?

Current is less relevant to noise than voltage.

The Lomo 19A-9 Mic used the soviet 6AK5 copy with 1.5mA+ current, 20k anode load and 90V+B.

Two sources for actual noise (not hum or microphonics) in tubes.

(1) There is inherent random noise from the flow (modulated by space charge) of discrete electrons.

The textbooks tell us that the equivalent noise resistance (a fictional resistor in series with the grid) decreases with transconductance:

RN = 2.5/gm, approximately, for a triode, in the audio range.

(2) There is also "excess noise" due to strange effects on the cathode surface, which are not so predictable, but the spectral density goes as 1/f.

It is also called "flicker noise" or "pink noise".

At higher frequencies, there is also induced grid current noise, due to impulsive effects from the electrons speeding past the grid wires.

There are possible defects in individual tubes, due to construction or deposition of unwanted material in annoying locations, over and above the inherent noise.

Noise (2) scales with cathode emission. Lowering heater voltage reduces this type of noise, as long as we do not exploit the tube near rated cathode current.

Noise (1) scales with Transconductance, which is lower at high voltage/low current operation and higher at higher current/low voltage.

A slightly more exhaustive and more math heavy discussion of tube noise plus actual data may be observed here:

Principles of Electron Tubes - Chapter 13 - Noise

There is also noise in gassy tubes, all older tubes that have been on the shelf a few years or decades should be baked to reactivate the gettering and capture the various molecules that leaked in over the years, do that before ever applying power to the tube.

Taking tubes out of the box after decades of storage and turning them on without first thermal treatment can permanently damage the cathode coating and result in noisy tubes.

I'm curious to see what difference this makes in reality.

Let's try. It will be easy to swap the CRD for a 100K resistor, 240R to 510R and drop the +B to 60V for @Emmathom .

PS: If you are wondering why your mods/design generates so little resonance here, that's easy to explain. Important keywords are missing in the thread title. Try Neumann, Neumann, Neumann and AKG and invent a completely implausible background story with maximum WWII and "Braunbuch" references, then there will be more interest here.😂

I am not doing this as a popularity contest.

And yes, I know actually a lot about all this. In former East Germany we still had System Eckmiller Coaxial Speakers in studio service in the 1980's, that originally had been built by Konskie & Krueger in the same factory that made the Enigma Cypher Machines for Wehrmacht, Luftwaffe and Kriegsmarine...



From the mist, a shape, a ship, is taking form
And the silence of the sea is about to drift into a storm

Sign of power, show of force
Raise the anchor, battleship's plotting its course

Pride of a nation, a beast made of steel
Bismarck in motion, king of the ocean

He was made to rule the waves across the seven seas
To lead the war machine

To rule the waves and lead the Kriegsmarine
The terror of the seas

The Bismarck and the Kriegsmarine

Plus we had Microtech Gefell (Neumann Ost) where they made microphones as it was still 1943. And yes, we had STGW 44 in reserves for the Paramilitary and I got to train with it and like it better than AK-47.

But that's like so much from the ancient days of 2019 BC (before covid) are lost in the mist of times.

Enough namedropping?

Thor
 
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We want 1.2V bias and have (say) 180mA heater current.

This means a 6.8R resistor will bias the tube correctly without introducing degeneration.

It means we make our heater voltage 6.3+1.2V = 7.5V and make sure it's very quiet.
What's the best solution ? a divider with 7,5v + 6,8Ω or a cathode R 240Ω ?
If divider is this schem. correct ?
 

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Hey Thor, thanks for the detailed explanations, I'm still chewing on it. (y)

I will build the mic circuit with CRD, I will order them today. The comparison will be interesting .
I am not doing this as a popularity contest.
I know I was just kidding around, but the background is real. Unfortunately, new designs that have nothing to do with vintage Neumann and Co have proven to have little resonance here.
Kriegsmarine
...and Sabaton😅 God damn, that's really bad! LOL. Is that the Swedish Ramstein?:ROFLMAO: # me speaks with AH voice: "Kriegsmarine"

An amazing amount of money and work went into the entire production of this piece of art. It's amazing that there is such a market for it, the Third Reich and the Kriegsmarine are still selling well. My suggested names for a mic for this target market would be U88 or Seelenfänger. 😅😅
 
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What's the best solution ? a divider with 7,5v + 6,8Ω or a cathode R 240Ω ?
If divider is this schem. correct ?

I think your math is right, beside that the heater current for the 5654/6AK5w is 175mA@6,3V

Just an another example, this is how Neumann did it in the U67. Kathode is grounded, heater voltage negative.
1000031433.jpg
Your example is more U47 style without the gigantic series resistor...
1000031434.jpg
 
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