6072a Tube Microphone Design (C12 & ElaM inspired)

[Jazzy_Pidjay]

Hi everybody !

I'm finish drawing a 6072a tube microphone design.
http://img.photobucket.com/albums/v439/jazzy_pi/MicDesign.jpg

Basically it's :

- the Capsule polarization pattern of the C12
- the 6072a is biased like in the ElaM251e
- the Capsule is the one of the U87 (K67)
- Add some DC filtering
- Cinemag CM-2480 Output transformer

I think the mic will work good.
But i'm not sure of the bias resistor (250M) and grid resistor (33M) values.

* The 250M bias resistor come from the C12 schematic
(That's a 8M on the ElaM251e schematic but from the biased front electrode of the CK-12)
* The 33M grid to ground resistor come from the ElaM251e schematic
(250M+30M in the C12)

What do you think of this values ?

What improvements of the design you see ?

thank you very much :thumb:

 

[analag]

33Meg at the grid is too low, try 250Meg or more if you want good bass response. Place a resistor before the 1uF cap to increase the filtering action of the cap, usually 10K is good enough. I have discovered from studying old schemos, in simulation and real world tests that using a big coupling cap from the tube to the transformer actually degrade the low frequency, you will find 1uF to be much better. And I often use as much as 500pF to cut the high end. That's my take.

analag

 

[Jazzy_Pidjay]

Thanks ! :thumb:

A lot of good informations !

Can you explain a little bit more !!

-> Why increase grid resistor, provide better bass ? leakage ?

-> If i put a 10K on the B+ ramp, i loose some Volts since there's a current
flowing through it ?? no ?

-> The more surprising is about coupling cap !??? can you explain
i expected the exact opposite, bigger µ, better low freq ??!

-> Ok for the 500pF.... it's about the corner freq.
But i don't know how to calculate it, since the Ra is 25k
but Jakob calculate the corner for this tube with 50k ????

-> What do you think of an 1G for the capsule biasing ??
(i have it in stock, but no 250M...)
i can put 4 in parallel, but, 1G is good or not ? what.
I have read the many threads about the 1G grid resistor
of the G7, but not many talk about the 1G capsule biasing.

thanks for all !!

 

[analag]

50pF getting bypassed by 33Meg resistance start losing gain at about 200hz, by the time you get to 20hz you have lost 18db.

As I have learned from the gurus on this here forum setting your tube to draw 1ma or a little less is the best place to be in terms of noise performance and it works. So if you use that formula, 10k will not hurt your B+ in any significant way.

I notice smaller coupling caps give better bass when the transformer is plate connected, but bigger caps give better bass when a CF is used before the transformer. The effect is slight, but careful attention will prove this. It's the plate resistance, capacitor and transformer reaction causing this...I hope will come along to back it up or disprove this theory.

A 1gig grid resistor will work just fine...

 

[Jazzy_Pidjay]

50pF getting bypassed by 33Meg

50pF is the capsule capacitance ?

10k will not hurt your B+ in any significant way.

OK

when a CF is used before the transformer.

a CF ? filtering capacitor ? :roll:

A 1gig grid resistor will work just fine...

OK, but 1G for capsule biasing ? in place of the 250M of my schematic


thanks !

 

[Gus]

Think about a few things

core size and alloy

tube used

voltages.

 

[Kit]

a CF ? filtering capacitor?

Cathode follower, i think.

The smaller cap=better bass theory does not make any sense to me.
But maybe im missing something...

 

[analag]

I think a careful matching of the capacitor reactance and the transformer inductance is required for a flat response. Too little capacitance and the low frequency falls and too much capacitance brings it down a little but not as much as too little would. 1.0uF is a good compromise in my experiments which are based around the Cinemags. Notice the classic schemos the coupling cap values vary somewhere between 0.5uF and 1uF, I don't think this is a coincidence. I think this is based around trial and failure, calculation and testing. Not unavailability of bigger caps.
The Ela M 251 uses 3.2uF cap but this mic is often described as being bright sounding.
A microphone is a remarkable thing in that electronics, eletro-mechanics and mechanics are brought together to form the sound of a particular mic.
I have made a few usable mics, but not before I thoroughly studied the Meta...too much to learn.

analag

 

[Jazzy_Pidjay]

Think about a few things

core size and alloy

tube used

voltages.

Yes i think of that, in the limit of my knowledge.

Some peoples say the CM-2480 is little, but i don't know
if i need 20Hz response for vocals (99% of use)

I'm still trying to understand why a transformer can do
the job and why another can't. but can't find the informations

Seems that all the 6072a trafo are little... ?

1.0uF is a good compromise in my experiments which are based around the Cinemags

Good to know.

-> Anybody can give a link or a website where i can understand
the matching tube/transformer ?

-> 1G for capsule biasing ??

-> Why in some mics, there's a 10nF between the two electrode of the capsule ? advantage ?

thanks !

 

[burdij]

The Cinemag CM-2480 is a 10.5:1 transformer that is small enough to fit into the rather confined space in a C-12 body, has two bobbins so it inherently cancels stray fields and is recommended by Cinemag as a replacement for the the T14 type transformer used in the original.

The important concept when using a transformer to couple a vacuum tube plate to a low impedance output is to achieve an impedance on the transformer primary that approaches the the impedance of the tube plate paralleled with the impedance of the plate load resistor and series impedance of the power source. Triode plate resistance is significantly lower than a pentode would be. For the 6072a the plate resistance in a class a amplifier circuit is approximately 25K ohms and the plate load in the standard C-12 circuit is 100K. Calculating the total resistance the load would be about 20K. If the transformer is going to drive a 200 ohm output load multiplying by the square of the turns ratio gives about 22K, a fairly good match to the calculated plate resistance.

 

[Jazzy_Pidjay]

very usefull informations

i think i'll try the Tab Funkenwerk BV314 also
but i don't know ratio and specs.

anybody for the 10nF and 1G questions ?

 

[analag]

A 1uF coupling cap will give a better low frequency response due to a resonant peak close to 20hz because of the transformer