MK7 - tube mic project

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ioaudio

Well-known member
Joined
May 11, 2005
Messages
2,087
Location
vienna/austria
as some of you might know, i was lucky enough to find a bunch of transformer lams originating from the seventies, the stuff used for the transformers of the classic neumann tube microphones.
i wound a couple of BV8 copies for U47 (clones), for people who run the transformer with the rare VF14 (EF14).
meanwhile i was working on a new headamp, using a tube which is more commonly available, the ef86 (pf86),  rewinding the transformers primary (and secondary) while varying the stack-height, setting the bias, the value of the couling cap etc to "emulate" the sonic qualities of the original u47 headamp.

i designed a pcb layout to fit many mic housings (i used the cheapest one from thomann link , a nice 3-layer mesh grille+psu & cable & shock mount)
edit: re: housing: disclaimer: this is not a beautiful mic.

the top pcb connects to the main pcb and will be soldered together.
pcb height is 74mm (+ height of the tube socket&parts), diameter >=45mm

the entire high Z part is build point-to-point upon the top pcb, polarisation voltages are fed through glass tubes. the tube socket is a hardwired type to ensure high insulation for the grid connection.

the "stiffened" biasing is done with the heater supply.
0,22F goldcaps are used for additional filtering along with 2,2R* resistor.
the 1µF and 0,68µF filmcaps are mounted on the back at the pcb side under the tube.

mk7_pcb_protobuild.jpg



MK7 microphone schematics with measured voltages for both the ef86 and pf86:
(click pic to enlarge)
thanks to axel/mad.ax for finding - pin number (g2/g3--pin8/pin1) error fixed.


preliminary checklist:
[some rs-part numbers]

1 x cheapo mic or mic housing + psu (used mic in the pics = http://www.thomann.de/at/the_tbone_sct700_roehrenmikrofon.htm)
1 x M7 or K47 capsule + holder
1 x ioaudio MK7 pcb
1 x ioaudio BV7 transformer

1 x ef86 or pf86
1 x noval tube socket chassis mount 18mm mounting hole
2 x M3 (4-40) screws/nuts to mount tube socket

1 x 1µF 250V filmcap tubular [117-987]
1 x 0,33µF 250V filmcap tubular - (0,47µF or 0,68µF, coupling cap)
1 x 47nF 160V film
1 x 10nF 160V styroflex
1 x 1nF 160V styroflex

2 x goldcap 0,22F 5,5V - upright standing version [377-372]

2 x 100 Megaohm 0,25W  [247-7834]
1 x 68 Megaohm 0,25W  [247-7806]
1 x 33 Megaohm 0,25W  
1 x 1 Megaohm 0,25W *updated, marked as 1,5Mohm on the pcb
1 x 910 Kiloohm 0,25W *updated, marked as 1,2Mohm on the pcb
1 x 100 Kiloohm  - plate resistor
1 x 20 Kilohm 1W
1 x 2,2 Kiloohm 0,25W
1 x 6,8 or 4,7 OHM 1W (marked 3,9R*)
1 x 2,2 Ohm 1W

MK7 PSU

a psu from scratch, all passive.
adjust voltages on test.
suggested choke for usa = triad C-40X @ alliedelec.com




Pattern selector :
1 x rotary switch 3x1 (lorlin or similar)
1 x 10µF 250V electrolyte
3 x 100 Kiloohm 0,5W
1 x 51 Kiloohm 0,5W

PSU mod:
*note that some users (mostly form the US) reported that this mod does not work completely hum-free when used in fixed bias.
the result seems to depend on mains voltage and the varying quality of the transformer in the original psu.
schematics for modifying the stock generic psu, again with measured voltages for both the ef86 and pf86, with the psu from sct700

preliminary checklist:

Heater upgrade:
2 x 10.000µF/10V
2 x 4.700µF/16V
1 x 22µF/16V
1 x 2,2 Ohm 1W
1 x 3,3 Ohm 1W
1 x 240 Ohm 0,25W
1 x 910 Ohm 0,25W
1 x trim pot 500 Ohm
2 x 1N4002
1 x LM317

--------------------------------------
thanks to greg and brad for the bom&word document:

http://www.twin-x.com/groupdiy/albums/userpics/MK7_max_s_mic.xls

http://www.twin-x.com/groupdiy/albums/userpics/Max_s_Mic_thread.rtf




IMPORTANT:

do not un-mount the transformer, do not tighten/loosen it´s screws/nuts.
be careful when soldering the wires to A+, it´s a bit tight there.

get rid of the stock innards.

solder a 10cm wire for the pattern on pin 3 of the xlr, use insulation


mount the rectangular pcb

insert the round pcb into the slot

check for 90° angle and for the correct position

solder one rectangular pin on both pcbs

check position & angle again, re-heat the soldered pin and adjust if nescessary

solder all pins

insert the glass tubes and fix them with a drop of glue

mount the chassis-tube socket, insert the solder lugs on top.

check for a good electrical connection from the pcb to the socket, use a tooth lock washer.

solder the parts on the rectangular pcb, check polarisation of the gold-/super-caps.
the 100M resistors are fed through the glasstubes as shown.

on the round pcb, use the solder lugs as ground connections for the 68Mohm, 3,9R* and 10nF instead of the associated holes in the pcb.
in this mic housing this additional clearance is needed to fit the slots.


alternative self bias wiring





mk7_pcb_protobuild1.jpg

mk7_pcb_protobuild2.jpg

mk7_pcb_protobuild_dale_m7_2.jpg


 
Tooooh....
Hey RedNoise , don't ask him for two kits...

Hi IOA !!!!
Two kits here please!!!!Ahahhahahahahahaahaaha!!!
 
No cap to ground at the 100K and 20K node in the plate/anode leg?

I like the use of the super caps in the extra RC after the stock supply voltage.

Like the fixed bias via the 3.9 ohm.

It looks like it sounds good. FWIW I have a circuit I like in one of my nady 1050 bodys and the linked microphone looks like a 1050.
 
[quote author="ioaudio"]more commonly available, the ef86 (pf86)
[/quote]
I look in my drawer, and it's not available...
:mad:
G7 uses ef86 too.. Is there a reason for that?
I got a bunch of 6ak5, would it work in a similar design?
:roll:
 
ioaudio just gave away a very good DESIGN IMO. That design with the right capsule grill looks like it be as good or better that some very expensive microphones. I have built microphones with the tweaks he has drawn and when you put them together the right way microphones can sound really good. A few things I would add but it looks like a very good design to me.


Being a "complete design" even down to the custom transformer, recommended microphone body and recommended capsule.
It will not work with a tube sub (part of the beauty of this design)without the circuit being redesigned a little. IMO It is one of the just enough parts and put together the right way circuit that every part and type/tube number was thought about, nice little things in that circuit.

I have built a tube circuit in a 1050 body that sounds very good and the linked microphone looks like the same body I would need to measure to be sure.

I can't wait until people post about wanting to remove mesh and stuff like that.
 
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