MK7 - tube mic project

[EEMO1]

im prob. looking at finishing this december. I finished the PSU last weekend. didn't explode on me so...

just need to measure/adjust the voltages, mod the capsule mount a bit... small things etc.

 

[roy]

how would a K87 capsule work in this?


-Roy

 

[ioaudio]

rather bright. the k87 has a build-in treble boost which is corrected in the u67/u87 amplifiers.
just an idea, try 33 or 47nF in series with ~1,2kOhm (adjust) across the transformers secondary (output). the result will relate somewhat on the receiving impedance of the mic preamp but it could work.

 

[mata_haze]

Hello Everybody,
I have just finished my MK7 microphone using all the suggested stuff from ioaudio:
-t-bone microphone+ psu
-ioaudio MK7 KIT
-Dale's capsule

the microphone sounds very good I really like the texture and body of it BUT:
I have a really high hiss and low gain which in conjunction makes it unusable for vocals and low level applications.
On the other end I have nearly no Hum.
I am using a Mullar EF86 which is supposed to be really quiet and the Psu is the T-bone modified one (following ioaudio description).

Question:
-is it normal to have an high level hiss on this mic (along with low level) or is there something I am doing wrong?
-is it worth upgrading the PSU and build a bigger one?

- I have checked the static voltages and they are all in within 3-5 %...

any thoughts?

Best,
Mattia.


PICTURES:

 

[Kingston]

Your tube is a bad. garbage.

You can't just buy one tube for this very critical high impedance fixed bias application. You buy 5 or more and hope that one of them is decent. I hope you didn't pay a stupid price just for the "mullard" name. There are EF86 mullards by like 4 different factories from like 3 countries. very different tubes, and these days probably all rejects by guitar amp enthusiasts. ie. garbage.

Just get a misc. selection of EF86 or PF86 tubes by various manufacturers from ebay, and cross your fingers. there's no other way these days. Don't be daft and get tricked by the tube salesmen pitching some brand with "good reputation".

 

[ioaudio]

the insulation between the backplate/capsule mount to ground is crucial.
to check this disconnect the capsule and place a small cap (1nF) between the connections to backplate and frontside.
if hiss is still the same as with the capsule try another tube.

 

[mata_haze]

Your tube is a bad. garbage.

You can't just buy one tube for this very critical high impedance fixed bias application. You buy 5 or more and hope that one of them is decent. I hope you didn't pay a stupid price just for the "mullard" name. There are EF86 mullards by like 4 different factories from like 3 countries. very different tubes, and these days probably all rejects by guitar amp enthusiasts. ie. garbage.

Just get a misc. selection of EF86 or PF86 tubes by various manufacturers from ebay, and cross your fingers. there's no other way these days. Don't be daft and get tricked by the tube salesmen pitching some brand with "good reputation".

thank you for the suggestion. I have actually paid good money for the tube...

the insulation between the backplate/capsule mount to ground is crucial.
to check this disconnect the capsule and place a small cap (1nF) between the connections to backplate and frontside.
if hiss is still the same as with the capsule try another tube.

ioaudio. if I disconnect the capsule there is already a 1nF 160V cap between back and front panel connections. do you want me to leave it on?
also the third connection on the capsule that goes doen to the 100M, 10nF should I leave that one floating?

thanks  guys,
Mattia.

 

[ioaudio]

throw the mic as is in your closet, record the hiss level.
dont change your preamp gain, connect the small cap instead of the capsule.
to do this, disconnect all three wires coming from the capsule on the pcb.
connect the testing cap on the points marked on the pcb as " capsule backplate" and "capsule front".
leave everything else as is and record again. compare the hiss level.
or, if you still have it , you can alternatively test your mic with the original capsule + mount.

good luck,
-max

 

[mata_haze]

Hello,
I have conducted the test, and the microphone is much more silent without the capsule connected.
I have posted the two audio files as well as some pictures of the mic itself on the capsule side.
there is still some hum, but I think I can get that fixed. the only thing I cannot quite grasp is the hiss...


http://www.angelswings.it/MK7-Capsule.mp3

http://www.angelswings.it/MK7_no%20capsule.mp3

 

[Gus]

As Max posted the capsule mount insulation is critical for the proper operation of the microphone.  It looks like all you have is a sheet of black rubber as insulation.  If you search here you will read posts about this issue.  Basically you need no leakage paths that "bleed off" the charge from the capsule, the rubber is most likely the cause yes the brass to the rubber to the metal frame can be a leakage path.  Remember the grid to ground in microphones can be 8 meg to >1,000 meg.  I would try a piece of plastic like Lexan or Plexiglas for the rubber sheet.  I sometimes do not use a shock mount capsule if the microphone will be in a shock mount.

 

[pH]

Congrats on finishing your mic Mattia! You're gonna love it.

If you read my earlier posts, I had trouble with a couple of things using the T-Bone body. Hum from the undersized power transformer made me go with self-bias (until I finish the upgraded PSU), and I had a hell of a time with capsule leakage. Your problem could also still be tube related - I recently came across a NOS Blackburn Mullard that I popped into the mic.....hissssss beyond belief, it was a defective reject. EF86 tube are tough that way, best to have several on hand.

-Paul

 

[ioaudio]

...and the microphone is much more silent without the capsule connected...

ok, so we found out that the source of the hiss is either the capsule or it´s mount.
the rubber looks like it contains carbon, which i only guessed from the first pictures, so try what gus suggested and let us know.

-max

 

[Emperor-TK]

A thread with details of my finished MK7 build:

http://www.groupdiy.com/index.php?topic=36857.0

 

[seavote]

this is probably a no brainer but i still dont have much confidence in my electronic knowledge so i have to ask. the 22ohm 5watt pot is for fine tuning the heater voltage,yes? if so builders are placing it inside their project cases as they would a trim pot.correct?
also submitted for discussion: has anyone experimented with slight underheating of the tube with this circuit. seems to easy(with the pot) not to try. anyone try it and have an opinion?
ps:i seem to have lost the tiny glass tubing that comes with the pcb. where would i start to look for that sort of item. im thinking maybe glass beads from a crafts store?

 

[ioaudio]

>the 22ohm 5watt pot is for fine tuning the heater voltage,yes?

yes. fine tune the voltage with a load attached.

>if so builders are placing it inside their project cases as they would a trim pot.correct?

i dont really get you here - the trim is placed inside the PSU housing if thats answers your question.

>also submitted for discussion: has anyone experimented with slight underheating of the tube with this circuit.

look at the schematics and the voltages, it´s already done.

>ps:i seem to have lost the tiny glass tubing that comes with the pcb.

i can send you some more, send me an email.

-max

 

[seavote]

Thanks Max.  as per the trimmer ,yes that answers my question.since i haven't gotten to the "check voltages stage" of this project  i failed to notice the underheated cathode. as for your generous offer to send me some glass tubing,i'll see if i can find some here to save you the trouble but will definitly take you up on your offer if my attempts are unsuccesful. thanks for a great project.

 

[mata_haze]

GOT IT WORKING!
after I have tried to isolta the capsule from the base I went into re-building the PSU.
new transformers for heater and B+ new stabilizer for the B+ and regulated 317 for heater and it is now fantastic!
virtually no hum ( need to put a plate between the trafos andd the psu boards) no hiss and AMAZING sound.
this microphone really is good!
thanks IOAUDIO for the project + pcbs and all the people here for the support!
I can see a lot of interesting recordings ahead!

Best,
M.

 

[ioaudio]

great!

to those pming me, i will soon have some news regarding the transformers/sets.

 

[EEMO1]

Fired mine the first time... No sound! Wtf.... Voltages are good coming out of the tbone modded psu. Oh crap.


edit. could it be something really stupid like not connecting the transformer lams to ground?  :-[

 

[ioaudio]

hi,

no sound at all? no "pff" ?
voltages on the tube are correct? checked all parts? the wiring including 7 pin xlr ? close up pics of your build could help.

measure the DC resistance of the output XLR pin 2 and 3 with the mic connected.
check all the wiring stuff twice - follow the schematic, measure voltages.

edit: edit.
-max