A tube mic finished (with photos)

[pasarski]

I finally finished the tube mic I've been working on for a while. It's a plate load circuit plagiarized from the G7 circuit, with a 5840 tube, 100K plate and 850 ohm cathode resistors (EDIT: B+ 130V, heater 6.5V). Capsule is a Thiercsh red line and the trafo OEP 1:6.45 backwards. Whole thing is built in a t.bone SCT700 (TCM 1050 type)  body and PSU is stock, slightly modified. There's nothing beautiful in the looks of it. I even had to use pliers to get the pattern selector to fit in the PSU, so no pictures attached!

I have a switch in the mic to choose between 100uF, 100nF bypass capacitors or unbypassed cathode resistor. 100nF was supposed to act as a kind of treble boost but I find it quite unusable. Maybe a little lower value for a "air boost" or a higher value for "anti-proximity effect" could be more usable. The other two choices are nice to have. I kind of like the sound of the unbypassed cathode resistor more, but the S/N ratio is not suitable for quiet sources without the bypass cap. The gain difference seems to be almost 10 dbs, which I find a little odd..

I made some experiments with the head basket, and am now settled for a two layer grill. It seems to emphasize the mid's a bit compared to one layer grill. I lowered the the plate where the capsule is mounted from the stock height and gained some extra air above 10khz or so. The rubber mount is the shortest one Thiersch offers. I'm still not sure if there actually was some useful sibilance rejection when the capsule was higher behind the ring, maybe gonna take a look at it sometimes. .

The sound is nice. I have only compared it to 414 XLII and Rode NT1a and it kicks their both a** big time, at least with my voice. Rode is brighter but smaller sounding, 414 is muddier and more nasal. Haven't had chance to test it yet with other than my own voice and room mic for drums but I have no doubts that it is going to be a very usable mic.

TO EVERYBODY WHO HELPED ME ALONG THE WAY AND HAD PATIENCE TO ANSWER EVEN THE STUPIDEST QUESTIONS, THANK YOU VERY MUCH!

 

[rodabod]

Photos or it didn't happen!

 

[gemini86]

Photos or it didn't happen!

+1

 

[tchgtr]

+2

 

[pasarski]

Ok, I'll take some shots next time I get to the studio, that's tuesday

Be prepared for some serious point to point jungle. I made a neat 3D layout in Sketchup but in reality it turned out to be a bloody mess.

EDIT: Something to wait for is also close ups of my head basket diffusor made of glue..

 

[pasarski]

Here is the ugly b*****d!

The jungle. It would have been prettier if I had only known that there's no use of star ground in tube mics..

Big stuff and THE switch. Could it be that the switch is making the mic noisier? I noticed a hiss today I haven't noticed earlier..

There's actually something beautiful in the mic (and I don't mean the glue diffusor)

Little bit ruff. Do you like the stickers?

 

[baadc0de]

I don't get you man, it looks beautiful to me. Maybe if you're a Karhu man, you suddenly get higher standards than some of us ESB lovers   Seriously, it looks grand. Good work

EDIT: and yes, I dig the stickers

 

[pasarski]

Thanks man ! It's mostly the mutilated PSU and the scrappy point to point work I'm not so proud of. The paint job is ok, though it could have been better, but I was little inpatient and sprayed too much on the first layer.

 

[tchgtr]

Certainly you know by now that all we care about is bare-naked microphones. This is the internet, after all.
Looks great to me, and let us know if you discover that the switch is causing noise.

 

[okgb]

mainly counts that it works and you enjoy it
good job  !

 

[Skylar]

I was surprised and a little disappointed to find out that Karhu III is a pale lager when the container clearly looks like it should hold a stout.

What are your B+ and capsule polarization voltages ?

 

[pasarski]

I was surprised and a little disappointed to find out that Karhu III is a pale lager when the container clearly looks like it should hold a stout.

What are your B+ and capsule polarization voltages ?

It's lager indeed. I love these pint sized cans..

B+ is 130V. 65V on the capsule.

 

[pasarski]

It was a 1 gig resistor touching the body of the mic causing the hiss. So the switch is ok. Though after second listen the unbypassed cathode resistor sounds thin and middley, rolled off lows. Maybe the output impedance of the tube goes too high? Anyway, not much use for the switch.

Got a chance to record a female rapper, sound was very very good, I'm happy.

Time to move on to the next project. My Tim Campbell CT12 capsule should arrive next week, yeehaw!