MK7 - tube mic project

[plumsolly]

While I was assembling my Mk7 amp today I managed to get some super glue inside two of the glass tubes. I was able to get the tubes clear, but will this compromise the isolation they are supposed to provide? Thanks, Ben

 

[ioaudio]

shouldnt be a problem.
-max

 

[plumsolly]

I just finished building my power supply and I have been checking my voltages unloaded. I am seeing ~12v for the heater. I know that 317-regulated power supplies (like the g7 ps) put out pretty close to the loaded voltage, unloaded. Does the nature of the passive ps mean that loading will have more of an impact on the voltages? I tried loading it with ~350 ohms and it didn't make much difference. My heater trim only adjusts about a volt. I am using a 10ohm pot in series with a 10ohm resistor because Mouser didn't have the 20ohm pot in stock. Thanks, Ben

 

[zebra50]

Does the nature of the passive ps mean that loading will have more of an impact on the voltages? I tried loading it with ~350 ohms and it didn't make much difference.

Your thinking is right, but your load is wrong (too big). Where did 350 come from? Did you measure the tube filament when cold?

Check the datasheet for the tube you're using. Most of those little tubes run at something around 300 mA and 6.3 V. EF86 is 200 ma .

R = V/I = 6.3/0.2, so you need about 31.5 ohms (33R) and a couple of watts to be safe. Or just use a tube! If you have 12 V unloaded you should be in the right ball park.

 

[plumsolly]

Your thinking is right, but your load is wrong (too big). Where did 350 come from?
Check the datasheet for the tube you're using. Most of those little tubes run at something around 300 mA and 6.3 V. EF86 is 200 ma .

R = V/I = 6.3/0.2, so you need about 31.5 ohms (33R) and a couple of watts to be safe. Or just use a tube! If you have 12 V unloaded you should be in the right ball park.

Thanks Zebra. I calculated that 31.5 ohms, I just didn't have any resistors around that were close to that value with high enough wattage. I was also unsure whether a purely resistive load is a good enough approximation to test for the correct voltage? But it sounds like I am close enough to try it with a tube and not damage anything.

Thanks again, Ben

 

[ioaudio]

I calculated that 31.5 ohms, I just didn't have any resistors around that were close to that value with high enough wattage.

so you took a resistor with factor of 11 
purely resistive is ok.


http://www.twin-x.com/groupdiy/albums/userpics/MK7_psu.JPG

we want 7,6V @ 180mA at the output of the psu which means we need a load resistor of 42,2 ohm

-max

 

[zebra50]

we want 7,6V @ 180mA at the output of the psu

Good point - i'd forgotten that this design has fixed bias and some series resistors in line with the heater inside the mic.

Anyway, it's a nice design and you should have a great sounding mic!

 

[EEMO1]

aaargh, fired the mic up again today after rebuilding the passive psu. still no frikkin sound.

well almost, if I boost the preamp I can hear me scrape the grill and tube noise comes up too. I guess it's positive that I have no hum. hehe.

 

[Skylar]

I had a similar problem with one of my builds which turned out to be a bad output capacitor.

 

[buschfsu]

i just picked up 12 Russion PIO caps (silver like skylar reviewed).  PM me if anyone needs one.

 

[buschfsu]

got your pms guys.  there not here yet but when they arrive ill respond.  they are the 0.51ufs that skylar wrote about a few posts ago.

 

[julien31]

Hello everybody, I finished a MK7 mic with self bias, Tbone SCT700 body and a Thiersch blue capsule.
It sounds fantastic, with a lot of details and a big low-mid but it lacks hi-end (10khz and higher like a low pass). I compared it with a lot of recent mics I use on vocals (SM7, 414, U87ai, RE20). Is it a characteristics of this "vintage" mic or is it a problem with my building? I suspected a lot of things :SCT grill, self bias or Thiersch PVC blue line.
Do you have an idea on what to do or change?

Thanks in advance.

Julien

 

[pH]

Congrats Julien. I did mine in a Tbone body as well, tried both biasing schemes and compared a Dale M7 and a Peluso capsule as well as some component swaps. The midrange is this design's signature (like the mic it was modeled after, from what we're told), it does roll off both the highs and the deep bass.

Cheers,
Paul

 

[TheGuitarist]

Im still building mine but I agree a lowpass at 10k sound quite odd. Could you put some pink noise out of your speakers and use an analyzer on it. Not the most scientific test but it will give you an estimation.

 

[rodabod]

You can measure the amp's response using an oscillator.

 

[julien31]

You can measure the amp's response using an oscillator

Can you explain me how to do this. I don't understand what's the amp you're talking about.
Thanks
Julien

 

[Sredna]

Probably the MK7 mic(amp) you just built. 

 

[julien31]

Probably the MK7 mic(amp) you just built

Probably yes but can you explain me where to measure it.
Sorry for being a noob!

Julien

 

[Sredna]

You can inject the oscillator signal to the grid and measure the frequency response of the
mic circuit.

 

[rodabod]

I don't have the MK7 schematic handy, but yes, attach a sine tone source to the coupling capacitor at the grid (where the capsule attaches, probably a ~1nF cap). Make sure that there is no polarising voltage reaching your tone source depending on the circuit.

Drive 1KHz at the grid coupling cap and adjust this level to get say 0dBu out of your mic preamp. So, say, dial in 30dB gain on your pre, and then adjust the oscillator level to get 0dBu out on your meters.

Now sweep the frequency from LF to HF, watching your meters, and take some notes.