MK7 - tube mic project

I should be seeing the same voltages at the front and rear capsule connection, when in omni, correct?
Edit: never mind on above question.

I haven't had time to open this thing up and search yet. After more basic speaking tests it seems the front circuit is more like 10db (pure speculation) quieter than the back. This is more noticeable with back side switched off, as I think in omni, I was hearing back capsule picking up, and making me think it was a smaller discrepancy.
Out of curiousity the voltage divider shouldn't be a problem if I used the original values of 1.5 Meg / 1.2 Meg instead of the updated 1Meg / 910 Kohm, correct? I'm measuring 60v at the junction here, so looks good I'd think.

I'm having issues with the low end roll off, my understanding is that cut off is set by the cap between pins 1,6 and 8.

Hey Guys,

On the attached schematic is the output cap is set to 0.33-0.68uF. I settled initially on 0.47uF but noticed there is a sharp drop off at 20Hz, like 20dB so I increased it to 0.68uF and it opened it up a bit, but still lacking in the low end a bit, which would be nice on kick drum/bass etc.

Is it safe to go even larger, or are there other places to tweak the circuit that are a better option?

I'm using Dale's MK7 and a Telefunken EF86, so those might be a hindrance or limitation?

If I compare it to an AT4060, which I know is like comparing apples and oranges, but there is more low end to the mic.

Thanks in advance


Cheers

Matt

Hey Matt,
I've used a .5uf Russian pio cap in mine and it sounds huge with great low end, even despite my front amp side acting up. I've been too busy with work and besides having limitations with pattern switching, this thing sounds remarkably great using the back side as a front. .
Edit: corrected value after double checking. I've go to much going on to remember what I've used!

@Matt

I had a low end problem and replaced the output cap before realizing it was a bad ef86. Just a thought.

Best,

Ben

I've corrected my post earlier to the correct value that I used. Sorry for any confusion. Have you tried a different tube like Ben suggested?

nashkato said:

finally i got rid of the hum  ;D
unfortunately i cannot say what i did exactly , cause i changed several things at a time , but i definetly was a grounding issue.
thanks to all who tried to help me .

Click to expand...

Could you please elaborate what you did? (even i you don't know the exact solution). I running into the same trouble.

matta said:

On the attached schematic is the output cap is set to 0.33-0.68uF. I settled initially on 0.47uF but noticed there is a sharp drop off at 20Hz, like 20dB so I increased it to 0.68uF and it opened it up a bit, but still lacking in the low end a bit, which would be nice on kick drum/bass etc.

Is it safe to go even larger, or are there other places to tweak the circuit that are a better option?

Click to expand...

So a 50pF capsule into a 68M grid resistor is a HPF at 43Hz.  You could increase the grid resistor up to 100M to extend this down below 20Hz and get more bass.

After much triple checking, I injected signal into amp circuits and the signal was equally level in both sides. I tried a 100M grid resister instead of the recommended 68M, and now I have the front capsule working as loud as the back capsule. It sounds great!

I recently got an unused pcb and transformer for this project, and got it all assembled. I am getting the same issue with my last build- no sound on front side of amp. I've reflowed all solder points on the pcb, changed the 68meg and  1nf, and still no luck. Can anyone suggest where to check?

Front side of capsule?

Maybe something is shorting out--capsule?

shabtek said:

Front side of capsule?

Maybe something is shorting out--capsule?

Click to expand...

Front capsule side works when I switch leads. It's so weird that my first mk7 did the exact same thing. Maybe I'm getting the 1nf cap too hot when soldering it?
Edit: I've just swapped out the 10nf and the front capsule is working now. I took it to my studio and It sounds great!

Hey, sorry for reviving this old thread. But I would like to build this mic. Unfortunately the schematics are gone since today and I did not save them  :
So does anybody still have them and can upload or send them to me? 

EDIT: Nevermind, found the mic and psu schematic on the last two sites.
Can anybody who has the original schematics tell me, if these are unmodified?

Those look like the originals. 

Hey, I'm building the power supply right now and I still have a spare power transformer from my G7 project.
But it has only 130V and 9V output instead of the recommended 160V and 12V...
Maybe by just omitting or lowering some of the dropping resistors?
Or am I better off just buying the right transformer?

After rectifier you will get ca 180V, so lower values of dropping resistors and you will get easily 150V, 9V is sufficient for heaters.

hmm, sounds good!
But are you sure I don't mess anything up considering the interaction of the capacitors and resistors?
Do I really just have to lower the first few resistors?

Everything would be fine, it's just a PSU and capacitances are pretty big.
Use 4.7k, 4.7k, 5.6k instead 2x10k and 13.3k.
For heaters use second schematic (with lm317)  from the first page.
That's it