MK7 - tube mic project

[ioaudio]

unfortunately, there is no way to raise the voltages coming out of your transformer.
B+ : try 10k resistors in parallel with the existing 10k's resistors.
Heater: try 4.7R resistors in parallel with the existing 4.7R's (maybe only one stage).
you might hace to add capacitance and double the caps if hum appears.

 

[Kingston]

139vdc B+ is not a problem, in fact it's probably close to ideal for M7 capsules. Plenty of times I have seen it suggested that M7 capsules like to see a little bit lower voltage than CK12 types. Too high voltage at the capsule (versus ground) will make the skin try to stick to capsule surface, resulting into a skewed frequency response.

The only drawback of the lower B+ would be that polar pattern responses might not be quite perfect, but this is a non-issue if you have a 5-way selector for example. Or you can adjust the capsule voltage dividers for a perfect match.

I would concentrate on raising the heater voltage to optimal, and leave the B+ voltage be.

 

[ioaudio]

The capsule polaristion is sourced from the voltage divider 1Meg / 910 Kohm (60v).
The diaphragm of a M7 would be attrackted to it's backplate with 139V.

 

[Kingston]

The capsule polaristion is sourced from the voltage divider 1Meg / 910 Kohm (60v).

I know, and lowering B+ takes down the result of this divider also. Wasn't it you who originally suggested perhaps lowering this 60V for M7 capsules?

Perhaps I remember wrong and it was related to some other mic.

 

[plumsolly]

Thanks guys.

I will leave the B+ for now and take some measurements of the pattern voltages and capsule polarization voltage and see if they match up - I'd like to have the polar patterns be pretty correct. I will lower that first R in the heater supply and see what I get - does having the heater voltage adjustment pot turned all the way down negate the effect of the last RC stage?

unfortunately, there is no way to raise the voltages coming out of your transformer.

I could always put a variac in front of the PS! Just kidding. The voltages coming out of the transformer are pretty right actually.

Best, Ben

 

[Kingston]

does having the heater voltage adjustment pot turned all the way down negate the effect of the last RC stage?

yes, but with the choke and the several RC stages things are very much DC at that point anyway.

 

[Grooveteer]

So...

Finished the mic and PSU today. I have to say that I was a bit disappointed at first. When I was testing the mic was noisy (wind like noise) and sounded hi-passed. Tried several tubes (PF86, I had some Siemens, Valvo and Telefunken that I bought off Ebay) but the sound didn't change much.

Tried a smaller output cap (had a 0.51uF Russian PIO at first) but that didn't change much either so I kept the 0.51uF

I left the mic on for a couple of hours, still not much improvement. This was all in my shed though a cheap *** behringer mixer and ditto headphones.

Then.. I took the mic into the studio and plugged it into my NV73 and I haven't stopped smiling since. What a great microphone.

Note to self: use decent equipment when testing new gear.  I still have a hard time believing that a preamp can make such a difference, so it might have been voodoo or whatever.

Bottom line: Happy, happy.

Thanks Max!

(MK7, Beesneez K7 capsule, EQU47 body)

 

[mata_haze]

hello has anybody tried the following mains transformer for this project:

http://www.musikding.de/product_info.php/info/p943_Toroidal-transformer-230V----185V--6-3V.html

otherwise what is the general opinion for a european main trafo?

Best,
Mattia.

 

[e.oelberg]

yes... not good if you want a brutal filtering..max's psu design wants 12V

 

[Apache5]

I built my mk7 psu with a custom made toroidal from www.audiophonics.fr   (http://www.audiophonics.fr/transformateur-torique-sur-mesure-toutes-puissances-p-3675.html). Easy to order or see costs (if you know what you want), not expensive (22€ or so, without shipping costs) and mine runs perfect on specs needed... I ordered mine overspecified in power ...
Hope it helps

Salut!

Arnau

 

[Kingston]

hello has anybody tried the following mains transformer for this project:

http://www.musikding.de/product_info.php/info/p943_Toroidal-transformer-230V----185V--6-3V.html

otherwise what is the general opinion for a european main trafo?

Best,
Mattia.

You can use that transformer if you make a voltage multiplier for the 6.3VAC. There's plenty of amps to waste. It's a bit oversized, but would work ok.

 

[Kingston]

max,

I'm a bit confused here. I have seen three different types of mic output transformer core configurations on this thread. Some transformers seem to have two screws through them, while some have four. Was there a new revision of the transformer at some point?

Also, some transformer cores seem to have a wire connected to them, while others don't. Where does this wire go? Grounded core? And why some builds here don't have it? (like your original on the first post of this thread)

Thanks,
Mike

 

[ioaudio]

Hi mike,

yes, the first batches came with laminations with two holes.
Eventually the grounding of the core did not make any measurable difference to me, so i left it out. (it could make a difference in RF poluted areas - near a transmitter it could help )

 

[guavatone]

Has anyone tried a winged C with this?  I have 156V on B+ but when the tube is in, B+ drops down to 2-4 VDC.

My HT PS is rectification then a 470uF cap > 2 RC stages of 47uF with a 22K and 33K resistor. 

Maybe capsule wired wrong?

 

[guavatone]

Ok. I measured the 22K RC resistor and got 69 V.  Then calculated a current of 3.14 mA.  I will try two 8k2 resistors.

But I thought it was 1mA?  Does this sound right as far as current draw?

 

[guavatone]

OK.  8K2's are in.  Still 2.7 Volts... hehe.... not really HT is it now

Must be a short somewhere...

 

[ioaudio]

maybe either the tube connections/pinout or the tube itself?

 

[TheGuitarist]

Hey i was suggested to change the 68mohm resistor to a 100mohm, and get rid of one of the caps (it was either the 1uf 250v one or the 1nf across the cap i think, it was a while ago)

and i think it was something to do with extending treble response.

Anyone have any thoughts on  that?

 

[volker]

All the bigger resistor could possibly enhance is the bass response, but I doubt you would hear a big difference.
And removing either of these capacitors is a bad idea to begin with.

 

[TheGuitarist]

Yeah fair enough... i think i'll leave the design

I've seen a couple of people get pretty funky with that 1uf cap (russian PIO like skylar etc)

Is it worth finding something cool or is it fine just getting a good tubular one. Looks like its just a power thing not in the audio path. Though i'm not that familiar with mic schematics.