MK7 - tube mic project

[ioaudio]

also check the 170mA vs 200mA heater rating @ 6.3V, you will have to adjust resistors.
in this circuit the ef86 runs 5,7V @ 180mA , so about 10% underheated.
i didnt play with the ef804s, so i cant say how the filament takes underheating. tube data says +/- 5%

-max

 

[EEMO1]

I'd recommend making the usual measurements; each of the PSU pins unloaded, plate voltage, cathode voltage, heater voltage, polarisation voltage before Hi-Z section. And transformer continuity as Max suggested. Probably something very simple.

unloaded pins from psu:
1= 148,6v
2= 6.06v
3= 0v, 50,8v, 102.2v
4,5,6,7= 0v

loaded:

dc resistance between output xlr pins 2-3: 30ohms.

tube socket pins:
1,6,8= 130,6v
2,4,7= swinging between 0~6v
3= 6v
5= 6v

polarization voltages before the 33mohm resistor or somewhere else?

no capsule connected btw.

 

[ioaudio]

tube socket pins:
1,6,8= 130,6v
2,4,7= swinging between 0~6v
3= 6v
5= 6v

ok, sounds like the lm317 cant regulate - your problem is definately the heater supply.

there is a group buy for the MK7 Psu in the black market right now.

 

[rodabod]

ok, sounds like the lm317 cant regulate - your problem is definately the heater supply.

Thermal shutdown?

Maybe try measuring the cold resistance of the heater circuit of the mic.

 

[kilmister]

Perhapas there is short circuit and regulator is limiting current due that.
Connect at least 2 watt 33 ohm resistor in psu's heather rails and check voltage again. If it drops or swings your psu is sick. Otherwise you'll find your problem in the mic. 

-Paavo

 

[EEMO1]

ok, sounds like the lm317 cant regulate - your problem is definately the heater supply.

there is a group buy for the MK7 Psu in the black market right now.

I thought of that one too. the psu is the problem in my opinion...


no problem building the real psu. I want to do it old school, no board.

the problem is getting the custom transformer for it... the company that kingston organized his custom transformer from in Finland, told me that their economics department don't want any new customer accounts, because it has an "employing effect". I guess they just want to sit on their thumbs all day long... geez!

I could organize a group buy for it though, if anyone else needs one.

 

[zebra50]

Hi!

Mine is up and running and sounding very good so far. I've used an AKG 'teflon' capsule, and am cheating slightly as I'm testing with a Drefahl audio power supply. This is spec'd for a UM57, so I get about 125V for B+ and 5.2V on the tube, which are lower than the parameters in Max's design. All the other voltages seem about right for this B+ - I've got 59V on the tube plate. This one is in self bias mode, with a .33 uF output cap.

Anyway, I had the test gear out for another project (See the UM57 thread), and thought I'd run a frequency response measurement on the amp circuit of the MK57 under these conditions - thought you might like a look...

I see a slight rise towards about 3-4K, and then down again, and sonically I'd say this is about right. Rather nice actually. I really ought to build it a dedicated supply.

z50

 

[rodabod]

I need to start doing these graphs for my mics. Nice one.

 

[Rossi]

I need to start doing these graphs for my mics. Nice one.

Same here! 

 

[zebra50]

Incidentally, and for what it's worth, it seems that the value of the output cap in this system makes very little difference, at least above 0.33 uF. I haven't gone lower than that because the way I put the mic together makes it slightly awkward to remove the existing cap. I did wire in my 0.1uF x 10 decade box in parallel and went up to 1.33 uF.

 

[rodabod]

Incidentally, and for what it's worth, it seems that the value of the output cap in this system makes very little difference, at least above 0.33 uF.

I might be generalising here, but I think it might be more noticable with designs that use lower Rp valves, as the filter response will be less damped.

 

[TheGuitarist]

Hey are there any major modifications to do if i use ef14 steel tubes? I'd probably do the self bias wiring... but are there any other considerations i need to make in changing the tubes from ef86?

 

[Kingston]

Hey are there any major modifications to do if i use ef14 steel tubes? I'd probably do the self bias wiring... but are there any other considerations i need to make in changing the tubes from ef86?

Why an earth would you want to do that? This isn't a U47 clone circuit...

 

[TheGuitarist]

Thats why i'm asking, any advantages or disadvantages to doing it, or are the ef14 tubes only handy in the u47 circuit? A guy here in Australia has a bulk amount and is selling them. Just a bit of curiosity really.

Isn't this mic suppose to be u47ish anyway? with the m7/k47 and the emulation of the u47 headamp?

 

[rodabod]

The transformer was designed to couple with an EF86.

 

[TheGuitarist]

that answers my question then! Awesome, saves me trouble and money!

 

[ioaudio]

Thats why i'm asking, any advantages or disadvantages to doing it, or are the ef14 tubes only handy in the u47 circuit? A guy here in Australia has a bulk amount and is selling them. Just a bit of curiosity really.

Isn't this mic suppose to be u47ish anyway? with the m7/k47 and the emulation of the u47 headamp?

sure - i encourage everybody to try different tubes.
i recommended the ef/pf86 more strictly at the beginning, because i wanted builders to understand the circuit to be able to calculate and adjust the heater - bias related resistors according to the tube´s needs.
the bv7 transformer is designed to play with the ef/pf86, but that doesnt mean it´s the only tube which works.
i recently tried the ef95 - sounds great.
different ef86 tubes vary quite a lot - i was lucky with my batch of valvo pf86 when i designed the mk7.

 

[zebra50]

I've seen EF86s dropped straight in as replacements for EF12 in old Reissmann mics (with an adapter), so it's not too much of a stretch that EF12 might work well here - IIRC the specs of those tubes are pretty close.

 

[seavote]

i lost 2 of the glass tubing pieces that are used to insulate the hi z part of the circuit. i took a trip to a craft store in my area and found all sizes and shapes to choose from. so if anyone would like to use this trick in another mic build its good to know of this source. go to a craft store and head to a craft store and ask for glass beads.
i understand the general theory behind this but im hoping to understand better. is the purpose of the extra insulation to stop leakage from the hi-z capsule connections to the ground plane on the pcb??
if so would it also be wise to use glass insulation where the capsule leads pass through the "top plate" of the inner mic assembly. since the mic enclosure is grounded and connected to this inner assembly insulating both ares seems wise. am i wrong about this? i know its not something i see DIYers doing with their MK7s.  thanks for any opinions

 

[ioaudio]

Emperor Tomato Ketchup finished his mk7 with the passive psu and posted his grounding scheme:
http://www.groupdiy.com/index.php?topic=36857.0