MK47 PCB tube mic kit - build thread

[The Frizzle]

I would like to use the capsule from the t-bone SCT 700 for the burn in. So I would just connect "front grid" to the front wire of the capsule and backplate to the side correct?

 

[burglar]

Hi folks,

When purchasing 6028 tubes, if I can have them tested is there anything that will signify good performance?

Is it best to have them matched if possible?

Many thanks

B

 

[Pip]

Hi folks,

When purchasing 6028 tubes, if I can have them tested is there anything that will signify good performance?

Is it best to have them matched if possible?

Many thanks

B

Low noise and microphonics. Matching is secondary to these qualities IMO. I would just buy a bunch and keep burning and listening until you get two that have those qualities. There are some GE 408a's on ebay that are great. The WE USA manufacture with the large type font fair well as well.

 

[Category 5]

The tubes run in parallel so there is really no advantage at all to matching them.

 

[burglar]

Thanks catagoty5.


- Also, the build pics seem to be gone from this thread.  Does anyone have them?

- my psu swings from 103vdc to 106vdc.  That seems a lot?

B

 

[Category 5]

The PSU is passive so output voltage is directly related to input voltage.  If the source fluctuates at the wall (and it will) then it will fluctuate on output too.  103-106 isn't anything to be concerned about at all.  Perfectly normal, and within acceptable range.

 

[burglar]

ok, great.

one other thing.  without the dummy load switch engaged my voltage is over 300vdc.  is this normal?

 

[Category 5]

ok, great.

one other thing.  without the dummy load switch engaged my voltage is over 300vdc.  is this normal?

Without a mic connected, yes.  Be careful.  It hurts.  Under no circumstance should you connect or disconnect the mic with the power supply on.  Also, make sure the led is no longer lit before connecting the mic since voltage is stored in the caps for a while after power is cut. 

If you connect the mic with power applied you will get a shock you will not soon forget. 

 

[burglar]

Do you still have the build pictures Max?

 

[ioaudio]

Build pictures are still in the first post, anybody else who can't see them?
Maybe try clearing your cache?

-Max

 

[burglar]

Cleared cache, no joy

 

[Category 5]

Cleared cache, no joy

Just checked and I don't see any either.  Text is there but no images. 

 

[ioaudio]

Interesting, it works on my PC with firefox, but not chrome. Will look into it.

 

[Bowie]

Interesting, it works on my PC with firefox, but not chrome. Will look into it.

Working fine on my PC with Chrome and on my Android.

 

[ioaudio]

Looks like Dropbox changed it's links to https, just fixed the links.

 

[The Frizzle]

Just tried out my mic. I had hum first. Then connected 0v to ground. Hum is gone, but now I get hiss noise.
It is passing audio though.
Can this also be the not burned in tubes or is this always the 1k5 dropping resistor?
I already ordered a aluminium chassis mount one, just figuring if I should just wait for it to arrive or if it makes sense to take the time and burn in the tubes.

 

[burglar]

How would I hook up the behringer b2 capsule into the mk47 for testing and burning in tubes?

 

[The Frizzle]

I've swapped out the 1K5 dropper resistor. Still white noise. Swapped the tubes out. Still white noise.
So the noise is probably the not yet burned in tubes. But my volume  is also super low. I can hear when I knock against the mic case but not much else.  I guess the white noise will go away after burning in but not the volume problem?
So I probably have an wiring issue. I am using the stock SCT 700 cable. Someone wrote here they are crap. So I'll solder a new one, maybe that'll help.

 

[The Frizzle]

I checked every pin on the cable and it should be fine for testing.
My mic is still not really passing audio, only if I knock against the mic housing.
If I make noise next to the mic it's not passing it.
I did some measurements and I'm gettig
B+ 105V
Heater Tube one 18,2 V
Heater Tube two  1,42 V
26R 1,41V
Backplate 56 V
And then as far as I understood the output capacitor is the russian one correct?
I get 44,5 V on the russian capacitor.

 

[Pip]

I checked every pin on the cable and it should be fine for testing.
My mic is still not really passing audio, only if I knock against the mic housing.
If I make noise next to the mic it's not passing it.
I did some measurements and I'm gettig
B+ 105V
Heater Tube one 18,2 V
Heater Tube two  1,42 V
26R 1,41V
Backplate 56 V
And then as far as I understood the output capacitor is the russian one correct?
I get 44,5 V on the russian capacitor.

That second tube heater voltage is wrong it seems to me. You should be seeing roughly 40VDC to the heaters after the 1500 OHM drop down resistor. Do you have more tubes if so replace the ones you have been using one at a time looks like you might have a bad heater

Pip