MK47 PCB tube mic kit - build thread

I'm getting 290V at the heater tube pin (after those solder joints) , but no voltage where it exits the tube at the 27R resistor.

Resistances were measured with the mic connected to the supply.

JW said:

Resistances were measured with the mic connected to the supply.

Click to expand...

I asked because if you are measuring resistance through the power supply resistors it doesn't tell you about continuity in the mic circuit.

Oops
Okay, without the p.s. connected, I get some pretty high resistance.
Between B+ and ground I get 4.6M ohms
Between the 1.5K resistor and the 27R, I get 5M ohms.

Thanks for helping me with this Bobine. It's probably something obvious.
I've resoldered the tube pins as well as the 90 degree angles.

Is there a suggested value capacitor to take place of the capsule for testing purposes? I'm using an old chinese Nady capsule right now that I haven't verified as working for a long time. But it's what I've been using for initial testing. Not that that is necessarily the issue, but I don't know.

So, looking at the schematic, that tells me that current is not flowing through the heater filaments. I think I'd take out the tubes and test continuity right up to the pins.

Then I'd measure the heater resistance between pins 3 and 4 of the tubes themselves. http://www.westernelectric.com/spec_sheets/408A.pdf

Maybe someone who knows more can help with capsule question.  

You can measure the capacity of the capsule with an LCR for an exact number.

But 80-100pf is a good place to start.

hey JW,

propably forgot the bridge here?

-max

Hey Max,
Nope. Got the jumper. Thanks everybody. I think I got a bad tube. Resistance between 3 and 4 on one tube is 92 ohms, the other is up around 8M ohms, so I got an order in on one of those 10 pc. lots on ebay. (Resistance measured on the tube pins themselves, out of circuit)
Thanks Bobine!

JW said:

I think I got a bad tube. Resistance between 3 and 4 on one tube is 92 ohms, the other is up around 8M ohms, so I got an order in on one of those 10 pc. lots on ebay. (Resistance measured on the tube pins themselves, out of circuit)
Thanks Bobine!

Click to expand...

Glad to hear the issue seems to be solved.
Would you be so kind and tell us when the mic is working and what the problem was?
That would help,including me as i´m about to build one (got one of max´s last kits ;D).

Thank you very much and good luck,

Udo.

JW said:

I think I got a bad tube.

Click to expand...

I'd send a free replacement as you wish - on the other hand having a box of tubes for the future is a good plan anyway.

Well this is interesting timing.  Fired mine up for the first time this morning and checked the voltages to find the exact same symptoms as JW.  Took the tubes out and found one bad.  Fortunately, I was able to take one from my second kit to test and with that the mic fired right up and works.

Thanks for your troubleshooting, JW.  Definitely saved me some headache here!  These tubes are fairly inexpensive so I'm going to hit EBay and buy some spares.  It will be good to have them on hand.

  Brian

Hi Max,

I'm new here, and I now offer my congratulations to the MK 47, very nice.
The kit and 'still available? It is possible to know the price?

Best

Anthony 

Hi Anthony

sorry the Kits are sold out at the moment.
I will have a new batch but i don't know when i get around it, lots of other things remained undone.
Pls watch the white market for availability.

Hey guys,
So, I didn't mention that I had a second kit, and was able to sub in another tube, partly due to embarrassment that I didn't try swapping tubes to begin with. Doh!
Anyhow, the mic is loading the circuit properly now, and the heater is working, and at the correct voltage. Yay.

However, all I'm getting is some really loud hum and microphonic noises. Tried both my original chinese test capsule, and then Dale's M7. Same result. Voltages are good though.

Any ideas?

Max, no need to send another tube. They're not expensive, and I'm gonna get a lot of 10. I'd rather just solicit your help on this thread.
I think it would behoove me to build up my second mic and supply to have something to compare to.

Hi, as suggested earlier in this thread, I am going to us this transformer:

http://www.tube-town.net/ttstore/product_info.php/info/p2192_Ringkern-Netztrafo-30VA-Preamp.html

What is an appropriate fuse to use?

Thanks,

Matteo

drgrateful said:

Hi, as suggested earlier in this thread, I am going to us this transformer:

http://www.tube-town.net/ttstore/product_info.php/info/p2192_Ringkern-Netztrafo-30VA-Preamp.html

What is an appropriate fuse to use?

Click to expand...

125mA

e.oelberg said:

drgrateful said:

Hi, as suggested earlier in this thread, I am going to us this transformer:

http://www.tube-town.net/ttstore/product_info.php/info/p2192_Ringkern-Netztrafo-30VA-Preamp.html

What is an appropriate fuse to use?

Click to expand...

125mA

Click to expand...

Thank you, should it be slow blow?

yes, slowblow.

OK... got a dumb one (probably audio electronics 101) :-[

I'm using Gotham cable with Skylars EQ47 body. I also followed bobine's suggestion for wiring up the cable.

Audio
1 = A- (gray) (connects to pin 3 of XLR at the power supply)
2 = A+ (white) (to pin 2 of XLR at the PS)
Center pin = audio ground (green)  (to pin 1 of XLR)

Power
3 = B- (black [blue in the cable])
4 empty
5 = B+ (red)

Pattern
6 = P (yellow)

Shield to the chassis.

Did the cable ends first and then went back to wire up the male connector at the body. Everything seemed straightforward until I realized that there is no solder tab for audio ground. What to do with the green (pin 1 of XLR) cable!!!

Do I just leave it? do I tie it to chassis ground?

Hi All
I have a strange problem with my MK47 build.I've used a BeezNeez capsule from the group buy and Skylars body.
The problem I have is that in cardioid both front and back of the capsule are on,although the back is duller sounding.If I switch to omni the sound becomes quieter and really thin sounding,again both front and back are on.In omni the pattern voltage drops from 48v to 30 volts but if I reverse the the front and back capsule wires I have the same problem but without the voltage drop.The relay checks out fine and all other voltages in the mic are are good.
I'm really hoping I haven't got a bad capsule.What do you guys think?
Many thanks
Rich

Is the diode in the correct way?