MK47 PCB tube mic kit - build thread

I replaced the dropper resistor with an aluminum housed 10w and so far the noise is gone! Let's see how it holds up after a few sessions. Thanks for the help.

MK burning for several days. 

Intermittent crackling is not what I would attribute to noisy tubes, but it may be a first.

The popping and crackling is violent, clipping the signal.  I'll post some audio clips, both of what I hear as normal tube futzing and this crazy violent popping. 

The microphone will be quiet for even long periods of time, and then it will go on for several minutes of the popping.

jordan s are you still around, is your microphone still behaving well after the resistor swap? 

Quick datasheet observation while hunting down an aluminum chassis mount 1.5K (aluminium for you folks across the pond  ;D )

Temperature Coefficient:

Power Resistors Wirewound Aluminum Housed  20 PPM / C

Power Resistors Wirewound Inorganic Potting Compound  300 PPM / C

MicDaddy said:

MK burning for several days. 

Intermittent crackling is not what I would attribute to noisy tubes, but it may be a first.

The popping and crackling is violent, clipping the signal.  I'll post some audio clips, both of what I hear as normal tube futzing and this crazy violent popping. 

The microphone will be quiet for even long periods of time, and then it will go on for several minutes of the popping.

jordan s are you still around, is your microphone still behaving well after the resistor swap?

Click to expand...

Sounds like the trouble I've had with some Western Electric tubes.  What kind are you using?  Are you using the cement style dropper resistor?

JAN Philips and what looks like the cement resistor that came with the kit.

The thing will work fine for hours on end and then blasts of popping, it's not like fizzy futzing in the noise-floor, it's like fireworks.

MicDaddy said:

JAN Philips and what looks like the cement resistor that came with the kit.

The thing will work fine for hours on end and then blasts of popping, it's not like fizzy futzing in the noise-floor, it's like fireworks.

Click to expand...

Yeah.  If you are using the cement resistor than that is likely the problem.  A good quality aluminum sinked resistor has solved the issue for more than a few MK47s.

Current kits are already including the newer resistors, that's why I asked.

I've got a bunch on the way.  I was curious as I'd seen IOaudio recommend replacing yet the photo for the mk kit at Chunger's place still shows the cement type..

Thanks guys!  I'll follow up with the audio and when the new 1.5K show up.

Hey,

yes the aluminium resistor should solve that problem - there was a batch of cement resistors showing the fireworks you describe.

-Max

Swapped through a few sets of tubes just to exhaust that possibility, popping followed the mic.

Swapped the 1.5k with a 10W aluminum, no more popping...  8)  thanks everyone for your help hunting that down!

Sad news, now my mic is stuck in omni...  did I kill the relay (how to test?), break a trace swapping tubes? 

I'm seeing ~90VDC before the output cap, this seems very high?  Thanks for any tips 

I had the same problem. The solder pad of the relais was disconnected to the rest. I had to bridged it with an extra cable (the red one).

Thank you duy_hoang, so you say you had a trace go open? 

I suspected I had broken one/several of the 90* solder pads where the main and daughter board meet from the stresses of swapping tubes. Would ideally like to secure that daughter board with something supplemental to those 90* solder joints.

From which contact to which did you apply the jumper?  (I suppose I could try to figure it out from the schematic but I don't have the mic in front of me.)

If I'm looking at the diode, would it be cathode to polarity, or anode to ground that was disrupted? 

When switching the pattern, I hear the normal pwop but no polar change...(can you hear the relais inside the mic?) 

I'll chime those two traces with the DMM and see if one is open.  Thanks again!!

Did you test the mic without the pattern wire connected?  just disconnect it inside the PSU and see if that changes anything.  You would normally be stuck in cardioid in such case.

Hi,
in my case, it was stuck in cardiod mode like Category 5 said. The faulty solder pad was the one of the lower relais pin (see picture)

And a picture of the wiring on the back side.

Excellent points and tips guys!  So sounds like my relais is stuck on/closed/shorting (term?)...

I'll try to both disconnect the polarity rail at the PSU and inside the mic (should go cardioid?)  If that works to get the mic to cardioid I'll hunt for a short, or replace the relais?  I suppose I could check the relais powered down for open/short?

I'll measure my switch for correct operation, perhaps it failed?

What else might cause the relais to not shut down... if the above fails I'll disconnect the rear diaphragm just for sanity check.  ;D 

If it's stuck in omni, it means the  relais is active /turned on.
Search around the pattern switch, maybe it's not correctly connected.
90 V is very high, what's your B+?

B+ at the PCB under load (mic) ~104+ 
I will double check.

:'( what a stupid problem...  the tubes which I was testing in the mic (northern electric) have some nasty white corrosion underneath. 

Swapped the tubes back for the JANs and my patterns are working, no crazy popping.  Working. 

Thanks again guys! 

MicDaddy said:

:'( what a stupid problem...  the tubes which I was testing in the mic (northern electric) have some nasty white corrosion underneath. 

Swapped the tubes back for the JANs and my patterns are working, no crazy popping.  Working. 

Thanks again guys! 

Click to expand...

I bought a 10 piece lot of northern electrics, about 1/2 of them are noisey, when you find a good quiet pair, they are quite nice.

interesting you say that tonycamp, I immediately found the "timbre" of these tubes more pleasing than the JAN, but the wooosh is much much higher than the noisefloor of the JAN, which are quiet yet microphonic