MK47 PCB tube mic kit - build thread

[baol]

Hi hellfire, I only have the 2 tubes sent with the kit. I checked the signals from each of the 7 pin on the  psu xlr, to the mic board (A-, A+, B+ etc...) and there's continuity. the tubes get red but there's no sound. I don't know what to check more than B+ and Pattern values... Here a few pics

 

[baol]

...and here the mic

 

[mrvision]

Make sure the bracket extensions under the tubes are not touching a tube pin, a trace or another component soldering. make sure there are no shorts due to the aluminum bracket.
Also double check component placements and resistor values.

 

[dmp]

Wow, baol, your psu is a nice DIY build.

I haven't built one of these (warning), but you could check the voltage drop across the 100k plate resistor. That will tell you if the tube(s) is conducting.
And check the voltage where the 2M and 3M meet and see if the capsule bias is getting set. Should be somewhere near 35v with a typical dmm.

 

[leitrim_lad]

Hey there, im having problem with my mk47.. It keeps making popping sounds.. Iv changed the tubes but it still is making these sounds. Im just wondering would anyone have any idea of what might be wrong? Ive included a linkto a recording of the pops for you to have a listen.

Regards

https://dl.dropbox.com/u/12155948/Output%201-2.mp3

 

[idylldon]

Hey there, im having problem with my mk47.. It keeps making popping sounds.. Iv changed the tubes but it still is making these sounds. Im just wondering would anyone have any idea of what might be wrong? Ive included a linkto a recording of the pops for you to have a listen.

How long have you burned in the tubes?  Mine was making the same sounds with the tubes that were included in the kit after countless hours of burning in, so I bought a bunch and put in another pair.  After a few days of burn in, the mic is dead quiet.  I know you've changed the tubes, but you might have to go through a few pairs to find ones suited for mic use.  Luckily, they are cheap and plentiful.

Cheers,
--
Don

 

[leitrim_lad]

Cheers don, was it the same kind of noise yours was making? The mic used to be dead quiet then started making the noise. I've only changed the tubes a couple of hours ago and have been burning in since !

 

[micaddict]

Quite a few have mentioned the popping noise, so this likely is more than coincidental. Tube burn in has helped some but not all. And most are using the JAN Philips 6028 that come with the kit.

A while ago I read a post by Oliver Archut, about JAN tubes and noise. Random noise was mentioned, too.
Here's a quote:

"JAN tubes, Join Army Navy tube key, specialty tubes made for military use. Most of them were upgraded industrial tubes, with the one purpose to insure function under very heavy-duty circumstances. Their construction was designed to insure the minimal function over a long period of time. To put it into better terms, there were pre tested on the edge of their rating to sort out early failing ones. It doesn't mean that they were tested for audio, and most of those tubes are very noisy, not microphonic but a lot of random and white noise. In circuits like the U67 most of it is suppressed because of the HF cut off feedback as you pointed out. The higher noise relates to the cathode construction, some of them used sintered ones others a double layer coating design, the first layer is fast activated at the factory, the second one starts activating over a long period of time. *This process was patented by Sylvania and use for most JAN tubes after 1963* That is what generates the higher noise floor if used for audio. Sure they nearly live forever with hardly no microphonics, but gain structure is quite different, most of them sound quite harsh. In the last few years so many JAN tubes were decommissioned and entered the audio market, but for my personal taste a standard RADIO GRADE or some industrial tubes are more desirable. Of course there are some exception, like the JAN tubes than were special made for audio use."

I've wondered if the JAN Philips 6028 (seventies era tubes if I'm not mistaken) could have had the treatment Oliver is talking about.
Just to be sure I bought some pre '63 6028s for my build.

 

[idylldon]

Yes, the noise was pretty much the same, but mine would do it in short bursts and not in such a random fashion as yours.  When I first powered up the mic with the included tubes, it was quiet but then developed these "pops."  I put a new set of tubes in, burned them in for a few days, and, like I said, it's dead quiet now.  I used it on sessions this past weekend and was extremely pleased by its performance.  Like someone else mentioned earlier in this thread, not all tubes are up to the performance level required in a mic circuit so you might have to go through some pairs to find just the right ones.  Once you get a good pair, though, they'll probably last a lifetime.

Cheers,
--
Don

 

[Volume11]

Yes, the noise was pretty much the same, but mine would do it in short bursts and not in such a random fashion as yours.  When I first powered up the mic with the included tubes, it was quiet but then developed these "pops."  I put a new set of tubes in, burned them in for a few days, and, like I said, it's dead quiet now.  I used it on sessions this past weekend and was extremely pleased by its performance.  Like someone else mentioned earlier in this thread, not all tubes are up to the performance level required in a mic circuit so you might have to go through some pairs to find just the right ones.  Once you get a good pair, though, they'll probably last a lifetime.

Cheers,
--
Don

I had the same issue a few months back. I ordered a set of ten nos tubes off of someone on eBay, popped them in and let them burn in. Mic has been perfect ever since. I was getting the same random pops and noise.

 

[baol]

Hi, so once I have my 105 volts on the psu output it means that the psu it’s ok, the problem is in the mic, isn’t it?
Well, the connections seems to be ok because there’s continuity between psu and the wires arriving at the microphone. DMP told me to check the voltage drop across the 100k plate resistor but I don’t really have understood what he means, I checked between ground and both 100K rheophores but I found something like a 0.30 volts, than I checked between 2M & 3M res and ground and I found 61 volts… May someone suggest me where and what (and maybe how) to chek on the mic? What kind of values should I have?

 

[leitrim_lad]

Iv been letting them burn in for nearly 20 hours on and off now and the pops have slowed down a good bit but are still popping every 30 seconds or so .. is this normal for the burning in process or do you still think my new tubes are jacked as well? 

Regards

 

[idylldon]

Iv been letting them burn in for nearly 20 hours on and off now and the pops have slowed down a good bit but are still popping every 30 seconds or so .. is this normal for the burning in process or do you still think my new tubes are jacked as well? 
Regards

I'd leave it on at least 48 hours continuously and then check.  If they're still popping after that, I would most likely try another pair.

Cheers,
--
Don

 

[Studio Mollan]

Hi again.
I'm hearing a bit of hissing but will wait a couple of days to let the tubes burn in thoroughly before pursuing that.
Is anyone else experiencing a clear high freq ringing when tapping and handling the microphone? Haven't heard any random noise yet. I still have only listened to my own voice through an mbox with headphones but it seems to be a killer sounding microphone! Thanks for this Max!
/
Emil

 

[baol]

Hi, so once I have my 105 volts on the psu output it means that the psu it’s ok, the problem is in the mic, isn’t it?
Well, the connections seems to be ok because there’s continuity between psu and the wires arriving at the microphone. DMP told me to check the voltage drop across the 100k plate resistor but I don’t really have understood what he means, I checked between ground and both 100K rheophores but I found something like a 0.30 volts, than I checked between 2M & 3M res and ground and I found 61 volts… May someone suggest me where and what (and maybe how) to chek on the mic? What kind of values should I have?

 

[Studio Mollan]

Also I have a question about the 6028 tube. I just bought 10pcs of the tube made by an old swedish telephone company LM Ericsson. Anyone tried these or have any preferences for the tubes? Searching eBay gives you quite a selection incl expensive gold pin versions. Any thoughts?
/
Emil

 

[Studio Mollan]

I notice I have some electrical noise, sounds like ground hum, low but audible when in cardoid mode. Omni is dead quiet.
Anybody have any experience on this?
/
Emil

 

[Andy Meyer]

I built this kit 2 years ago and I had issues with the tubes. After 3 pairs I found some good ones. Shortly after, the popping came back. I swapped out the 5W resistor and it has been perfect ever since.

 

[Studio Mollan]

Actually the 1k5 resistor is a new one as the one with the kit could not be removed from a different donor body which I ended up rejecting. I think it might be an issue with the relay or the 48v to it. Seems reasonable looking through the schematics. It's really not loud but with headphones on its clearly audible and will be a problem after compression. Any help on this is greatly appreciated!
/
Emil

 

[Songguy]

Hi all,
I have a question on 7 pin gotham cable wiring..
I have been reading a few posts on how to wire the mic/mic body to the psu using the 7 pin connectors. It seems that there are a few different ways to this when it comes to ground wiring ie pin 3/4 and bridging of pins.
I would be grateful if someone could let me know what seems to be the best way to wiring the grounding regarding mic body and 7 pin connector.

Many thanks,
Chris