Arrrrrrgggh! Tube mic noise. Any ideas?

So, I built a mic to take an old, bayonet fitting M7 for a customer.
Used an AC701 ( tested and working fine ) with a C12 circuit apart from the multi-pattern side of it.
Sounds very nice indeed. Nos hiss but little bump noisess like fireworks going off in the distance.
Have changed ALL the components ( including the tube ) and.....it's still there!!!!
Anybody know what it might be?

Thanks

how did you build the circuit? On a PCB etc. Make sure every thing around the grid part of the circuit is very clean,grid to ground resistor,input cap if used etc. You might have a leakage problem.

What type bias did you use, fixed like a c12 or cathode bias?

I have not used a ac701 but I have read all I can find on them. Is your tube(s) NOS that have not been used for years or used? Maybe the cathode and oxide materals need to "cook" for some hours to get the oxides in good shape.

http://recforums.prosoundweb.com/index.php/t/2927/0

Hi Gus

It's all very clean as per your advice last time.
It's on a pcb using a 22uf cap and 3.6k bias resistor.
I had thought it might be another grid problem but it's just
FINDING IT is so difficult.
The grid resistor has been replaced and I haven't put any fingerprints
on any part of the grid components or board.
Very frustrating 'cos apart from this noise it sounds lovely!

Does it get worse when you blow a little moist air on it?

If the parts changeout does nothing it would seem to come down to the PCB or the capsule itself. It does sound like a leakage mechanism, one in which the charge accumulates to "breakdown" and then produces the noise.

Then again if it is a part the whole batch could be suffering from the ailment.

clean the board.
clean the board again.
clean the board again.
clean the board again.
clean the board again.
clean the board again.
clean the board again.
clean the board again.

one more time.

Remember you are dealing with 100's of megaohms
oils from finger prints will kill you.

Use 99% isopropanil (spelling) drugstore alcohol
it is very hydroscopic keep it in a closed container.
Never use some and add the left over back to the container.
Use a brush to scrub the circuit board and everything including the tube and use plenty of alcohol to wash the stuff away. Yes let it drip off the board.
Do that several times let board natural dry and retest
the microphone.

Ok.

I'm going to start again from scratch with a new board and LOTS of cleaning and leave the tube running overnight and if it STILL doesn't
solve the problem, I shall stop making microphones and make furniture
from sticks instead to pay my mortgage.
I shall get back and let you know how I get on.
Huge thanks for your help.

Oh and I used the AC701 before and it was great.
I swapped it out for an EC1000 and the problem persisted.
Both tubes have been used in nice sounding test mic circuits I have built.
I might also try a simpler circuit as the M7 is only cardioid and I am using boards I made for C12 - type mics.
I'm sure I have a circuit or two somewhere.

Not sure, but I thik it can be a grounding issue also...

when my m149 goes crackling hissy, I need to open the power supply and clean the spring contacts in there. Also it needs a little extra tension on the springs that tranfer voltage to the pcb of the PS. After hooking up again there is no more noise anymore. I think bad contacts PS is the biggest noise maker in tube and condensor mic circuits.

Tony

..remember to clean the tube as well..

You could be radical and solder the wires directly to the tube. I do it all the time, and have seen it done by commercial companies inside of mics.

10,000 hour tube = no big deal.

There are 8742 hours in a year, so just ration accordingly.

At klaus's forum Oliver posted about the problems with the 149 the tube heater oxides and metal do not work correcly when underheated. I believe there is "fix" in the thread.

IIRC the ec1000 and the 6011 dual triodes were used in a charge amp circuit(so they could call it a tube microphone) they could have used a fet

The 701 is a small wire leaded tube design, just for microphones. The grid exits opposite of the other wires.

and where the heck did you get a 701 from?
:?


since you found the tube isnt your problem, the 701 primer on klaus's forum says that only 1 out of 10 good-tested 701's is fesable noise wise to use in mic applications.
thats a pretty small % of the little 701's out there.

Or Klaus Heyne has another agenda with claiming this?

I don't know.

He says some wierd things sometimes - like it's not possible to reskin/repair AKG CK12-capsules, and yet he has had a deal with AKG for many years that he collects all the broken CK12-capsules that they routinely replace..

I don't quite get it - maybe there's another explanation - but some of the info over there smells like indirect smokescreen. Which may be completely understandable, trying to protect business. Just not good when disguised as information.

Anyway, it's a great forum - with some very hardcore microphone guys hanging out and contributing..

Jakob E.

[quote author="CJ"]
There are 8742 hours in a year, so just ration accordingly.[/quote]

??? :shock:
You don't work on January 1st???

My year counts 8760 hours, every 4 year even 8784 :green: :green:

actually, oliver achut wrote that primer, its just on klaus's forum.

i hear ya on the capsules... im pretty sure klaus is a FIRM believer of the 'even though its a ck12, if you reskin it its going to sound NOTHING like a ck12, and since capsules are aged, NO original ck12 sounds like a ck12 30 years ago'.

which, in fact is very true.

[quote author="gyraf"]Or Klaus Heyne has another agenda with claiming this?

I don't know.

He says some wierd things sometimes - like it's not possible to reskin/repair AKG CK12-capsules, and yet he has had a deal with AKG for many years that he collects all the broken CK12-capsules that they routinely replace..

I don't quite get it - maybe there's another explanation - but some of the info over there smells like indirect smokescreen. Which may be completely understandable, trying to protect business. Just not good when disguised as information.

Anyway, it's a great forum - with some very hardcore microphone guys hanging out and contributing..

Jakob E.[/quote]

I read Klaus's forum I don't understand some of what is posted there as fact. I alway read with a "filter" sometimes you can't clean a capsule something happens at the screw term in neumans and at the edges. I have even taken the skin off a stretched capsule skin and placed it in a temp oven at 80C to see if it would shrink it did not.

The 701 link was posted as some possable information more about stamped codes on the tube.

I seem to remember reading ( perhaps on Klaus's forum ) that the older AC701'S were selected for noise and marked with the 'k' for klingarm or low ringing noise ( sounds too much like klingon for my liking - perhaps they were used in warp drives as well as mics! ) but that AFTER a certain point ( 1960something ) they did not mark them as the quality control meant that more of the tubes they made from that point were good.
Hmmm! Anyway, mine has worked well in the past.
Lots of helpful stuff in this post thanks!!!!!