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Author Topic: Ac701k motorboating in M367  (Read 2787 times)

granger.frederic

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Ac701k motorboating in M367
« on: August 26, 2013, 06:03:24 am »

Dear friends,

i have changed a ac701k in my M367 , at the beginning everything was perfect but now, i have some motorboating at startup(which accelerate)
the tube was NOS (never soldered)
the new tube is doing the same oscillation in another mic
the previous tube solve the problem so i think it's not the mic or psu

I've mounted it very carefully with no soldering overheat.
the tube was burned in during 80h with no success

the voltages are: h=3.9v, b+=118v , 35v at the plate instead of 45, bias=1.6v, 60v at the capsule.

BUT the strange thing is that after a fast polarization change: cardio-) figure8-) omni-) cardio, the tube becomes stable and the motorboat goes away.
after that, the mic sounds quite good...

is there a way to fix that, or regenerate the tube (by overheating or ionization...)?

thanks
Kindest regards
Fred
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klaus

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Re: Ac701k motorboating in M367
« Reply #1 on: August 26, 2013, 01:10:55 pm »

Hello Fred,
Motorboating caused by a tube cannot be remedied through regeneration, because the underlying cause is a vacuum leak that can't be plugged or reversed by any means I am aware of.

You will need to switch out tubes (even if it's just re-soldering the older tube you replaced), to verify that indeed the tube is the culprit. I have had this a few times with AC701s, but typically motorboating in condenser mics is caused by leaky capacitors, i.e. a contaminated capsule, for example.

Therefore, before you remove the suspected tube, unplug the head and listen for the motorboating: if it's gone, the motorboating was caused by a contaminated or moisture-laden capsule.
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Klaus Heyne
German Masterworks®
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granger.frederic

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Re: Ac701k motorboating in M367
« Reply #2 on: August 26, 2013, 05:15:51 pm »

Dear Klaus,

thanks for your answer
i've unplugged the capsule and the motorboating is still present
the tube is clearly the culprit.
After a little overheating (4.2V) during 24h the plate tension has increased to a more normal 38V ....very strange...

but the weirdest thing, is, that the tube is long term stable and sounds good, BUT ONLY after a quick variation of the capsule polarization...
you said that there is no regeneration possibility
however, if it was a vacuum leak, the tube should always oscillate without a possible fix, isn't it ?


regards
Fred
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granger.frederic

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Re: Ac701k motorboating in M367
« Reply #3 on: August 28, 2013, 04:36:18 pm »

Hello,

i'm back with my bad ac701k.
i've tested a desperate regeneration but in vain: no fix possible.
the tube could work with some tweaks, but it would not be professionally usable.
I'm tired with buying some very expensive and rare tubes, so i actually experiment for a substitution.
I've decided to take my M367 for that....!
After one week, a lot of tests/recordings and various subminiature tubes, the closest thing to a good ac701 is .....;) : FOR MY EARS a 5718 with h+=5,9v bias=1,45v Va=48v
Sonically i find it extremely close, even with a loud source.
The input grid insulation of a 5718 is lower, so the very low end (below 40 hz) is reduced.
Even after tube selection and underheating, the tube is a little bit more noisy but very usable.
I know that Klaus will disapprove that, but i think that the day will come soon, when all our beautiful mics need a second life.
After all, it's better to have a near perfect mic, than no mic at all...
But if you like poker, like me, you could still buy an ac701(k) which has been rejected by Neumann in the 60's...
regards
Fred
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klaus

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Re: Ac701k motorboating in M367
« Reply #4 on: August 28, 2013, 04:43:52 pm »

Please (re-)read my AC701 primer: http://repforums.prosoundweb.com/index.php/topic,1150.0.html.
Plenty of AC701, especially during the last decade of production, cannot simply be identified as "rejects". And, while the number of Neumann-tested and approved select AC701 is very limited, chances are very good that at least one out of three non-selected specimens of AC701 or AC701k will pass muster.

As mentioned in the primer: get a decent money-back warranty from the seller, and you will still be better off with installing a quiet version of the tube for which the microphone was specifically designed. (You already verified the shortcomings of installing substitute tube versions.) In my opinion, A $10,000.- mic deserves a $600.- tube that, especially if you are conscientious with preventative maintenance, can last for the rest of the mic's life.
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Klaus Heyne
German Masterworks®
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granger.frederic

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Re: Ac701k motorboating in M367
« Reply #5 on: August 28, 2013, 06:21:58 pm »

Dear Klaus ,

I agree with the fact that a M367 deserves an original tube.
I' ve read the AC701 primer post BEFORE purchasing a tube that hasn't passed Neumann's validation.
But the fact is that my "New Old Stock" tube is unusable.
Sonically, i've found a substitution that suits me very well, and i don't wan't to buy a bad tube again (at 500€/600$ .......what crisis?).

Thank you Klaus for your interest and your precious help.
Best regards
 
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