Neumann M49C repair causing problems...

[Khron]

Ever heard of "marketing"? ;D You gotta sound "right" (no pun intended), if you wanna make it, in the audio world...

 

[ln76d]

It’s a solid state circuit, is it not? Why test them for 300 hours?

Simple marketing trick?
It's typical type of advertisement for pricey tubes, something what  many people would like to read paying few hundred bucks for single tube. It looks like it have no sense in case of solid state circuit but many people will believe in that.
Tube is a tube.

 

[rmburrow]

Hi Ruud !

did you ever used a russian 6S6B in a m49 or similar circuit ?

Looks like the 6S6B uses a 6.3 volt heater.  No problem to modify a NKM or NSM supply to 6.3 volts.  Few component changes.  In the mic, can't use bias from H+ unless you use a divider.  The 7586 nuvistor works nicely with some component changes; the 5703 triode works, the triode connected 5840 also works.  The late Oliver Archut liked the triode connected 5840.    Note of interest... Neumann used the 7586 nuvistor in the export U64 mics...the KM64 retained the AC701...  If you substitute out a AC701, make sure first the tube is the problem!  (Resistors, etc. can be noisy!)  Then proceed with the modification.  You may need to try a few tubes to find a quiet one, but a few 7586's won't approach the cost of a AC701..  Be careful substituting tubes where the capsule bias is obtained by IR drop off the cathode.  Excessive voltage on the capsule can lead to problems.

 

[rmburrow]

One more comment:  If you use a 7586 in place of a AC701, and the grid bias is obtained from a divider,  use 100 k metal film in the anode in place of the 200 k, and 2.7 k from cathode to ground.  I did not try bypassing the cathode.

 

[RuudNL]

Hi Ruud !

did you ever used a russian 6S6B in a m49 or similar circuit ?

I recently ordered 25 pcs. NOS in Russia.
Very curious how they will perform!

 

[micaddict]

Is that really you, Ruud? 

 

[RuudNL]

Never say never...  ;D

 

[granger.frederic]

6S6B or "6C6b" is the equivalent of 5703

https://frank.pocnet.net/short/054/1/129.pdf

be careful to not burn the psu trafo (200ma instead of 100mA)
transconductance (S) is twice higher also
grid leak is the major issue
don't expect it to sound like an AC701

for 7586 the tube is even more different, electrically and sonically.

for me, after A LOT of tests in my M367 , nothing can completely replace the AC701 which was design specifically for microphones (unlike VF14...)

various tubes works and pass sound but the magic is never completely here...If someone has a new idea....
i always founded that the U67 is a somewhat "undergraded" version of the M269c/M367

i didn't try the fet solution, but i'm curious, maybe a shootout....

my opinion is : everybody should go with substitutes for diy clones , and let the stock of genuine AC701s for original vintage mics
after all 300 to 500 euros for a 10k€ mic is not that crazy...

for what it worth...

 

[RuudNL]

The problem is that you can't be sure that a AC701k is a good one, if you can find them.
There are 'fake' AC701's in the market and even if they are not fake, you never know if they meet the original specifications, or if they are rejects.
And apart from that: most of my clients are not willing to spend hundreds of Euro's/Dollars to get an old microphone working.
(The microphone lovers will of course, but most people say: if it works, it is good enough for me!)

 

[granger.frederic]

i understand but the resale value for a vintage classic (10k for a M49c or m269c...) mic is highly degraded with a tube substitution (perhaps unsaleable) and the most important, the sound as well...
Ac701 (k or non k) are not extincted :

http://www.ebay.fr/itm/Original-TELEFUNKEN-AC701K-Microphone-tube-fur-Neumann-Schoeps-etc-NOS-/322316092766?hash=item4b0b89455e:g:gFgAAOSwcUBYGzRk

http://www.ebay.fr/itm/Original-TELEFUNKEN-AC701-Microphone-tube-fur-Neumann-Schoeps-etc-NOS-/322326740448?hash=item4b0c2bbde0:g:mo4AAOSw4shX39-X

http://www.ebay.fr/itm/TELEFUNKEN-AC701K-TUBE-FOR-VINTAGE-TELEFUNKEN-NEUMANN-SCHOEPS-MICROPHONES-/132013789045?hash=item1ebca28f75:g:UXAAAOSw7n9XEg63

even at 800 euros that's 10% of the price (nearly what Neumann charge for a M149 tube) and all items are returnable...

i add that i have a NOS ac701k selected by neumann (with certificate) ,  i installed a couple of non selected ,and i never found a difference...
however nothing hurts to find the better substitution...

 

[Khron]

Confirmation bias, anyone?

 

[rmburrow]

i understand but the resale value for a vintage classic (10k for a M49c or m269c...) mic is highly degraded with a tube substitution (perhaps unsaleable) and the most important, the sound as well...
Ac701 (k or non k) are not extincted :

http://www.ebay.fr/itm/Original-TELEFUNKEN-AC701K-Microphone-tube-fur-Neumann-Schoeps-etc-NOS-/322316092766?hash=item4b0b89455e:g:gFgAAOSwcUBYGzRk

http://www.ebay.fr/itm/Original-TELEFUNKEN-AC701-Microphone-tube-fur-Neumann-Schoeps-etc-NOS-/322326740448?hash=item4b0c2bbde0:g:mo4AAOSw4shX39-X

http://www.ebay.fr/itm/TELEFUNKEN-AC701K-TUBE-FOR-VINTAGE-TELEFUNKEN-NEUMANN-SCHOEPS-MICROPHONES-/132013789045?hash=item1ebca28f75:g:UXAAAOSw7n9XEg63

even at 800 euros that's 10% of the price (nearly what Neumann charge for a M149 tube) and all items are returnable...

i add that i have a NOS ac701k selected by neumann (with certificate) ,  i installed a couple of non selected ,and i never found a difference...
however nothing hurts to find the better substitution...

Neumann used the 7586 in the U-64 export version of the KM-64...Neumann must have thought enough about the 7586 at the time to build product using the tube.  AKG also used the 7586 in some of their product...  I agree... a work around for the AC701 needs to be found since it appears no one with deep pockets will fund JJ Electronic or some other tube manufacturer to build new AC701's from the old prints....

 

[useme2305]

a work around for the AC701 needs to be found since it appears no one with deep pockets will fund JJ Electronic or some other tube manufacturer to build new AC701's from the old prints....

that would be heaven. lets do it!

 

[e.oelberg]

I recently ordered 25 pcs. NOS in Russia.
Very curious how they will perform!

ah great ! I did that too, maybe we should open a thread on m49 with 6s6b, I forgot all my russian by I can still read Cyrillic if needed

 

[micaddict]

that would be heaven. lets do it!

Didn't Oliver Archut buy the old equipment?

Old prints or equipment even won't easily bring back the old quality AC701, mind.
And I'm afraid that's an understatement.

 

[RuudNL]

Yesterday I received my 25 pcs. 6S6B-V tubes.
I have tested them all, in a 'real world' situation (a microphone circuit).
Noise is very low and more important: the noise is very consistent.
The best ones have 0,5 dB less noise than average, and I have only one with a noise level 1 dB higher than average.
(Noise was measured with an "A" weighting network.)
I compared the noise of the 6S6B-V microphone circuit with my own low noise solid-state circuit, and the results were very close.
The 6S6B-V circuit was only 0,8 dB 'noisier' than the extremely low-noise FET circuit.

 

[Khron]

So the old adage is true - there IS more than one way to skin a cat, eh?

 

[granger.frederic]

Yesterday I received my 25 pcs. 6S6B-V tubes.
I have tested them all, in a 'real world' situation (a microphone circuit).
Noise is very low and more important: the noise is very consistent.
The best ones have 0,5 dB less noise than average, and I have only one with a noise level 1 dB higher than average.
(Noise was measured with an "A" weighting network.)
I compared the noise of the 6S6B-V microphone circuit with my own low noise solid-state circuit, and the results were very close.
The 6S6B-V circuit was only 0,8 dB 'noisier' than the extremely low-noise FET circuit.

you have soldered 25 tubes in a M49c  ?  oh boy....  :-[
noise is important but how does it sound ?

 

[RuudNL]

A microphone circuit doesn't necessarily mean a (M49) microphone.
And there are test sockets, you can push the wires of a miniature tube in...
Flea uses the 6S6B in their M49 remake, so I suppose it isn't the worst choice!

 

[rmburrow]

Did you try the 7586?  What results did you get?  Took about 5 of them to get quietest, couldn't tell difference between 7586 mic and AC701 mic sonically...