Neumann M49V tube mistery...

[kingkorg]

Guys, am i just tripping or is this just a variation on c12 type circuit? I see 100k plate resistor, 2.2K cathode resistor. If this is indeed 6s6b-v tube or a close sibling, and my assumption on the topology is correct, the tube is not properly biased with these values. Is this even a m49 related mic, or just approximation? Seems close to Oliver Archut's alternate take on m49. Grid resistor is 470Meg, i'd say that might be too high!
Does anyone see a pF range feedback cap anywhere? 22uF cathode cap doesn't help with low end.

Edit:
It seems to be m49c topology, but i suspect biasing and high grid resistor might still be an issue.

 

[GeorgeToledo]

Does 6s6b-v give impedance in the circuit that can drive a real M49 output transformer properly?

 

[Purplenoise]

I was under the impression by reading here that the tube runs at 4V but the manual states H+6.3v and B+120v

 

[GeorgeToledo]

Most all of these related subminis run at 6.3, some people here have just been underheating it.

 

[GeorgeToledo]

 

[granger.frederic]

Does 6s6b-v give impedance in the circuit that can drive a real M49 output transformer properly?

6S6B-V has around half the Ra of the AC701, this induces a variation on how it drives the transformer, not necessarily in a bad way, all other parameters considered...
But the 6S6B-V is quite different from a AC701 and may need some adjustments to perform OK in a AC701 circuit. By the way, I'm puzzled by the way Neumann has biased it...
Anyway, the heat and the grid leak are still an issue and can be problematic for some applications.
The grid current can change the proximity effect and can be worse when the tube's getting old.
But according to Klaus , it's not a 6S6B-V...

 

[kingkorg]

Anyway, the heat and the grid leak are still an issue and can be problematic for some applications.

Whatever tube it is, it glows like crazy, and i guess heats like most of the others close to 6s6b. The filament current and voltage in the specs confirm this. That cathode cap won't be happy. Higher value grid resistor having high grid leak in mind isn't a great idea either i guess...

 

[mrbutterskin]

6S6B-V has around half the Ra of the AC701, this induces a variation on how it drives the transformer, not necessarily in a bad way, all other parameters considered...
But the 6S6B-V is quite different from a AC701 and may need some adjustments to perform OK in a AC701 circuit. By the way, I'm puzzled by the way Neumann has biased it...
Anyway, the heat and the grid leak are still an issue and can be problematic for some applications.
The grid current can change the proximity effect and can be worse when the tube's getting old.
But according to Klaus , it's not a 6S6B-V...

Datasheet values for the 6s6b and ac701 show that in the m49c circuit (~43V, .73ma) these two tubes have roughly the same plate resistance and gm. Measurements confirm this. It might be different in other circuits though.

As far as grid current goes, the datasheet value for the 6s6b is a worst case scenario. I've measured dozens of these and the vast majority of them have less than 100pA grid current, which is the datasheet value of the AC701 for grid current

 

[kingkorg]

Datasheet values for the 6s6b and ac701 show that in the m49c circuit (~43V, .73ma) these two tubes have roughly the same plate resistance and gm. Measurements confirm this. It might be different in other circuits though.

As far as grid current goes, the datasheet value for the 6s6b is a worst case scenario. I've measured dozens of these and the vast majority of them have less than 100pA grid current, which is the datasheet value of the AC701 for grid current

This doesn't seem like m49 circuit though. 22uF cathode cap, are they trying to get rid to the low end?

Ignore, it's a m49c variant.

 

[mrbutterskin]

I agree, there's a number of differences in this new mic by neumann. I just meant for the vintage m49c circuit.
Edit: seems to only be component value differences. circuit actually looks the same/very similar

 

[Purplenoise]

This doesn't seem like m49 circuit though. 22uF cathode cap, are they trying to get rid to the low end?

Looks like an m49c cirquit. If you look at the schematic of the c version you’ll see a 20uf cathode bypass cap

 

[kingkorg]

Looks like an m49c cirquit. If you look at the schematic of the c version you’ll see a 20uf cathode bypass cap

Thanks, i'll edit my posts to avoid confusion. Still pretty high grid resistors.

 

[Gus]

Maybe a the tube used in the m49v could be used in the microphone in this thread at another forum

https://www.tapatalk.com/groups/prorecordingworkshop/u47-b-t19415032.html

 

[GeorgeToledo]

Maybe a the tube used in the m49v could be used in the microphone in this thread at another forum

https://www.tapatalk.com/groups/prorecordingworkshop/u47-b-t19415032.html

Neumann looked into whether or not they could build one of these in their “MadeToOrder” shop for me. Their main guy checked twice, but they can’t put it together. I think more than anything it is because these decks were point to point construction and they aren’t up for that.

I was somewhat interested because it would have sidestepped any comparison to M49 while being an “official” Neumann built mic, using k47/49. I figured I would have had no problem flipping it if I didn’t like the result.

One could probably gut an M147 and have a headstart to something similar. I’d probably go ahead and put a decent full size tube in any build like that though.

 

[granger.frederic]

Datasheet values for the 6s6b and ac701 show that in the m49c circuit (~43V, .73ma) these two tubes have roughly the same plate resistance and gm. Measurements confirm this. It might be different in other circuits though.

As far as grid current goes, the datasheet value for the 6s6b is a worst case scenario. I've measured dozens of these and the vast majority of them have less than 100pA grid current, which is the datasheet value of the AC701 for grid current

I unfortunately had a different experience.
From a batch of 20 6B6B-V, only 18 (great performance) were noiseless and low microphonic (tap on it , it will sometimes ring like hell),
But none can go easily to rumble frequencies, unlike the AC701 i have.
I thought it was the grid current .
Maybe i did something wrong in the settings...How did you bias it ?
How did you test the grid current and measure the pA ?

 

[Gus]

Neumann looked into whether or not they could build one of these in their “MadeToOrder” shop for me. Their main guy checked twice, but they can’t put it together. I think more than anything it is because these decks were point to point construction and they aren’t up for that.

I was somewhat interested because it would have sidestepped any comparison to M49 while being an “official” Neumann built mic, using k47/49. I figured I would have had no problem flipping it if I didn’t like the result.

One could probably gut an M147 and have a headstart to something similar. I’d probably go ahead and put a decent full size tube in any build like that though.

That is odd
Buy some thin garolite (G 7, G9, G10) from McMaster Carr or other companies
https://www.mcmaster.com/garolite/Machine the garolite(garolite dust is nasty I wear a N95 when I have cut it and also run a HEPA vac for the dust) It also rapidly dulls tools
Drill some holes install some keystone tab or turret etc. connections.
Wire it up

Or cut up no copper pads Vector perf board. I build microphones with perf

Buy one of made in china 47 bodies shorten the body and 3 supports drill the needed holes

 

[RuudNL]

I don't see the well known 'Neumann quality' in the assembly.
I was also surprised that the grid wire was connected directly to the PCB, no teflon feedthroughs.

 

[Matt Nolan]

Probably the core alloy is not the same as the vintage Bv.11. So, generally speaking, ratio, inductance, DCR and capacitance can be "to the spec's", but it's hard to replicate the magnetics resistance of the alloy.

I would tend to agree. Having compared old stock AKG T14/1 with newer, AKG produced T14-1 a few years back, the frequency response and headroom were vastly different in otherwise the same circuit. Alloy formulation for the original laminations is just not available any more. It would not surprise me if the same was true with Bv.11 trafos.

 

[Tubetec]

Does each mic come with its own NDA clause to sign before they accept your bid ?
Has anyone had a stab at possible circuit diagrams for the AC701 version in the pics , looks like most of the components are on the top board .

 

[Tubetec]

Teflon washer pipe seals come in a variety of sizes , miniature pcb rivets can be installed to support the components according to mounting style and pitch , only hand tooling required , its not rocket science ,well it kind of is actually .
Agreed Ruud ,strikes a bum note for me on the construction ,not properly using the insulation properties ,why bother spending the money on the standoffs in the first place.