investigating the russian 6S6B-V tube

[aazaa]

Isn’t the 6s6b half of the 6n16b?
Which voltages and current did you get with the 1k8 cathode and 100k plate resistor?
Was it around 43v, 1,2v and 0,73ma?

The 6s6b-v drives the same load. My choice is to let it draw the same current.
Low signals at the grid, no issues with distortion.

Measurements with 1k8 cathode resistor:
B+ 119.6V
anode 44.7V
current 0.75mA.

I like the sound of 6S6B-V a lot but it differs a bit from AC701. AC701 is a bit more relaxed in the bass, generally more detailed and more extended in the top.

If you want more detail and extension I would suggest to change the value of the 600pF cap or to omit it.

Anyway, it will be interesting to see the choices of Neumann.

 

[tomas.borgstrom]

If you want more detail and extension I would suggest to change the value of the 600pF cap or to omit it.

I either omit 600pF or use a lower value. I depends on the capsule as well.

 

[RuudNL]

The Neumann M49-V uses a cathode resistor of 2.2K. Plate resistor 100 K. Capacitor (plate-to-ground): 470 pF.

Picture: ©2022 Klaus Heyne.

 

[Khron]

Has anyone researched the effects of that cap going to ground, versus that cap across the plate resistor? Just thinking that simply shunting the high frequencies to ground would induce more distortion, than just "making them go away" in a less brute-force manner (as in, not having them amplified in the first place).

Surely there are no feedback-like effects going on, are there? I'm definitely no expert, that's why i'm posing questions

 

[RuudNL]

Yes, I did measurements and the plate-to-ground capacitor increases indeed the distortion.
(A capacitor in parallel with the plate resistor would do the same, since the supply voltage is supposed to have a low AC impedance.)

 

[Murdock]

The 6s6b-v drives the same load. My choice is to let it draw the same current.
Low signals at the grid, no issues with distortion.

Measurements with 1k8 cathode resistor:
B+ 119.6V
anode 44.7V
current 0.75mA.


If you want more detail and extension I would suggest to change the value of the 600pF cap or to omit it.

Anyway, it will be interesting to see the choices of Neumann.

Thanks for the measurements.
They seem to correspond with the 6N16 load line calulator figures. Triode / Pentode Loadline Simulator v.1.0 (20161216 www.trioda.com)
There it seems as the 1k8 ohm cathode bias produces less distortion than 2k6.
When going for "vintage correct" biasing, 1k6 cathode resistor and 116V as per C revision seems to be the best fit.
That would be 43v on anode and 0,73mA as per schematic.
But instead of 1.6V on grid it would be 1.2V...
This configuration has even less distortion than 120V and 1k8 cathode resistor.
Also plate impedance is lowered and fits a 7:1 transformer for 200 ohm output impedance.
For a 9:1 or 10:1 transformer the first M49 revision would be more suitable with 200k plate resistor and 3k2 cathode resistor.

 

[tomas.borgstrom]

Thanks for the measurements.
They seem to correspond with the 6N16 load line calulator figures. Triode / Pentode Loadline Simulator v.1.0 (20161216 www.trioda.com)
There it seems as the 1k8 ohm cathode bias produces less distortion than 2k6.
When going for "vintage correct" biasing, 1k6 cathode resistor and 116V as per C revision seems to be the best fit.
That would be 43v on anode and 0,73mA as per schematic.
But instead of 1.6V on grid it would be 1.2V...
This configuration has even less distortion than 120V and 1k8 cathode resistor.
Also plate impedance is lowered and fits a 7:1 transformer for 200 ohm output impedance.
For a 9:1 or 10:1 transformer the first M49 revision would be more suitable with 200k plate resistor and 3k2 cathode resistor.

My testing was very basic. I used sine waves all over the spectrum aiming for symmetric clipping and the least amount of second and third harmonics. I use a 10:1 transformer and 100K plate resistor. To be honest I can't really hear any clear difference changing to 2K4 or 2K2.
It makes no sense to me that Neumann is using the exact same circuit with a different tube.

 

[aazaa]

It makes no sense to me that Neumann is using the exact same circuit with a different tube.

You're probably right about Neumann. In the past they changed resistors with a new different tube or a different transformer.

Neumann's choice not to change the circuit seems strategic. (Anyway, 2k2 is ok. Differences are minimal, barely or not audible.)
The M49 is hyped, people want to buy a M49 identical to the old one. There may have been a concern that, with a changed circuit, many will consider the old microphones as better. Now, with the same circuit, most will assume that the ac701 and the new 'secret tube' (unlabeled) are completely interchangeable and have the very same sound.

Strategy was probably also one of the reasons to consult Klaus Heine and follow some of his (good!) advice before releasing the new microphone. The danger of the 'ultimate guru' rejecting the new microphone was gone. He is on their side. He also follows them in hiding the information about the new tube. Would have done the opposite had he not been involved in the development process.

Strangely enough, Klaus had no objection against the position of the cathode capacitor close to the very (too?) hot tube (not sure, but it seems not underheated?).

 

[aazaa]

Low heat inside the vintage Neumann M49.

The Telefunken ac701 is designed as a microphone tube with low heating of the filament. Underheated filament is part of the concept.
The tube is used in the M49c at a very low wattage.
(43-1.6)x0.73= 30mW

At only 30mW you can fearlessly do the same and underheat the filaments of replacement tubes (e.g. the 'Neumann mystery tube', 6s6b-v, or others). Under this low power conditions these underheated tubes will perform fine and last a long time.

Low temperature in the microphone is beneficial for all components, the capsule included.

 

[Papa Tango Charly]

The Neumann M49-V uses a cathode resistor of 2.2K. Plate resistor 100 K. Capacitor (plate-to-ground): 470 pF.

Picture: ©2022 Klaus Heyne.

Hello Ruud when you are mentioning the Plate resistor is it R3 ? or another place? / Cathode resistor of 2.2K on R7 / Capacitor (plate-to-ground): 470 pF is C6 Is this right?Is there any other changes between the M49C and the M49V Thank you Best. Merry Christmas!

 

[Khron]

Low heat inside the vintage Neumann M49.

The Telefunken ac701 is designed as a microphone tube with low heating of the filament. Underheated filament is part of the concept.
The tube is used in the M49c at a very low wattage.
(43-1.6)x0.73= 30mW

At only 30mW you can fearlessly do the same and underheat the filaments of replacement tubes (e.g. the 'Neumann mystery tube', 6s6b-v, or others). Under this low power conditions these underheated tubes will perform fine and last a long time.

Low temperature in the microphone is beneficial for all components, the capsule included.

Doesn't most of the heat come from the heater, anyway, in most tubes? Maybe except for power tubes...

400mW for the AC701 heater kinda dwarfs the 30mW plate dissipation...

 

[My3gger]

Miniature tubes like 6SN7 or ECC88 can have pretty high anode dissipation compared to heater dissipation, it depends what they are used for.
Input stages run them at low anode currents for high gain, drivers at high anode currents (translating to higher power) in order to drive heavy loads. JJ datasheets are quite useful for this kind of calculations.

I wish all of you guys a marry Christmas

 

[aazaa]

Doesn't most of the heat come from the heater, anyway, in most tubes? Maybe except for power tubes...

400mW for the AC701 heater kinda dwarfs the 30mW plate dissipation...

You're right!
My point is that you don't need much heat in the 6s6b-v when you only want to draw 30mW. Why not follow the excellent example of the ac701?

 

[Khron]

Which "excellent example" are we talking about?

 

[aazaa]

Which "excellent example" are we talking about?

???
The ac701 is intended by Telefunken for use with low heated filament in microphone applications. Other low noise miniature tubes can as well be used in microphone applications with low heated filaments. I can advise to do the experiment, try it...

 

[Papa Tango Charly]

Could anybody tell what changes between i should make on the the M49C Schematic to buy a M49V with a 6S6B-V Thank you Best. ( Can you mention the placement on the M49C schematic R... C....etc...) thank you to you all and Merry Chistamas

 

[soapfoot]

I know Klaus Heyne said definitively that the tube in the M49V is not a 6S6B.

Is a 6S6B-V considered a different tube type? Or is it more like “12AX7A” compared to “12AX7”?

 

[rock soderstrom]

Is a 6S6B-V considered a different tube type?

It is technically the same tube. The letters after the dash stand for quality selected or ruggedized long life tubes for military or special industrial applications.

 

[kingkorg]

I know Klaus Heyne said definitively that the tube in the M49V is not a 6S6B.

Is a 6S6B-V considered a different tube type? Or is it more like “12AX7A” compared to “12AX7”?

It's a military spec 6s6b. There's yet another variant with even lower tolerances, and even better performing, identical physically to 6s6b-v and in all other aspects.

 

[soapfoot]

Thanks! That would seem to suggest, then, that the 6S6B-V is not the tube in the M49V, internet speculation notwithstanding

Incidentally, our studio just got a pair of M49V, and we have another pair en route.

I’ve only heard them very briefly, but can confirm they sound fantastic on piano