investigating the russian 6S6B-V tube

[KrIVIUM2323]

Except that i ve got the same plan?  ;D
Well, why change cathode resistor to 3,9k?
It may work the same as original (i would say it will depend of heater voltage you ll use for the 6s6b).
Which kind of capsule you are going to use? This could dictate rgrid value, maybe lower than 150M could work too, 8M seems low but 30M like in Elam e? It may depend of output transformer too.

 

[e.oelberg]

I do have two  diy Elam 251 E  with 6072. On has a very good sounding ge 5 star inside. for the other one I went through lots of tubes with no success. Quality wise a Telefunken ecc801s is comparable, if not better to my ears. Nevertheless they do sound different, not much though. So I want to change for the same tube. The  6s6b is consistent low noise and sounds great. I can by 100 6s6b for the price of one ge6072 5 star. I use Tim Campbell ct12


the russian anyway made  the best tubes

 

[KrIVIUM2323]

I probably wasn t clear i do agree that 6s6 is a nice cheap tube (and most last russian tube produced for military are amongst the best ever made) and may be nice as substitute to ac701 in an Elam (non E  version).

As you are using a 'real' ck12 (clone) what i meant about rgrid is to start from the value of an non E version and then go up in value if you find it too noisy or lacking low end. Maybe even try the 8M value if the mic purpose is for vocals it can work great and could make the mic even less sensible to moisture noise.

Anyway i do think this is worth trying.

 

[e.oelberg]

I give it a try with lower mega ohm resistors. Right now I have 200M inside. I once bumped into Andreas Grosser and he said contrary to the schematics ELAM dropped much higher grid to ground resistors into their mics, he literally said: it's a mistake in the schematics. Anyway I will start with the least modifications to my mics, leave the 200M in place and just change the bias

let's keep up with our builds !

 

[kingkorg]

I did this with 6n16b in paralel and as single triode, two mics. I did have to lower plate resistor to 50k, don't know if its necessary with 6s6b.

Didn't notice any diference soundwise above 51meg for that resistor. I did null tests after every change.

 

[KrIVIUM2323]

Maybe it is error in schematic or no, i dont know... i remember a thread where MsVienna said he had a discussion with former Akg engineer which said the low rgrid was on purpose to lower sensitivity to 'moisture noise' when mic was used for vocal duty.

Well anyway nothing to loose to try.

Kingkorg, for me 6s6b-v and 6n16b are different tubes. They can be close (on paper) but the one i ve got are different in practice. Both are great but for different purpose imho.
About rgrid it will depend of capsule too, it is a system including tube/resistor/capsule. How high in value did you tryied and with which capsule?
From my experience with k47 difference past 200M are audible, k67 up to 500M.
I never tried lower than 100M with both, i will in the future.

 

[kingkorg]

It was c12 style from 797 audio that ended up in that mic. K67 (797) had the same response down to 10Meg, where c12 (Rayking) started showing differences there both in high and low end.

Which of these tubes would you use in which case, besides obvious triode/pentode difference, and how do you feel about how they sound in respect to each other. Off course 6n16 used as a single triode,

I ordered both at the same time, and 6n16 came first, so i ended up installing that one in all of the mics by the time 6s6b came. So 6s6b are just lying around now.  It was interesting to experiment with it as i could reconfigure it in single triode/paralell/cathode follower. Additionally no one mentions it in microphone world, so... But i guess you prefer 6s6b as we are talking about that one here

 

[KrIVIUM2323]

You talk about triode/pentode difference but i don t see why, both are triode, maybe we talk about different tubes?

Anyway, the 6n16 i have are close relative to 6021 tube, and i think you can see them as small 6sn7 (with smaller allowed b+ ratings). I like them in CCDA (obviously), CF or used as GC (but not paralleled, i ve seen practical issues long term with parallel tube so i usually don t use it) in tube mic.

Overall i think they are quite ok but a little bit more microphonic that the 6021 (at least for the one i ve got) and sometimes a bit metalic/colder in the high end (i do think this is related to microphonic too, maybe a mechanical resonnance in the tube build or structure). Maybe this is related to the one i own only i don t know, but the 6021 i own are better with this.
Beside this and the highish heater current needed, they are quiet tube with a nice low end and mid range for my taste.
I had good results in CCDA or as more classical GC capacitor coupled to a CF (a la Groove Tube).

6s6b are close relative to 5703. I use only as GC and in circuit which originally need an ac701. I tried in CF but i prefer the 6n16 for that.
I do prefer the 6s6b to 5703 i own as they are quieter.

Difference between the two in sound is difficult to explain clearly because they don t need the same transformers in circuit so it is a bit apple to orange comparison in my view. But in GC i would say i prefer the 6s6.

I would say try the 6s6 with a 7:1 transformer and tell us what you think of it, i m curious of what you ll think of it.

 

[Murdock]

Just wanted to ask, what transformer ratio/impedance is the best match for the 6S6B. 7:1 is the original BV11, right?
I also read somewhere it's 11:1. Don't know which one is correct.
Did the ELA M251 with AC701 have a different T14/1? Because the "stock" one has around 12:1 ratio. So 11:1 BV11 seems to be more correct...

 

[KrIVIUM2323]

Hi,
Yes bv11 is 7:1 ratio.
And yes T14/1 is 12:1.
In fact there is no best match... there is a lower limit below which the transformer will not have enough inductance so you ll have not enough low end, but you can use higher ratio than that. The drawback in using higher ratio is that it will lower the max output voltage the circuit is able to produce, but it ll present an easier load to the tube -in other words it lower distortion-(and more current to drive cable capacitance).
So it will depend of goal of circuit designer, what he want to achieve and the choice made.

For the 6s6 i do think the lower limit (given the transformer have enough inductance, is good quality) is around 7:1 (put differently i didn t try with lower ratio than 7:1... and in a circuit with feedback, which lower impedance too (m49b/c circuit)).
Elam (different than most Neumann that use ac701 which does use feedback one way or another) doesn t have feedback involved around headamp (cathode resistor is bypassed by a cap) this explain in part the need for higher ratio output transformer too.

About bv11 specificaly, there have been multiple variation of it during life of m49 and other type of transfo have been used too in the 4 versions of m49 i ve seen (the nwdr one with msc2 tube, the first generation with ac701 (both of them without feedback or lesser amount), then the b and the latter c version) and probably as much variation of the out transformer... Maybe the 2 first version used higher ratio transformer?
From the info i gathered, 7:1 is the correct ratio for the b and c version.
In fact i think the confusion about bv11 ratio come from the conversion proposed by the great O.Archut where he used an headamp without feedback (more or less based on the two first version) using his own version of bv11 or t49 (don t know the ratio of bv11r but i suppose it is the same as t49, around 10:1)...
But i may be wrong.

 

[kingkorg]

You talk about triode/pentode difference but i don t see why, both are triode, maybe we talk about different tubes?

I really shouldn't be posting that late at night. I was thinking triode/double triode not pentode. Thanks for great answer.

 

[RuudNL]

For what it is worth: I recently replaced a defective AC701 in a Neumann M269C with a 6S6B-V tube.
Of course I changed the heater voltage and slightly adjusted the cathode resistor for optimal bias.
The microphone sounds great and the self noise is extremely low!

 

[Murdock]

For what it is worth: I recently replaced a defective AC701 in a Neumann M269C with a 6S6B-V tube.
Of course I changed the heater voltage and slightly adjusted the cathode resistor for optimal bias.
The microphone sounds great and the self noise is extremely low!

Sounds promising! How did you adjust for optimal bias?

 

[kingkorg]

I did change 6n16 used as single triode for 6s6b in a mic with 12:1 3u audio transformer.

I injected various full range signals with various levels and recorded before and after.

Just swaped tube with nothing else changed (circuit optimized for 6n16) 2.1vdc cathode, 69vdc anode, 6s6b had more 2nd order harmonics, and less 3rd order. Also slightly higher noise in midrange compared to  6n16. It seems like it has softer higher mids, is a bit less hifi sounding and has slightly more low end than 6n16. While frequency response remains the same. Almost like some kind of multiband compression

I guess it's a game of harmonics. Can hear it, but can't put a finger on it.

I can't believe how non-microphonic 6s6b is!

When i did null tests i heard sweet and very controlled overtones it creates. It reminded me of some null tests i did with UA LA610 vs solid state preamp.

I guess i could conclude from the test that 6n16 is slightly cleaner and transparent. Maybe bit brighter without FR showing any difference. I have before experienced testing it against FET circuit with same capsule and body that it sounded almost the same as FET. Nothing spectacular happened in null test either.

When bias adjusted for less THD with cathode resistor about 1.7k i felt like part of the magic disappeared. Also, i got slight low end roll off, -1db @50hz. So i settled with 2.2k bias resistor. 6s6b will remain in this mic as it for lack of a better word realy does sound 'warmer' and is so quiet that i can't believe it's a tube.

All of this being said, they are close and in real world i wouldn't mind using any of them. For me it comes down to how they are used, what type of circuit, if one needs double or single triode. And yes, heater current.

I am though puzzled that these tubes don't attract more attention, they are awesome.

Question to Ruud, do you always adjust bias for minimum THD?

 

[stelin]

"I can't believe how non-microphonic 6s6b is!"
Same as my experience. 
Ruud: I also would like to know how you adjust the bias for minimum THD.

 

[e.oelberg]

I once did it by  injecting a signal via a capacitor, put a 40pF cap  in place of the capsule, crank it up till you see the distortion (or hear it) and than adjust the pot for minimum distortion.

 

[RuudNL]

This is how I do it:
I connect a low distortion sine wave generator to the grid of the tube (through a small capacitor).
The output of the microphone is connected to a distortion analyzer.
Then I use a trimpot as cathode resistor and adjust for minimal distortion, while observing the plate voltage.
I noticed that with tube circuits the adjustment is way less critical than with FET circuits.
With a FET circuit I usually get a sharp 'dip' in the distortion figure at optimal setting, but in a tube circuit there is a much wider range. Which is good, because tube parameters change (a bit) with ageing.
In the past I was never a big fan of tube microphones, because they were always significantly more noisy than solid state microphones, but after I 'discovered' the 6S6B-V, my opinion has changed...
By the way: did someone ever use the EC70/71 triode?
I found that Oliver Archut once wrote that this tube would be a perfect alternative for an AC701 tube...
http://www.neumann.com/forums/view.php?bn=neumann_archive&key=994314599&v=f

 

[e.oelberg]

Ruud you say it with better words, but basically the method is the same, I just have no real tools, so I use audio analyzer on my mac
as well as a sine or square wave . Nevertheless I move towards using my ears instead. So my bias in my m49 is 2.7k, low distortion would be more 3.5k

 

[Recording Engineer]

By the way: did someone ever use the EC70/71 triode?
I found that Oliver Archut once wrote that this tube would be a perfect alternative for an AC701 tube...
http://www.neumann.com/forums/view.php?bn=neumann_archive&key=994314599&v=f

Very interested in anyone’s results with a 5718 as well! I remember Scott Humphrey’s post, but that’s about all I’ve heard from anyone:
https://groupdiy.com/index.php?topic=55012.0

He mentions the 6533 too.

 

[e.oelberg]

here is a small test. 2 almost identical elam E 251 one with 6s6b and one with my best GE 5 star  6072, no adjustments are made to bias. I do think adjustments have to be made though, but just for the fun pick the 5 star
https://wolke7.grandprixdamour.com/index.php/s/3jKjFhibYhG78Ek