investigating the russian 6S6B-V tube

[KrIVIUM2323]

I breadboarded the same Elam circuit as E.Oelberg, and ive got 53v at plate and cathode around 2v, aprox 0,65 ma through the circuit.

With 50k plate resistor and 3,9k cathode plate is at 85v and cathode at 3,26v so around 0,8 ma.
Heater at 6v and b+120v.

Sadly i'll have to wait a bit to hear how it perform this way... i miss the 12:1 transformer which is in one of my other mic in use at the moment... (i simulated the load with 29k resistor which should be close to t14/1).

I m curious to hear how the tube perform with this ratio and such a small core (i prefer big core and high nickel content for the 6s6b without feedback).

 

[kingkorg]

Try to inject different full range material through it at different levels. Acc guitars, drums, distorted guitars, pink noise! Eye opening experience.

 

[e.oelberg]

channel 1 is 6s6b channel 2 6072. I use fat BV14 by Max Kirchner ratio 1:11..

 

[e.oelberg]

I breadboarded the same Elam circuit as E.Oelberg, and ive got 53v at plate and cathode around 2v, aprox 0,65 ma through the circuit.

With 50k plate resistor and 3,9k cathode plate is at 85v and cathode at 3,26v so around 0,8 ma.
Heater at 6v and b+120v.

Sadly i'll have to wait a bit to hear how it perform this way... i miss the 12:1 transformer which is in one of my other mic in use at the moment... (i simulated the load with 29k resistor which should be close to t14/1).

I m curious to hear how the tube perform with this ratio and such a small core (i prefer big core and high nickel content for the 6s6b without feedback).

interesting, what do you expect to be the advantage ? I ran a 6s6b in a m49oid with a 6.5:1 tx without bass loss

 

[KrIVIUM2323]

So you used a big core transformer in your build. Ok i thought it was a t14/1!
Definitely there is some biasing in how i hear things in blind listening... Anyway, i think i will use your circuit value as starting points for  251 circuit. I like the results you have.

Well i have m49(b and c) kind of circuit already built too using 7:1 transfo (a sowter mic input transformer) and didn t experience loss of bass too. But there is feedback involved in both circuit i build so output z of stage is lowered  and it change the way the transformer is loaded...so in a bypassed grounded cathode amplifier the ratio needs to be higher.
I do think that for 6s6b with one between 8 /10 to 1 it should be ok, even better if it is one big core/high nickel content because there is low end with this tube. This is what i think make the difference in transformer needed for 6n16 and 6s6b ( and i prefer minimum 10/1 for 6n16 too).

Anyway i don t have any transformer with his ratio at home for now so it ll be speculation until i can afford some to test.

Back to m49, this discussion make take a detail i overlooked in my build: interelectrode capacitance.
There is difference between ac701 and 6s6b too, so c4 should be modified in value to compensate for this if you want to have close behavior. I will in my next build of m49b!

What do i expect from mod in bias, well... differences but i can t tell what they will be without test! From my experience with grounded cathode when you increase current (up to a limit) there is more details coming through, probably due to more second harmonic distortion or a different ratio of the different overtone created.
The circuit usually do seems to be quieter too. But i must say most of the test i made about that was in conventional preamp stage, not in mic impedance converter. Here the design are a bit spooky in using the tube at low current/voltage and with so high input z and results may well be differents!
Overall higher bias current give for my taste (so subjectively )better results.

But Kingkorg comments about his observation seems to agree with that idea...

 

[e.oelberg]

I'm not at all the one with proper electronic knowledge, I just wonder in the ac701 data sheet, the lower Ra seems to result in less distortion as far as I understand ?

If you want to start with the circuit I built, I used 100M grid to ground btw

 

[KrIVIUM2323]

Yes with same bias point in the ac701 datasheet, i didn t specified i change bias point but i have done (by ear) until i m pleased with what i listen to. And i don t dislike some amount of second harmonic distortion.

But maybe my first assumption is just plain wrong and the details i heard are just the opposite, less distortion (or a different ratio of even/odd)?

I m still learning too...

 

[e.oelberg]

great, we seem to be at similar places right now !

 

[KrIVIUM2323]

E.Oelberg,
What kind of m49 have you built? A,b,c or a variant (o.Archut)?
Does the circuit you used have feedback?
I ve tried with a 6,5:1 i ve got here (a cheap one that may not have enough inductance compared to a nice one ) and without feedback i do find it light on bass, but it is usable.

I m lazy to dismantle my 47ish mic using cinemag nico 2461 to test...

Kingkorg, i'll try this.  You did using your schematic (120v b+, 50k anode load, 3,9k cathode/47uf, 10:1 transfo) for this test?
At which level and through which capacitance did you sent signal, and with which value or rgrid?

I decided to redo a better psu for my bench to test all this. I ordered parts and hope to have them in weeks to come.
I m now curious do to more serious exploration than i did about tube mics headamp.

 

[kingkorg]

You are right, i settled for 50k, but 2.4k bias resistor. However while experimenting with values i had potentiometers in place of these two as ln76ed recommeneded for testing. I did try cheap 5:1 trafo from sterling audio st51, and it worked great with 50k plate resistor! It didnt work well with 100k.

I used 50meg grid resistor, i am going for vintagey values. And 50pf cap to inject signal, up to 400mv output signal from my audio interface. The key is to inject lower level signals as well, i rarely use tube mic where so high level is recorded. And properties change with levels. Vocals are i think nowhere near that level.

I even thought about leaving a switch that changes 50k>100k and maybe two biasing resistor values switch as well, to manipulate character on the go. However one member even made fun of idea. His opinion was that as clean as possible is the best solution. But then we would get close to Gyraf mic values, and that would be it.

And yes. If that 50pf cap is used for injection, the whole thing needs to be shielded, and that can be tricky. If you can't pull it of, just inject signal directly without cap. You don't need shield then.

 

[KrIVIUM2323]

Kingkorg, thanks for the info!
Your observation about changing ra and output transformer does make sense: more current through the circuit lower rp (internal plate resistance) and this being in //with ra this lower circuit output resistance so a lower ratio transformer can be used.
The more i think about it the more i think there is lower distortion with more current (the opposite of my first assumption bout that). This may well do change the harmonic distortion profile too.
The issue that may arise could be with grid leakage current though...

400mv seems indeed like a lot, but this may well happend with close miking vocals and an 'hot' singer.

I still wonder what is the "typical" output voltage of LDC (i know it does vary with capsule type, rgrid and bias and probably tube spec -probably cgp, cgk dependent).

Your idea of changing parameters of mic using switch is valid in my view. Probably not allround with each settings but that is the point. Afterall, we sometimes switch to colored mic preamp for some duties...

And you can t please everyone, someone will always critisice your choice, that wont mean this is right or wrong way of doing things.
If you are pleased by outcome keep the feature! If not you gained experience...
I do think i ve seen this kind of feature on some mic from known brands... (a bit different than what you are doing but Chandler's 'abbey road' mic have different modes availlable trough a switch).
And Iaudio does include a 'deemphasis disabled' feature on his u87 to 67 mods... some may argue this is technically wrong, other will be pleased to have bonus 'c800g meet neumann' mode...

 

[e.oelberg]

girls'n boys here is a stereo recording with my elam clones both with 6s6b equipped. I's a steinway played with my robot. If you think the piano player is a little stiff you are absolutely correct
https://wolke7.grandprixdamour.com/index.php/s/N8fpHBbZVTE66CZ/download

 

[kingkorg]

Sounds awesome!

 

[e.oelberg]

I think so too. I adjusted the positions of the mics now for even better balance of sound,  I'm pretty blown away.  AND I cannot pick one mic from the other, they sound the same. Recorded are the mics with a sound devices mixpre-3

 

[KrIVIUM2323]

I agree it sound awesome!
I suppose you changed cathode bypass capacitor value, the mics seems less sensible to low end.
With which values did you end for the cwhole ircuit? Have you modified biasing (related to previous llast sample you give us)?

Nevertheless i like your robot too! Yes it is a bit stiff but not really different from some players i ve heard!
I ll have to wait one more month for my new psu to come, custom transformer build is delayed. I hope this is worth the wait.

 

[e.oelberg]

only bias resistor 3.6kohm, but I will change the 47mF panasonic fc to a 22mF tantal. The change of bass response is due different microphone positioning. Of course omni.

 

[e.oelberg]

here comes another file stereo recording, one channel is a akg 414EB with Telfon capsules, the other one is the diy elam with 6s6b.
guess which is what ?
https://wolke7.grandprixdamour.com/index.php/s/li30TnJSgcHAekX/download

 

[KrIVIUM2323]

Well, it is difficult...
First it seems there is a level diference between both chanel, right chanel is lower level.
Maybe this is because of what is played: range of notes is localised more to the center/left of the keyboard?

Anyway for me impossible to spot the tube mic from noisefloor cues!
All in all, i prefer the left chanel but it could be only level related as it appears louder (maybe a bit more compressed). I would say this is the tube mic, but i ve been proven to be wrong previously.
There is enough difference between them to not sound like a pair to me though.

E. Oelberg, it could be easier to determine with same track recorded two time with  pair of each kind of mic, rather than a mix of both type together.

 

[e.oelberg]

E. Oelberg, it could be easier to determine with same track recorded two time with  pair of each kind of mic, rather than a mix of both type together.

that's true, but I sold my second 414eb due to a dry summer a couple of years ago, so I'm afraid I can't do that. I didn't properly match the levels, for myself I split the stereo and normalized both. Good enough for jazz. Resaon why I chose the EB is that it sounds similar and it does. The mics are three meters away from the piano and maybe 25 cm apart from each other, I doubt microphone position is the main factor why they sound different.

 

[KrIVIUM2323]

Ok i understand.
Well all i can say for now is you ve got a very nice sounding room!
I don t know the eb well, i ve used a pair maybe 10 years ago and it was a ck12 brass capule in it.
Will try to match level and play again later...