Voltages in an original AKG ELA M251 AC701 version?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

rock soderstrom

Tour de France
Joined
Oct 14, 2009
Messages
4,282
Location
Berlin
Hi guys, I am currently working on a cardioid only DIY ELA M251 style mic with a 6S6B tube. I'm thinking about the correct biasing of the tube, I'm orienting myself on the AC701 version, I currently have a B+ of 120V, Ra=200k and a cathode resistor of 3.6K (it's actually a 5k pot).

I would be interested in the voltages in the original ELA M251 with AC701 tube. Is the B+ really 120V? (I can't read it in the attached schematic)
What is the voltage drop across the cathode resistor, what is the voltage potential at the anode?

The total current in the microphone appears to be 1.1mA according to the schematic.

I searched for it on google, but found nothing. Can anyone help?

1000044543.jpg
 

Attachments

  • Elam-251-251E-Schematic.pdf
    149.5 KB
Last edited:
@rock soderstrom

As far as I search, this one is the only one 6S6B (6C6B) tube datasheet that I could find with grid curves, so I attached here with the hope that maybe helps you for drawing the load line…

Also as you know, there isn’t any extra help for those Soviet Russian tubes by the online tube calculators, so if you know any American or European Sub. Tube very similar to the Soviet Russian 6S6B (6C6B) it will helps more to calculate the Grid Bias Voltage (V), the Iq(mA at 60 Vq(V) and by them the correct cathode resistor if the Vb is 120 Volts…
 

Attachments

  • 6S6B.gif
    6S6B.gif
    114.6 KB
Okay, the user manual confirms the B+ of 120V for the AC701 version. The total current consumption is therefore a more realistic 0.5mA!
1000044808.jpg

Screenshot taken from attached german/englisch manual
 

Attachments

  • akg-elam250.pdf
    918.2 KB
ToastyTone (TM) 😎
ToastyTone (TM)is good, it not only warms up the sound, but also keeps the vocal booth nice and warm! A clear recommendation (y)

Joking aside, man these 6S6B-W tubes are really low-noise. I'm so glad I bought 10 of them when they still cost €3 each. My EAZYMOD M251 sounds really good now, wow.🥳

1000044812.jpg
1000044815.jpg
 
Plate voltage in the original is probably something around 40v. Similar to the earlier M49‘s and the km5*c series. Pretty similar circuit with 200k plate and 3k9 ohm cathode resistors and 10:1 ratio transformer.
Although 0.5ma through 200k with a B+ of 120v would mean 20v on the plate which seems a bit on the low side.
0.1mA is „lost“ in the voltage divider for the capsule polarisation. So I think the stated 0.5mA is total B+ and not total plate current, which is probably more around 0.4mA.
 
Plate voltage in the original is probably something around 40v. Similar to the earlier M49‘s and the km5*c series. Pretty similar circuit with 200k plate and 3k9 ohm cathode resistors and 10:1 ratio transformer.
Although 0.5ma through 200k with a B+ of 120v would mean 20v on the plate which seems a bit on the low side.
0.1mA is „lost“ in the voltage divider for the capsule polarisation. So I think the stated 0.5mA is total B+ and not total plate current, which is probably more around 0.4mA.
This roughly corresponds to the situation with the 6S6b and sounds realistic to me. Today I tried various operating points in the range Rk= 1.7k to 3.6k, the anode current was more or less over 0.4mA, the anode voltage from just over 30V to 43V.

At the moment I am leaning towards an Ra of 100K instead of 200K to get the anode voltage more in the direction of B+/2.
Rk I think will be somewhere between 1.7k-2.7k, we'll see.

I wonder why the old guys at AKG/Neumann went for 200K anode resistor in some cases?
 
Last edited:
Yes, I also have 5 of them and I'm curious to see how they perform in an exactly identical microphone in comparison. The 6S6B has already set the bar quite high in terms of noise.
I compared them some years ago. I remember that it was hard telling them apart sound wise but the noise levels were more consistent in 6s6b, but if I remember correct it was not a huge problem in the 5703. Let us know what you find.
 

@rock soderstrom

Hi guys, I am currently working on a cardioid only DIY ELA M251 style mic with a 6S6B tube. I'm thinking about the correct biasing of the tube, I'm orienting myself on the AC701 version, I currently have a B+ of 120V, Ra=200k and a cathode resistor of 3.6K (it's actually a 5k pot).

I would be interested in the voltages in the original ELA M251 with AC701 tube. Is the B+ really 120V? (I can't read it in the attached schematic)
What is the voltage drop across the cathode resistor, what is the voltage potential at the anode?

The total current in the microphone appears to be 1.1mA according to the schematic.

I searched for it on google, but found nothing. Can anyone help?

@tomas.borgstrom

5703 is the western equivalent
As I was looking around I fund that there is this older thread here in this group about the 5718/EC71 vs 5703/6S6B

I couldn’t found any help in the online loadline calculators especially about the 5703 / 6S8B tube, but as I was looking around the grid lines of the tube models on the online loadline calculator, I found that the grid lines of the 6N16B (6Η16β) tube is very similar to the 5703 / 6S8B tube, in other words the 6N16B (6Η16β) tube is the double “big brother” of the single 5703 / 6S8B tube, something similar like with the 12AT7 / ECC81 tube which is the double “big brother” of the single EC92 tube

Now, if we have the V+ (V): 120 DC Volts and the Resistive (RL) Load (Ohm): 100KΩ and we choose the Vq(V): 60 DC Volts as center bias, we will have Iq(mA): 0.60mA and the Grid Bias Voltage (V): -1,98 DC Volts

-1,98 DC Volts / 0.60mA = 3300Ω
aka 3k3 Cathode resistor…

And I believe that the 100KΩ is the best value for Resistive (RL) Load (Ohm) for this tube, because If we would like to be too much “tutorial” about the 200KΩ as Resistive (RL) Load (Ohm) for the typical AKG-Telefunken ELA-M 250 & AKG-Telefunken ELA-M 251 (non E, with AC701k tube):

We will have the V+ (V): 120 DC Volts and the Resistive (RL) Load (Ohm): 200KΩ and if we choose the Vq(V): 60 DC Volts as center bias, then we will have Iq(mA): 0.30mA and the Grid Bias Voltage (V): -2,23 DC Volts

-2,23 DC Volts / 0.30mA = 7443,33333… Ω
aka 7k45 (nearest value) Cathode resistor and so big value is a little bit “overkill”

Personal I wouldn’t preferred the 5703 / 6S8B tube as the regulated 6,3 Volts / 650mA for the heaters means that the current heater’s value at the Power Transformer should be at least 6,3 Volts / 1,5A, but everyone is free to make his own choices…
 

@rock soderstrom


@tomas.borgstrom


As I was looking around I fund that there is this older thread here in this group about the 5718/EC71 vs 5703/6S6B

I couldn’t found any help in the online loadline calculators especially about the 5703 / 6S8B tube, but as I was looking around the grid lines of the tube models on the online loadline calculator, I found that the grid lines of the 6N16B (6Η16β) tube is very similar to the 5703 / 6S8B tube, in other words the 6N16B (6Η16β) tube is the double “big brother” of the single 5703 / 6S8B tube, something similar like with the 12AT7 / ECC81 tube which is the double “big brother” of the single EC92 tube

Now, if we have the V+ (V): 120 DC Volts and the Resistive (RL) Load (Ohm): 100KΩ and we choose the Vq(V): 60 DC Volts as center bias, we will have Iq(mA): 0.60mA and the Grid Bias Voltage (V): -1,98 DC Volts

-1,98 DC Volts / 0.60mA = 3300Ω
aka 3k3 Cathode resistor…

And I believe that the 100KΩ is the best value for Resistive (RL) Load (Ohm) for this tube, because If we would like to be too much “tutorial” about the 200KΩ as Resistive (RL) Load (Ohm) for the typical AKG-Telefunken ELA-M 250 & AKG-Telefunken ELA-M 251 (non E, with AC701k tube):

We will have the V+ (V): 120 DC Volts and the Resistive (RL) Load (Ohm): 200KΩ and if we choose the Vq(V): 60 DC Volts as center bias, then we will have Iq(mA): 0.30mA and the Grid Bias Voltage (V): -2,23 DC Volts

-2,23 DC Volts / 0.30mA = 7443,33333… Ω
aka 7k45 (nearest value) Cathode resistor and so big value is a little bit “overkill”

Personal I wouldn’t preferred the 5703 / 6S8B tube as the regulated 6,3 Volts / 650mA for the heaters means that the current heater’s value at the Power Transformer should be at least 6,3 Volts / 1,5A, but everyone is free to make his own choices…
Yes, 6S6B seems to be half of the double triode 6N16B.
But it does not draw 650mA heater current. It's 200mA.
 

@Murdock

Yes, 6S6B seems to be half of the double triode 6N16B.
But it does not draw 650mA heater current. It's 200mA.
Yup, you are right…

As I check the datasheets again the 5703 / 6S8B tube draws 200mA…

My “typo” as I confuse it with the Svetlana 6N1P tube, which is an another good & valid choice & option for a “AKG-Telefunken ELA-M 251 non E AC701K like” soundscape microphone (more like a E88CC...)
 

Attachments

  • Raytheon CK5703WB 1957.pdf
    921.5 KB
  • Tung-Sol 5703WA - 5703WB 1963.pdf
    197.2 KB
  • Svetlana 6N1P 1999.pdf
    37.7 KB
I am building a 251. I think 6072 should be more suitable for M251, 5703/6s6b/5718/5840 besides AC701. I have also tried them all, and the qualified tube noise is very low. The main difference is the sound.
 
Back
Top