Voltages in an original AKG ELA M251 AC701 version?

[rock soderstrom]

Hi guys, I am currently working on a cardioid only DIY ELA M251 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?

 

[Khron]

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)

People seem to think / say so..?

https://groupdiy.com/threads/akg-ela-m251-with-ac701-neuman-m49-circuits.48399/

 

[rock soderstrom]

People seem to think / say so..?

Thanks. Yes, it seems to be so, but unfortunately no hard proof in this thread (which I knew)

B+ is not so important to me, the voltage at the cathode and anode would interest me more.

 

[Accelerator]

@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…

 

[rock soderstrom]

Okay, the user manual confirms the B+ of 120V for the AC701 version. The total current consumption is therefore a more realistic 0.5mA!


Screenshot taken from attached german/englisch manual

 

[Khron]

The total current consumption is therefore a more realistic 0.5mA!

... As opposed to the 0.5A(!!!) of the 6072 version

 

[rock soderstrom]

... As opposed to the 0.5A(!!!) of the 6072 version

big tube version for extra warm sound!

 

[Khron]

big tube version for extra warm sound!

ToastyTone (TM)

 

[rock soderstrom]

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

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.

 

[Murdock]

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.

 

[rock soderstrom]

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?