"Manleyish" Tube Microphone (ST51 + SCT 800)

[Mauritius89]

Hey,

I stumbled over ther wonderful detailed Manley Reference Cardioid analysis thread from @kingkorg and his various rebuilds based around the origial capsule also used in a Sterling Audio ST51. My st51 laying around unused since years and because my electronic knowledge is not high enough to build a tube microphone from scratch I decided to combine the capsule with an t-bone SCT 800. KingKorg shows that the tube circuit of a Manley is very clean, the SCT800 is the complete opposite. My goal is not to rebuild the Manley in general but getting a good sounding tube microphone with a "Manley" capsule.

The biggest change I did so far, beside the capsule and microphone body swaps (manleyish lookalike), is the replacement of the the original tube to a 6072. The new tube is working so far and brings some improvment to the sound but I don't know if the pcb and psu should be adjusted to a 6072 (original tube is a 12AX7).

Besides, I would like to do some changes of the pcb to improve the performance of the microphone based around the modding gtc did. It is the most complete and well thought out modding of a SCT800 I could find (the whole thread about the SCT800 is quite interesting). Still I don't know if there are some changes I should do to the change of a 6072? I also read a lot about tuning a circuit to a spezific capsule. Is that necessary in this task? The original capsule is a k67 and the felio/st51 capsule is a k67, aren't they?

I attached some pictures. First is the original schematic of a sct800 from @In76d. Second is the modded schematic from @gct.

​

 

[Accelerator]

@Mauritius89​

Change the 6072-12AY7 tube with the ECC81 – 12AT7 tube.

(Original “50’s AKG C-12, Telefunken TF-51, BSA Microphones C-12, M-251)

If R2 = 47KΩ + R1=3KΩ, then the Cathode Resistor R7=891,666Ω aka you will have to go to the closest 900Ω

The C12 (10pF/200V) from the modded schematic from @gct. Should been changed to (at least) 100pF/250V or higher...

(Typical AKG-Telefunken ELA-M 251E…)

C9 + C10 get ride them both out, at once…

 

[RuudNL]

The C12 (10pF/200V) from the modded schematic from @gct. Should been changed to (at least) 100pF/250V or higher...

In the M251 100 pF is connected from anode to ground.
Here the 10 pF is a feedback capacitor between anode and grid.
With 100 pF between anode and grid the sensitivity will be very low...

 

[Accelerator]

@RuudNL

In the M251 100 pF is connected from anode to ground.
Here the 10 pF is a feedback capacitor between anode and grid.
With 100 pF between anode and grid the sensitivity will be very low...

Good point RuudNL but why someone’s wouldn’t preferred the oldie and trustful microphone way of ELA-M 251 E with 100 pF connected from anode to ground for a microphone rather, than preferred the Marshall JCM800 Guitar amp. way, that the 10 pF is a feedback capacitor between anode and grid?...

Are we planning to make a tube microphone with 120 Vb or a Guitar Amplifier with 280 Vb for the Pre-Amp. tubes?

 

[Khron]

why someone’s wouldn’t preferred the oldie and trustful microphone way of ELA-M 251 E with 100 pF connected from anode to ground for a microphone rather, than preferred the Marshall JCM800 Guitar amp. way, that the 10 pF is a feedback capacitor between anode and grid?...

The 100pF from anode to ground rolls off top-end.

The 10pF from anode to grid, reduces overall gain / sensitivity.

Just because two components have values around the same order of magnitude, and happen to have one of the terminals connected to the same node, does NOT necessarily mean they have the same role / effect.

 

[Migs 31]

Hey,

I stumbled over ther wonderful detailed Manley Reference Cardioid analysis thread from @kingkorg and his various rebuilds based around the origial capsule also used in a Sterling Audio ST51. My st51 laying around unused since years and because my electronic knowledge is not high enough to build a tube microphone from scratch I decided to combine the capsule with an t-bone SCT 800. KingKorg shows that the tube circuit of a Manley is very clean, the SCT800 is the complete opposite. My goal is not to rebuild the Manley in general but getting a good sounding tube microphone with a "Manley" capsule.

The biggest change I did so far, beside the capsule and microphone body swaps (manleyish lookalike), is the replacement of the the original tube to a 6072. The new tube is working so far and brings some improvment to the sound but I don't know if the pcb and psu should be adjusted to a 6072 (original tube is a 12AX7).

Besides, I would like to do some changes of the pcb to improve the performance of the microphone based around the modding gtc did. It is the most complete and well thought out modding of a SCT800 I could find (the whole thread about the SCT800 is quite interesting). Still I don't know if there are some changes I should do to the change of a 6072? I also read a lot about tuning a circuit to a spezific capsule. Is that necessary in this task? The original capsule is a k67 and the felio/st51 capsule is a k67, aren't they?

I attached some pictures. First is the original schematic of a sct800 from @In76d. Second is the modded schematic from @gct.

​

Check R7 - should be 3K or so on the Thomann T. Bone SCT-800 (I have two at home) If that is the case, change for 1.8K or 2K Max resistance 1/2 or 1watt. Your valve will run cooler.

Of course change that cheap valve/tube for a quality 12ay7/6072 or 12at7/ecc81. (which you have already done )

Change C8 for same value but better quality capacitor, 250V.

C4 equally important - very important in fact. I went for 1nF (same as 1000pF) Styrene cap.

Clip/cut C9-C10 don’t replace them just leave sockets empty.

There was a 10 pF cap in mine, C12. I just “lifted” (unsoldered) one leg to try it and left it like that. That was my choice.

You’ll hear the difference with these quick mods on the pcb and have a more usable SCT-800.
Good DIY.

PS: I know/read what’s your goal, but this is far from a Manley Reference, because of the head basket, transfo, dimensions etc. But enjoy & good luck to you Sir.

M

 

[Accelerator]

The 100pF from anode to ground rolls off top-end.

The 10pF from anode to grid, reduces overall gain / sensitivity.

Just because two components have values around the same order of magnitude, and happen to have one of the terminals connected to the same node, does NOT necessarily mean they have the same role / effect.

If you want to reduces the overall gain / sensitivity in a tube microphone, you can simple choose a tube with less μ factor, lower gain and lower input sensitivity such as ECC82 – 12AU7 or even a 12BH7-A today rather than using local feedback loops that introduce un-pleasure “solid-state” characteristics in the F.R. like bass boost and that’s will answer you “why” Georg Neumann chose the AC701 tube (μ factor = 17) for his M-49 (a,b,c), M-50(c), M-250(b), M-269, KM-54, KM-56, SM-2c, SM-26, SM-69 and also the AKG chose the AC701 tube for the AKG – Telefunken ELA-M 250 rather than using local feedback loops…

The usage of local feedback loops in High Gain Guitar amps with the high gain ECC83 – 12AX7 – 7025 pre-amp. tube (μ factor = 100) it’s a whola lotta different “story” and has a sense, but I don’t consider someone microphone electronic designer – maker will put his money, his effort, his time to design – make a tube microphone to sound as “tubey” as it can be and then ruined it to sound like a nonsense cheap solid-state microphone by adding a local feedback loop…

You can read more about the feedback loops at Chapter 9: Feedback Theory (page 212 – 228) at:

Designing Tube Preamps for Guitar and Bass, 2nd edition By Merlin Blencowe

 

[Accelerator]

Check R7 - should be 3K or so on the Thomann T. Bone SCT-800 (I have two at home) If that is the case, change for 1.8K or 2K Max resistance 1/2 or 1watt. Your valve will run cooler.

The value of the Cathode Resistor R7 is related with the values of R2 + R1…

If Vb = 120 DC and we will like to use the 12AY7 – 6072 tube and biasing it at 60 Vq(V) volts center bias and we choose R2 = 100KΩ + R1 = 30KΩ (Typical Neumann U-47), then we will have Grid Bias Voltage (V) -1.36 / 0.46 Iq(mA) = 2956,5217…Ω so the nearest Cathode Resistor R7 value will be 3KΩ …

If Vb = 120 DC and we will like to use the 12AT7 – ECC81 tube and biasing it at 60 Vq(V) volts center bias and we choose R2 = 100KΩ + R1 = 30KΩ (Typical Neumann U-47), then we will have Grid Bias Voltage (V) -1.76 / 0.46 Iq(mA) = 3826,0869...Ω so the nearest Cathode Resistor R7 value will be 3.8KΩ …

 

[Migs 31]

The value of the Cathode Resistor R7 is related with the values of R2 + R1…

If Vb = 120 DC and we will like to use the 12AY7 – 6072 tube and biasing it at 60 Vq(V) volts center bias and we choose R2 = 100KΩ + R1 = 30KΩ (Typical Neumann U-47), then we will have Grid Bias Voltage (V) -1.36 / 0.46 Iq(mA) = 2956,5217…Ω so the nearest Cathode Resistor R7 value will be 3KΩ …

IF .... If ....
And there are quite a few ifs, in there.
In practice ...open a SCT-800 if you have one lying around & find a few big surprises. Starting with HT voltage.

I was also surprised to find part value variations from one mic to another (same brand/model, close serial numbers) And totally crap valves.

@Mauritius89 have fun & welcome to the world of DIY & Made in China products.

M

 

[Accelerator]

@Migs 31​

IF .... If ....
And there are quite a few ifs, in there.
In practice ...open a SCT-800 if you have one lying around & find a few big surprises. Starting with HT voltage.

I was also surprised to find part value variations from one mic to another (same brand/model, close serial numbers) And totally crap valves.

I don’t have any SCT-800 in my hand to opened it up and un-fortunally for me I don’t have any cheapie as a cheap “Made in China product” with the crappie as a crap valves and problematic P.S.U. with surpriselly high HT voltage(s).

I just see & analyze a microphone’s electronic design and the values that should be on it, no matter if this microphone it is a P2P (point to point) Neumann U-47, an AKG – Telefunken ELA-M 251E, a Neumann – Gefell UM57 or even in our case today the hobble and cheap “Made in China” SCT-800…

If you believe that only the cheapie as a cheap “Made in China product” with the crappie as a crap valves and problematic P.S.U. with surpriselly high HT voltage(s) have uncorrected values, let me inform you that the 1K8 Cathode Resistor of the AKG – Telefunken ELA-M 251E is totally uncorrected and you have to replace it with the proper value of 2050Ω, or even also that the 12AY7 - 6072 tube in an AKG C-24 is totally uncorrected and you have to replace it with a 12AT7 – ECC81…