Schoeps 221B preamp + CK61 capsule + what tube?

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LevinGuitar

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Jun 13, 2019
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I've bought the preamp because of the transformer with the idea of using it in a LDC microphone but since the preamp is so beautiful and well built I'm tempted to use it with my CK61 spare capsule. The preamp fits well in the 460 body (even it's way longer) so I could even use a bigger and better output capacitor. Other options are modern and shorter SDC body but they are also wider.
The main question is what tube to use and what biasing. I guess the transformer ratio is around 10:1? so any of the tubes I have should work well: 5718, 5719, 6S6B-V, EC70, 5840W, 5703WB. But I imagine that the biasing should be completly different with a 6.3v heater voltage from modern PSU.
This will be a long project so any ideas about the body or circuit are welcome :)
 

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rmburrow

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I've bought the preamp because of the transformer with the idea of using it in a LDC microphone but since the preamp is so beautiful and well built I'm tempted to use it with my CK61 spare capsule. The preamp fits well in the 460 body (even it's way longer) so I could even use a bigger and better output capacitor. Other options are modern and shorter SDC body but they are also wider.
The main question is what tube to use and what biasing. I guess the transformer ratio is around 10:1? so any of the tubes I have should work well: 5718, 5719, 6S6B-V, EC70, 5840W, 5703WB. But I imagine that the biasing should be completly different with a 6.3v heater voltage from modern PSU. Also I'm tempted to change the connector to the standard 7pin.
This will be a long project so any ideas about the body or circuit are welcome :)
The late Oliver Archut liked the 5840 triode connected. A 5703 (triode) selected for noise should also work. I haven't tried any of the other tubes and don't have one of the 6S6B-V to try. Assuming the preamp output traffo is for a AC701, your tube choice should have anode impedance near that of a AC701.

Now for the bias: Apparently -1.6 volts on the grid (with respect to the cathode) is optimum. I like the fixed bias version where the cathode and H+ are tied together, and two resistors for a voltage divider off H+ for the bias. No bypass capacitor for the tube cathode is needed inside the mic. Apparently Scheops used 100k and 150k resistors in series with 4 v H+ (in the M221a); at the junction of those resistors -1.6 v was present (with respect to cathode) and the 180M resistor went to the AC701 grid. A 6 volt tube can be fixed biased; a different resistor ratio is needed to get the desired -1.6 v bias. Keeping the 100k resistor off H+, the closest standard value for the bottom divider resistor will be 270 k. With 6 volts across the divider, the current through the divider will be 16 uA, the bias will be approximately -1.6 v with respect to the tube cathode.

Note the biasing arrangement for a M221b preamp looks different. There was a M221b preamp on ebay recently but no mention of size or version. The resistive divider is taken off the tube anode voltage rather than the DC heater voltage. Assuming the "operating" anode voltage of the AC701 is 40 vdc, the M221b divider places approximately -2 v bias on the grid. (The 3.9 M resistor should drop 38 vdc.) One comment: Before the tube heats and draws current, the voltage across the divider is almost full HT, and 5% of full HT could be at the tube grid.

Look carefully at the parts, they will help identify your "version" of the preamp. PS I am looking for a similar preamp which will fit a 95 mm by 20 mm body.
 
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LevinGuitar

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Thank you for your answer!

Yes, I bought it from eBay (France), it's the exact version of the schematic above. I have 53ohm between pins 1 & 5 so it's the -6db preamp version? Could it be rewired to 200ohm in the way of attachment image?
Yes, it fits the 20mm body like C460.
If you really need it let me know, I have no need, just fun & learning.

I guess the 6S6B-V is the closest thing to AC701 since a lot of top M49 clones use them, but I still haven't tried them yet. I had positive experience with the rest of the tubes, except EC70 that I haven't tried yet too. How can the 5840 be wired as triode since it have internally connected G3 with cathode? The 6205 don't have that internal connection.
I like Royer circuit with 5840 anyway.

I'm waiting to 6pin amphenol conenctor, will wire it to the 7pin cable and see what voltages I have in the preamp without tube.
 

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bremusound

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The polarization voltage in the schematic is 120V. If you want to use the CK61 capsule, be sure to have a voltage divider installed to get 60V.
 

rmburrow

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Thank you for your answer!

Yes, I bought it from eBay (France), it's the exact version of the schematic above. I have 53ohm between pins 1 & 5 so it's the -6db preamp version? Could it be rewired to 200ohm in the way of attachment image?
Yes, it fits the 20mm body like C460.
If you really need it let me know, I have no need, just fun & learning.

I guess the 6S6B-V is the closest thing to AC701 since a lot of top M49 clones use them, but I still haven't tried them yet. I had positive experience with the rest of the tubes, except EC70 that I haven't tried yet too. How can the 5840 be wired as triode since it have internally connected G3 with cathode? The 6205 don't have that internal connection.
I like Royer circuit with 5840 anyway.

I'm waiting to 6pin amphenol conenctor, will wire it to the 7pin cable and see what voltages I have in the preamp without tube.

Connect the output traffo secondaries in series as shown for 200 ohm output. Hopefully the connection is accessible.

The triode connection of a 5840: Connect the screen (g2) to anode, connect the suppressor (g3) to cathode. Wire up the remainder of the circuit as usual. I keep the high impedance g1 lead short with the tube leads facing the capsule and dress the remaining leads away from the grid lead. The "coiling" of 4 turns of lead over a 1 mm drill close to the tube (as used with AC701) helps stop heat "shock" to the glass envelope when soldering in tube leads. (Also grip the leads with needle nose pliers as heat sink when soldering the tube leads!)

If my memory is correct, the 200 ohm M221's had a red dot on the case, and the 50 ohm M221's had a blue dot. Fingernail polish type change.

I have a empty 95 mm x 20 mm body with a shorted M934b capsule, and a small piece of 1 u nickel membrane material. Attempting a reskin is tempting. Getting my shop ready for winter activities. If I repair the capsule, I will still need a preamp that fits fwiw.
 

rmburrow

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The polarization voltage in the schematic is 120V. If you want to use the CK61 capsule, be sure to have a voltage divider installed to get 60V.
Good point with the 120 volt polarizing voltage of the M221b capsule. A series divider using two 360 M resistors should work for the lower voltage capsule. (The 180 M resistance at the capsule should be preserved since that impedance may affect audio; time constant of the polarizing resistor and the capsule capacitance!)
 

LevinGuitar

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Connect the output traffo secondaries in series as shown for 200 ohm output. Hopefully the connection is accessible.

The triode connection of a 5840: Connect the screen (g2) to anode, connect the suppressor (g3) to cathode. Wire up the remainder of the circuit as usual. I keep the high impedance g1 lead short with the tube leads facing the capsule and dress the remaining leads away from the grid lead. The "coiling" of 4 turns of lead over a 1 mm drill close to the tube (as used with AC701) helps stop heat "shock" to the glass envelope when soldering in tube leads. (Also grip the leads with needle nose pliers as heat sink when soldering the tube leads!)

If my memory is correct, the 200 ohm M221's had a red dot on the case, and the 50 ohm M221's had a blue dot. Fingernail polish type change.

I have a empty 95 mm x 20 mm body with a shorted M934b capsule, and a small piece of 1 u nickel membrane material. Attempting a reskin is tempting. Getting my shop ready for winter activities. If I repair the capsule, I will still need a preamp that fits fwiw.
I have the grid connection with capsule and grid resistor as short as possible (but not so beautiful as you describe) in my SCT700 with Royercircuit and K103 (great combo btw!). Will have a look for the way AC701 grid is connected for next time. Plate and G2 (leads 5&7) connected together. But since leads 2, 4 and 8 are connected internally in 5840 I only have connected one of them, could be any issues with it?

I don't have the original body so no blue or red dots. Rewiring should be possible I guess.

Why using so high resistances for the divider? Can't I use 1M?
Also in my "Ami M49" experiments with 5703 and 5719 tubes I found 100K plate resistor was better than going 150K or 180K (adjusting bias in each case). Don't know if the original 200K here will be the best for my ear but this is a different circuit.
Your body is the original Schoeps?

Thank you!
 

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rmburrow

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You should be OK with a single lead connection for the 5840 g3/cathode internal connection. You will have to connect the screen (g2) lead to the anode.

The curves for the 5703, 5718, etc. may be different than the AC701 curves. This will of course affect components used to determine operating point.

Consider the single 180 M resistor off HT used for capsule bias on your M221b. This biases the capsule at 120 volts. The supply side of the capsule polarizing resistor is effectively at audio ground. The tube grid must always remain at high impedance because of the low capsule capacitance and the resulting time constant. The tube grid is also isolated using a 180M resistor connected to a voltage divider for grid bias. The supply side of this resistor is also effectively at audio ground. Audio is coupled from the capsule through the fixed dc blocking capacitor to the grid (at -2 v bias). The way I see it, two 180M resistors are effectively in parallel with respect to audio.

In order to use a 60 volt capsule and preserve the effective impedance looking into the grid, the series resistors in the polarizing divider should be 360M apiece or higher. (The 120 v HT is effectively at audio ground, and ground is ground so with respect to audio, the 360M resistors are effectively in parallel.)

Using a lower resistance (1M as you suggested) would be a divider with little or no audio generated; 20 pf capsule doesn't like low resistance load!

There are no markings engraved on the mic body I have. The finish is similar to the M934b. The dimensions are 95 mm long x 20 mm diameter (minus the capsule). The body threads the M934b capsule perfectly, has the Tuchel connector at the bottom with threads for the connector.
 

LevinGuitar

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Have some news here!

Already made my 7pin-6pin amphenol cable, tried a 63v B+ from psu to avoid a divider in the body and original SCT800 psu 6.6v heater voltage. Installed a 5840 tube and tried in a Pronomic body+capsule. Guess using the original heater voltage from this psu for the 5840 cathode is not the best idea, have hum and low gain. The hum dissapears and the gain rise up as soon I turn off the psu (see audio test).
I tried a 750ohm on cathode-ground (like Royercircuit) and the rest without modification, but with 63v from B+ the resulting plate voltage goes lower than 30v.

By the way is ok to use a 62v zener to lower the 200v voltage?

So the version 2 is what I was wondering to try with the classical 1M divider using 120v B+. I still don't get the way I can use 360M divider.

So in case of version 2 should I adjust the cathode resistor to get 1-1.6v between cathode and grid or cathode and ground?

Open to ideas that keep most of original circuit and parts :)
Thank you all and sorry for the multiple edits!
 

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LevinGuitar

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I made a divider of the heater and now have 1.5v between Cathode and Grid. For the first test seems a good biasing, good gain but still have the hum that dissapears when turn off the psu.

BTW I tried a TBone SC140 mic body but its too thin in the inner diameter and the pronomic is too wide :(
 

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Matt Nolan

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Using the divided heater voltage to bias the cathode is putting any ripple on that voltage effectively at the input of the tube gain stage. You are amplifying that hum. To use a mic topology like this your heater voltage has to be very well smoothed. If the gain is, say, 10x then it needs to be 10x smoother than the ripple on the 63V rail. That's why the hum goes away when you turn the PSU off, because it is just running for a short while on residual voltage in the reservoir capacitors and there is no mains AC superimposed.
 

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