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[David Satz]

Thank you for pointing out that the threads of the M 155 don't match those of the original M 221, which I wasn't aware of. [Edited later to add: I have since verified this directly.]

The question (for me) now is whether the threads of the original M 221 match those of the M 221 A. [edited later to add: No, they definitely don't.] There's an indication in one Telefunken document that they don't--actually, a literal statement that the capsules of the original M 221 series can't be used on M 221 A amplifiers. But to me that statement seemed as if it could have been a warning not to do something that was possible, rather than a warning that to do it was impossible.

So I will try to find that out first-hand if I possibly can--I hate relying on hearsay. (For example, even though I knew that an M 201's capsule head couldn't possibly fit onto an M 221 A amplifier, I actually tried it, for the sake of being able to say that I had tried it.)

Until then, I think it's best if I scale back the first of the two messages that I posted yesterday. I don't want to create false understandings or false issues. I also removed what I wrote about errors in earlier messages, since obviously I can and do make mistakes as well, sometimes right out in public.

--In general I can only surmise why Schoeps made some of the decisions that they made in the mid-1950s or, indeed, at other times. I have a fairly long list of "Schoeps mysteries" that I would like to solve. And even though I do occasionally find real answers (or what seem like real answers), that list just keeps getting longer.

--best regards

[panman]

I just found out, that the capsule in my M221 is the same as was used in the older model M201/ Telefunken ela M921. Comparing my capsule, that only has the sn with the capsule pictured here it looks exactly the same; just missing the "Umwegscheibe":

 https://www.ebay.de/itm/Telefunken-Ela-M921-Schoeps-M201-tube-microphone-with-capsule-Ela-M921-A/383335090745?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

I copied some of the text for the furure, because the link is not going to last forever:

“Please consider my description. You find only little information overall the world. This is all I found out.- -Offer for one original:- -1 x Telefunken Ela M921 >> Schoeps tube microphone M201 - -Serial number 0634- -On pic 2 you see 2 little holes in the body: Here was the telefunken label before- -There is scratched in the body: RIVA No 9- -This mic is equipped with the original capsule Ela M921- -The connector is a vintage 5 - pin plug- -The transformer for this mic is not inside. It must be outside. I have none of it.- -Once such a mic was modified with another plug and transformer by Andreas Grosser in Berlin- -Function:Capsule cannot be tested by myself.The body has a new Telefunken 6AU6 tube. - -Beside this tube there are only 4 resistors and 1 capacitor inside.- - If here is something defective, no problem to repair it. - -Also nothing is broken or does look damaged.”

The capsule here is labeled Ela M921/A, but I am sure there is a Schoeps name too. I am guessing it is M934, because I have seen pics of similar looking capsules having that marking without "A or B".

[David Satz]

panman, no, that would be a false conclusion. The M 921/A and M 934 were different capsules, not just different names for the same capsule. They were physically interchangeable, but the M 934 was built for 120 V polarization while the M 921/A was built for 150. There may also have been other differences between them; I hope to find that out sooner or later.

--Designations such as "Ela M 921" and "Ela M 201", etc. were clearly Telefunken nomenclature, but in these instances and a few others, Schoeps' internal designations were applied only to the "innards" of the amplifiers and capsules, and the public never saw them. Even in internal correspondence and notes, Schoeps often used the "M ..." designations for these models.

At the same time, however, Schoeps made other microphone types which were labeled with the company's own designations, such as the CM 51/9 and later the CM 61 and CM 60 series. And Telefunken distributed these microphones as well, using Schoeps' names for them (as they previously had done with the CM 51/3). I haven't discovered any guiding principle yet that determined which microphones got an "Ela M ..." designation and which ones didn't.

--best regards

[RuudNL]

I know that this is an old topic, but at the moment I have a defective Schoeps/Tele M221.

I found this schematic: https://repforums.prosoundweb.com/index.php?action=dlattach;topic=36988.0;attach=2568;image
Pin 2 of the connector has the filament voltage: +4 volts.
There is a voltage divider (#6: 100K and #5: 150K) that produces the grid voltage.

Assuming 4 volts filament voltage, the grid voltage would be 3/5 * 4 = 2.4 volts but....positive!
This seems very unlikely to me. In all cases I know of the grid is meant to be negative in reference to the cathode.

Am I missing something, or should te voltage on pin 2 be a negative 4 volts?

[afterlifestudios]

I am new to learning all this, but I think it's like the M49c which is grid leak biased, or "self biased" through a resistor from grid to cathode.  (I think that's #7, the180Mohm resistor on the m221 schematic.)

M49c schematic. https://io.groupdiy.com/file/gdiy-2017/u/39511/58d1402a040aa.bmp

This post I came across makes some sense for understanding grid leak bias:

"The key to understanding how grid-leaking biasing works is to dig deep into the physics of how a tube works. The heater heats the cathode. The cathode has a coating that, when heated, creates a cloud of electrons around it. Some of these electrons fall back into the cathode, some escape, and some bang into the nearby grid wire - not many, because, after all, nothing is attracting them (compared to plate B+). But there are some with high enough energy and direction to hit the grid wires and stay there. So now we have a grid with more electrons than it started with. If it were not for the resistor between grid and ground(cathode) the electrons would keep accumulating. But with this resistor the extra electrons have somewhere to go -- through the resistor and back to the cathode, so they are constantly bleeding off. Electrons flowing from the grid to the cathode (through the resistor) make the grid negative with respect to the cathode. Making the grid negative is biasing the tube."

Feel free to delete this if I'm way off track!

[David Satz]

Ruud, there were at least three different M 221 circuits, and Telefunken distributed them all--so the first thing you need to do is figure for sure which model you have. If it's an M 221 B, that should be engraved on the outer sleeve near the Tuchel connector. If it's an original M 221 (not A or B) then the outer sleeve will have a kind of "cigar band" carrying the Telefunken logo, embedded in a groove or depression within the housing.

[panman]

M155 schematics redrawn and modified from M221b schematics. No idea what number 16 is.

Two resistor-values 5 and 6 corrected.

[RadarDoug2]

The cathode is more positive than the grid, because it is connected to the +4 volts.

[RuudNL]

Yes, you are correct! I overlooked the cathode connection. I had assumed that the cathode was connected to ground.
My wrong...