4 lead capacitors? "nM"?

[Murdock]

Hey folks, another part I can't identify.
I have some nice DIY mic's which seem to be out of the 50s or something. I'm trying to draw a Schematic for them but there are two components I don't know. They look like those old ceramic capacitors but have 4 leads. Two coming from the rings surrounding the tube at each end and two coming out of each end. One has the value "10 nM 500 =#" written on it and the other only "2,2 nM". The smaller one is connected to the capsule diaphragm and the bigger one is connected to the tube.
Does anybody know what this could be? What is "nM"?
I'll try to upload some pictures later.

 

[Tubetec]

Hi DOc ,
Im not sure exactly whay your talking about without a picture ,what I have seen on some old european radios are caps that seem to have a small inductor included in the package ,looks like a cap with 4 leads on it . I have an old radio here at the moment ,I try and take a few pics of these 'odd ball' components later and see if there similar to what your trying to indentify ,Ive only ever seen these in German sets so far actually .they might be custom made components more or less unique to each set.
I'll try post a pic or two later in the evening ,
Best o luck .

 

[Murdock]

Hey, thanks for your answer!
But I think I saw it wrong. It seems, as the builder has just put some wire through this component... as ceramic tube capacitors are exactly that, tubes, you can see through it. So in fact, it has only two leads and look exactly like these old ceramic tube capacitors. But what does "nM" stand for??

I'll probably open a new dedicated thread for these mics, but can someone tell me, why someone would Ground one wire of each, the primary and secondary of a transformer?

 

[rackmonkey]

As for the value, the “M”makes me think it’s a variant on the early nomenclature used for caps. Back in the day, “M” was equal to uF (one millionth of a farad or M(icro)Farad).  So a 1uF cap would be 1M. Picofarads (pF) were expressed as “mm” or “mM” or “MM” (micromicrofarads).  I’ve never seen it, but it’s not crazy to think some cap company might have used “uM” for pF, mixing the two conventions a little.

If it looks like a ceramic cap, measure it. If it’s remotely close to 10pF, then I think you’ll have your answer.

BT

 

[CJ]

nF is nano farads, nM might be the same thing,

100 nf means 0.000000100 Farads, or 0.1 uF, or 100000 pf.

 

[Murdock]

Hey guys, thanks alot for the answers!
I also thought it probably is an old nomenclature for caps. But with "M" being 1uF, what is "nM"?  NanoMikroFarad?
I don't have any tools to measure capactitance... Should be next on my tool shopping list.

The whole mic circuit is really weird. One wire of each, primary and secondary, is grounded.
Inside the mic there is no connection between the housing and ground.
Both halves, of what I'm quite sure to be a ECC81 or 6072, are used but both cathodes are directly connected to ground. Would that even work?
One plate resistor is 150k and the other is 200k. Grid number two goes to ground via a 30M resistor.
I'll upload my redrawn schematic and a picture of the unknown component in the next days.

 

[gyraf]

TLR - draw up a schematic and take a pic, that's the only real way to phrase this sort of question..

Jakob E.

 

[analogguru]

.....
One has the value "10 nM 500 =#" written on it and the other only "2,2 nM".
.....
Does anybody know what this could be? What is "nM"?
.....

As for the value, the “M”makes me think it’s a variant on the early nomenclature used for caps. Back in the day, “M” was equal to uF (one millionth of a farad or M(icro)Farad).  So a 1uF cap would be 1M. Picofarads (pF) were expressed as “mm” or “mM” or “MM” (micromicrofarads).  I’ve never seen it, but it’s not crazy to think some cap company might have used “uM” for pF, mixing the two conventions a little.
.....

Thats nonsense.

To solve the "secret" of the letter "M" is really easy:
the values are 10nF and 2,2nF and as I already wrote here nine years ago:

Styroflex coding:

voltages (at 40 deg C):
blue: 25V DC
yellow: 63 V DC
red: 160V DC
green: 250V DC
black: 630V DC

tolerances:
B: 0,1 %
C: 0,25 %
D: 0,5 %
E: 0,5%
F: 1%
G: 2%
H: 2,5 %
J: 5 %
K: 10%
M: 20 %
N: 0,05 %
P: +100%/-0%
Z: +80%/-20%

analogguru

..... stands the letter "M" for the tolerance: M = 20%

 

[Murdock]

Hmm, the weird thing is, that there is another 10nF ceramic capacitor... Which is also a little bit bigger. And both mic's have those 10nM caps in the same place and the 10nF also. Could be that the builder had only those 4 caps available and matched it but I doubt that. Why did he not also use those 10nF caps in the other places?
Anyway, here is my redrawn schematic. I'm sorry for the messy drawing and writing. I hope everything is readable.



Does this make any sense to you? Would the mic even work in that configuration?

 

[abbey road d enfer]

Hmm, the weird thing is, that there is another 10nF ceramic capacitor... Which is also a little bit bigger. And both mic's have those 10nM caps in the same place and the 10nF also. Could be that the builder had only those 4 caps available and matched it but I doubt that. Why did he not also use those 10nF caps in the other places? 

Electronic components were rae and expensive at the time, so they used anything that was available as long as it worked.

Anyway, here is my redrawn schematic. I'm sorry for the messy drawing and writing. I hope everything is readable.

Does this make any sense to you? Would the mic even work in that configuration?

Yes it does; draw them as capacitors and you have a 2-stage configuration. I would think the output to be pretty hot.
The unbalanced output is not such an oddity since many microphones were used with unbalanced tape recorders.

 

[Murdock]

Hey abbey, thanks for your thoughts!

Is there a name for this kind of configuration? The only circuit I found to be quite similiar is the "long-tail pair". But there also is at least one resistor between cathode and ground.
I read a bit about "grid leak bias". Is that the case here?

What do you think about the "quality" of this circuit? Is it any good? Would I it be a good idea to make it balanced or would that mess with anything in the circuit?

And what do you think would the B+ and H+ be?
B+ is probably higher than the "normal" 120V, right?
And H+ should be 12V?

 

[PRR]

> similiar is the "long-tail pair".

So-named because it has a "long" (high value) common cathode resistor.

This has NO cathode resistors! The ultimate "short-tail"!

It is indeed as Abbey says. Two grounded-cathode gain stages, resistor-loaded. Yawn.

But since we usually come out of condenser mikes with one gain-stage, sometimes just a unity-gain buffer, this one may be intended for "low line level" output rather than mike level.

 

[abbey road d enfer]

I read a bit about "grid leak bias". Is that the case here?

Yes. Also named grid-current bias.

What do you think about the "quality" of this circuit?

Almost textbook application; nothing wrong, nothing fancy.

Would I it be a good idea to make it balanced or would that mess with anything in the circuit?

It's feasible; there may be an issue with the cable/connector but in terms of operation/performance, yes.

B+ is probably higher than the "normal" 120V, right?

Very likely, since B+ is divided by 8 for delivering bias to the capsule. I would think about 250V.

 

[Murdock]

Hey Abbey and PRR, thanks alot for the infos. Much appreciated!
I'll probably just use the capsule, transformer and body and build another circuit with it.