New year, new mic...RCA type MI-10002 Uni-Angular Electrostatic (experimental build)

[pinchemotherloaf]

Most likely will sound rough, but I've always wanted to try this drawing anyways! RCA MI-10002 Uni-Angular Electrostatic. Could be a bit harsh, there is no grid or output coupling cap. I used the new cheap u47 case, replaced the saddle with a better one than provided. Modified top plate for tube standoff and added cheap E.H. tube. 500m grid leak. Will probable use an edge terminated single sided capsule. Just fun and games, nothing too serious. Any opinions if this will work?

 

[emrr]

Sure, this is a lot like the Altec mics. There’s an output cap, look again.

 

[pinchemotherloaf]

Hmm, I see 2 caps on the B+ section (ac decoupling) and 2 more, one between ground to capsule and the other between ground to anode. This is a cathode follower so I see the cathode going straight to transformer. Which cap are you thinking is for the output coupling cap? I may have missed something!

 

[emrr]

Yep, I wasn’t seeing it all on my phone. It’s a DC capable transformer.

 

[pinchemotherloaf]

Yep, I wasn’t seeing it all on my phone. It’s a DC capable transformer.

Do you think the 3U 12/1 transformer, here, will work in this configuration? or burn up ?
Maybe I should add one!

 

[emrr]

I don’t know anything about that transformer but this looks like less than 1mA worst case shorted tube, so well less than that if my calc is remotely correct.

 

[pinchemotherloaf]

Well, it'll be interesting to see what happens when it fires up!

 

[Gus]

Found two pictures of the M1-10002 A little lower than 1/2 down in the pictures
https://hollywoodsound.com/mic-collection
page 19 https://worldradiohistory.com/Archive-Catalogs/RCA/RCA-Audio-5th-1964.pdf

Have not found any information on the transformer used in the cathode follower circuit. I would guess greater than 4:1

the 150k as the last R in the R C R C going to the plate makes me think the tube is running at lower current because the circuit is powered with 135VDC
1mA would be 150VDC drop
.1mA would be 15VDC drop
etc.

It is a cathode follower with a transformer in the cathode section. I would guess it is a gapped SE type or maybe has 2 or 3 lam interleaving

 

[alphasnk]

Power it on that's the only way to know

 

[abbey road d enfer]

Could be a bit harsh, there is no grid or output coupling cap.

What do you mean? Do you think the absence of coupling caps is a cause for "harshness"?

 

[pinchemotherloaf]

What do you mean? Do you think the absence of coupling caps is a cause for "harshness"?

Don't know, I'm just used to seeing one in the circuit for either blocking dc or filtering. Not sure why RCA didn't put one there. Maybe they inserted one post transformer. But might still sound sweet, I should remain more optimistic about this build!

 

[Gus]

The transformer looks to be the most interesting part of the microphone.
Was the RCA 303502 gapped, Si steel lams, interleaved etc.? what was the ratio, 4:1 or10:1 etc.?

Some things to read

https://www.tubecad.com/2005/June/blog0049.htm
https://www.tubecad.com/2005/June/blog0048.htm
https://www.tubecad.com/2012/05/blog0230.htm

 

[pinchemotherloaf]

The transformer looks to be the most interesting part of the microphone.
Was the RCA 303502 gapped, Si steel lams, interleaved etc.? what was the ratio, 4:1 or10:1 etc.?

Some things to read

https://www.tubecad.com/2005/June/blog0049.htm
https://www.tubecad.com/2005/June/blog0048.htm
https://www.tubecad.com/2012/05/blog0230.htm

The transformer looks to be the most interesting part of the microphone.
Was the RCA 303502 gapped, Si steel lams, interleaved etc.? what was the ratio, 4:1 or10:1 etc.?

Some things to read

https://www.tubecad.com/2005/June/blog0049.htm
https://www.tubecad.com/2005/June/blog0048.htm
https://www.tubecad.com/2012/05/blog0230.htm

Thank you for the info. I haven't found much on the transformer. I used the 3U just as a kludge. I suspect the xformer was a can of some type. Will see how it sounds with the 3U and compensate.
The question remains why an output cap is not used?

 

[mhelin]

The circuit is designed around the transformer, your one (3U) is not a good one for this application.

The transformer also sets the operating point of the tube (bias).

 

[Gus]

Maybe something like a 1680 transformer

http://www.lundahl.se/wp-content/uploads/datasheets/1680.pdf

 

[emrr]

The question remains why an output cap is not used?

It’s not really a question. It’s a direct feed circuit, while most are parallel feed. Look at preamp circuits, you’ll see both. Being a cathode follower makes it look unusual perhaps, but it’s no different. RCA is more often than not direct feed which has its own advantages.

 

[pinchemotherloaf]

Maybe something like a 1680 transformer

http://www.lundahl.se/wp-content/uploads/datasheets/1680.pdf

I have this Collins type. It says 100-10K but the person I bought it from says they test +/-2dB up to 30K. It will fit in the U47 body, barely!

 

[emrr]

At mic level and current, it may well be full range.

 

[emrr]

Another page tells us the MI-10003 is the windscreen, and the MI-10005 is the bit with the transformer and B+ filter elements.

MI-10002 is mic without windscreen, MI-10004 is with.

 

[pinchemotherloaf]

Another page tells us the MI-10003 is the windscreen, and the MI-10005 is the bit with the transformer and B+ filter elements.

MI-10002 is mic without windscreen, MI-10004 is with.

I see that the RCA Mixer MI-10279, A, B had anode and heater power supply taps as well.