emrr

Carrier Microphone Company model 94 condenser, USA, 1932
« on: February 05, 2014, 07:49:47 AM »

Carrier Microphone Company model 94 condenser microphone, approximately 1932, based on product announcements in magazines.  This would be pretty close to the end of the first generation of American condenser mics, with the RCA ribbons just announced and starting to take over.   Made in Inglewood California, primarily for the film industry. 

I've had this since about 2000, rebuilt it and tested since 2002.  It's been shelved while I looked for the unobtainium Cannon mating connector.  I've since learned it's 'extra special unobtainium' and I might as well forget it, so I made a connector recently. 

This thing sounds far better than expected.  It has a peak in the 4-5K range and much less above that, goes well low enough.  Sounds really great as ambient drum kit pickup, the kick drum quality is pretty special (read: the right distortion contribution).   I can see cutting some lead vocals with it, some horns and guitars would also be suitable uses. 

Unlike reports of the early Western Electric condensers crapping out with high volume, this microphone does not.  The amplifier goes into limiting type overload with rock drums 6 feet away, and you can see typical single ended class A flatlining against the power rail side in the waveforms.  You can't really hear it as distortion though, so it must be fairly strong 2nd harmonic relative to anything else. 

Output level would be on par with low output condensers, or medium/high output dynamics. 

Power supply would have been battery originally, four 45V B batteries and 6 volt filament.  Today there are arguments about the effect on sound quality with various types of DC supplies feeding direct heated triodes like this, which were never considered for use with AC:DC PSU's in any high quality installations.  I haven't been down that rabbit hole, other than to note that any and all ripple will appear in the audio, since cathode and heater are one in the same.  I'm using a modified chinese tube mic PSU at the moment.   In one ambient test highest noise peak with gain set for voice use was 180Hz (PSU hum) at -74dBFS, or roughly -56dBm.  60/120Hz are equal, and slightly lower.  The highest noise peak above 180 is about -90dBFS, most lower than -95dBFS, baseline above 1.5kHz being about -108dBFS.  The PSU can definitely be better than it is, and I've not tried a 6V lamp battery for comparison yet. 

This mic was designed for a 200 ohm load.  In the early days it would be normal to have a condenser mic panel that might provide power, and also have a 200 ohm ladder attenuator to control level to the program amp or mixing panel.  I changed the pattern selector on my PSU to be a load switch, ranging from 230R up to 1K6 in 200R steps, and open.  This affects both response and level, and gives some useful tailoring. 

No great pics yet, but here are a few. 

The original water decal was starting to flake off.  Most of these I've seen are totally gone.  I gave it a localized coating of Polycrylic to preserve what remained. 







Pics before the recap; on film! 









Pics showing the recap.  I left all original caps in place, and worked around them, so any anal future collector can return it to original condition if desired.
The strangest thing I've ever found when drawing out a circuit was a few of the resistors here; they are mounted inside the block of wood in the middle, which appears to only be terminal connection points at first glance. 









tube sockets



capsule front and back





disassembled



The Cannon connector saga.  My home made connector is cut from a 1.5" dowel, pins were robbed from some old stereo speaker press-on connectors, and reshaped.  Pin size is 1/8", the same as the filament pins of 4 pin triodes. 
















Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g


micaddict

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #1 on: February 05, 2014, 10:03:19 AM »
 :o

Gus

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #2 on: February 05, 2014, 10:11:49 AM »
Very cool, something different to look at

looks like 200VDC capsule voltage
I see a 15.3ohm resistor that helps soft start the tubes direct heated filaments and also looks to be degenerative feedback at the first tube.
Found this http://www.radiomuseum.org/tubes/tube_ux864.html looks like 1.1VDC filaments .25amps 
.25amps x 15.3ohms =3.825VDC
3.825 VDC +1.1VDC + 1.1VDC = 6.025
so it sums to 6VDC
Now the china PS often have the regulator set to about 6.6VDC.
You could use the extra .6VDC to your advantage by adding a resistor in series with the reg to drop about .6VDC
.6VDC x .25amps =.15ohms
this would help soft start the tubes a little more

Now you like the sound so lets think about the operating points
Yellow as ground
The first stage grid is bias at 3.8VDC to 4.9VDC from end to end of the filament cathode
the 2nd stage grid is biased at 4.9VD to 6VDC from end to end of the filament cathode
I have not used direct heated tube I would guess the grid bias is the Voltage average ANYONE know about direct heated tubes?

What are the plate voltages referenced to yellow this will help find the plate current they are run at
found this http://www.tubebooks.org/tubedata/R10.pdf   pages 93,115
I have not found any curves yet
EDIT found this http://frank.pocnet.net/sheets/049/8/864.pdf

Someone with a lathe and mill could make the connector parts out of plastics

Are the plates at about 45VDC and 55VDC?
« Last Edit: February 05, 2014, 10:29:46 AM by Gus »

emrr

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #3 on: February 05, 2014, 10:49:41 AM »
Hi Gus,

I tried the connector in plastic first, failed horribly.  Decided with my tools, wood was better, and.....that's what the original connector is anyway. 

A few new notes on the drawing.

+200 is stamped on the inside of the connector bakelite, as is +6, L9(?), (+), and (-). 

Some of the WECO manuals suggest what must be five 45V batteries with a rheostat for trimming B+ value.

186V in gave 48V at V1 plate and 67V at V2 plate.  That's what I wrote down in an early test, I managed 193V in the end.  The chinese PSU has 6 RC stages, zeners removed, and is really running at current limit.  It was originally set by zener for roughly 150V.  I halved the first two R values to get B+ up.  Loading the B+ output with 1M5 (close to pattern control load value) sucks it down to 193V from 198V, that all with mic load present.  Each C value is 22mfd, so 22mfd pool at B+ connection to mic. 

Black to Yellow measures 40 ohms cold, before filaments heat.  For curiosities sake. 

Correct on the tube data, and dropping purpose of the 15R3.  This looks fairly typical for heater/cathode connection and supply in that era.   I saw 6.5VDC from supply, 6.2V at the mic connector, in tolerance values at the tubes.  Another 0R15 in line to another filter cap may help, worth trying.  Current filament is 2200mfd/2R/2200mfd/LM7806CZ/2200mfd. 

These filaments heat differently.  Sound comes on full blast fairly quickly, none of the fast ramp up I normally hear with indirect heated tubes.

I can't address the grid bias question.  I have seen comments out there about which feed orientation sounds best, since filaments are marked + and -. 

Some more thoughts on PSU's for direct heated tubes.  I don't have my head around this stuff much at all. 

http://www.hificollective.co.uk/pdf/diyhs_filament.pdf
http://www.bartola.co.uk/valves/dht-filament-regulator/
http://www.diyaudio.com/forums/tubes-valves/38248-new-dht-heater.html?perpage=30&pagenumber=2
http://www.tentlabs.com/Products/Tubeamp/Tubefilament/index.html
http://www.diyhifisupply.com/opcart/index.php?route=product/product&path=179_136_139&product_id=559
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

Gus

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #4 on: February 05, 2014, 11:10:56 AM »
186VDC - 48VDC = 138VDC
138VDC/150K = 1.24ma
186vDC- 67VDC = 119VDC
119VDC/150K = .79ma
So about 2ma B+ current
Did you disconnect the pattern control and any other shunt paths to ground in the B+ supply

you could adjust the B+ resistors without the microphone with a 100K load for 2ma at 200VDC

emrr

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #5 on: February 05, 2014, 11:16:57 AM »
The pattern control was disconnected and repurposed for output load usage.  I stuck 1M5 back across the output, so there'd be some sort of bleeder present. 

I did initially listen to the mic at 150V.  It definitely sounds better closer to 200V.  It does not sound noticeably different anywhere in the 185-200 range. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #6 on: February 05, 2014, 11:28:19 AM »
Hi Doug,

 Thanks for sharing the photos and notes.

 It's fascinating.

Gus

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #7 on: February 08, 2014, 11:37:52 AM »
This post reminded me of something I read in the past on the web.
I found the link about direct heated triode
http://diyaudioprojects.com/mirror/members.aol.com/sbench102/dht.html

Another possible reason to reduce the heater voltage or add a series resistor

Do you know what type of transformer construction and lam type was used?

emrr

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #8 on: October 12, 2014, 10:53:08 PM »
This post reminded me of something I read in the past on the web.
I found the link about direct heated triode
http://diyaudioprojects.com/mirror/members.aol.com/sbench102/dht.html

Another possible reason to reduce the heater voltage or add a series resistor

Do you know what type of transformer construction and lam type was used?

That is super interesting.  Thanks for the link.

No idea about the transformer used here.   It's clearly huge, and it's unbranded.  I imagine like many transformers of that era the size is predominately the cast casing, which is probably very thick single layer.  You don't see multiple layer shield cases in the early '30's that I am aware of, at least in America.   Kenyon gets credit here for the multi-case nested shield, a few years after this time. 

I keep sticking this mic up around drum kits, and it's cool. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

PRR

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #9 on: October 23, 2014, 10:03:57 PM »
Thanks for rescuing this monster.

Reminiscent of the Western Electric 47-A Amplifier with 394 Condenser Capsule, but very different amplifier.

"The 6 volt supply should preferably be a storage battery", you have one in your tractor, right?

Peak around 4K-5K is the front cavity resonance. This can be cured. I'm sure you don't want to.

More info:
http://www.tab-funkenwerk.com/id55.html
http://www.coutant.org/we47/we47.pdf

> guess the grid bias is the Voltage average

That's what the electrons think, at higher currents.

However for spec-sheet purposes, the bias is measured down from the "+Fil" pin.

So V1 runs at nominal -4.9V, V2 runs at nominal -6V.

(Yes, the input stage runs hotter.)

I found a Raytheon sheet at Franks (sorry if this is already on-thread):
http://www.mif.pg.gda.pl/homepages/frank/sheets/081/8/841.pdf

For 200V supply and 150K plate loads, I make it out as 60V-70V at plate, nearly 1mA. Rp around 16K (nice fit for high-price transformer), Gm down around 450uMhos. Mu at this point is near 7 (not 8 as on the sheet because we are running much less than 3mA).

Ah, I forgot the 24K dropper, so Vp and Ip will be a little less. 48V and 67V is not wrong.

Voltage gain of first stage about 6. Second stage is *probably* loaded in its plate resistance (by DCR guess-work, and because that sets the 200 Ohm line impedance). So V2 voltage gain is 3.5, OT has about 9:1 ratio. Total gain is 6*3.5/9 or about 2.3. Looks like V2 can swing near 15V peak to the OT, so ~~ 1Vrms maximum output. I think it will clip in cut-off, and have another 3dB-6dB of up-swing before grid current, so it may have sweet overload. (I had an electret FET biased this way when I worked for saxmen. The sax overload was wrong but not unpleasant.)

I'd say, in 1930 studio practice, this thing would never overload. And yes in this day of jazzed-up drums, large amplifiers, and spit-the-mike vocals, you might meet overload.

« Last Edit: October 23, 2014, 10:08:23 PM by PRR »


emrr

Re: Carrier Microphone Company model 94 condenser, USA, 1932
« Reply #10 on: October 24, 2014, 12:27:46 PM »
Thanks.  I still mean to burn down a tractor battery and see what life/cost/sound benefit ratio it gives.  So far with loud sources it apparently matters little. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g