R/E/P Community

R/E/P => Klaus Heyne's Mic Lab => Topic started by: Michael O. on June 17, 2016, 02:46:25 am

Title: Hartsfield C-2 Tube Condenser Mic
Post by: Michael O. on June 17, 2016, 02:46:25 am
Hey again everybody,

     I recently bought a Hartsfield C-2, a very small tube condenser microphone from the mid '50s, ostensibly somewhat akin to the Altec lipstick mics and Stanford/Omega Condenser mic. There's virtually no info online about it other than the rather technical (and tangentially Neumann related, more on that later) U.S. patent info (http://www.google.com/patents/US2910539), so I thought I'd contribute what I know in hope that one of the more informed forum members might be able to enlighten me further.

     First, the mic itself is minuscule, almost lavalier sized (see attached pic for scale). I'm assuming that no tube could fit in the housing, and I chose not to fully disassemble it to confirm that assumption. It's very well-machined, and houses an apparently gold-skinned capsule that (due to the lack of rear vents) is likely omnidirectional. It has a removable pocket clip and features a captive cord with the same 6 pin connector found on the similar Altecs.
     The power supply has an on/off switch, Music/Voice switch, and a power indicator light. Inside there's seemingly just 4 huge capacitors, the power transformer, an orange relay(?), and the tube. The tube is an enigma to me: it's either metal (copper?) encased, painted a copper color, or has an atypical metal shield on it. Either way, I couldn't identify the tube without pulling it. Finally, I have to assume the output from the power supply is high-impedance due to the circuit's lack of audio transformers.

     Sound wise, mine works, but is thin and notably quiet (the signal that is, not the noise, haha). It's clearly not up to spec, but I haven't done anything to remedy this (e.g., retubing, checking the power transformer, etc.) due to my lack of expertise. I'd love to get this thing working, but have no leads on who might be able to do the tech work.

     The mic was designed by the same William Hartsfield who designed the nearly-legendary old-school JBL speakers that bear his name. As for the patent, the drawings of the capsule's backplate make it look remarkably like that of the kk84 (to this layman, at least). Further, it lists a 1965 Neumann microphone patent that makes reference to the C-2's patent. Maybe the existing IP was an inspiration for a Neumann capsule design?

Anyway, I hope someone out there has some additional info on this obscure and interesting old mic, and any lead on who might be able to return it to its former glory are welcome. I'm thinking if anyone out there knows anything about this mic, it would be here.

Thanks again for your seemingly endless collective knowledge,
Michael O.
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: Michael O. on June 17, 2016, 02:47:40 am
(extra post to include power supply internals photo)
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: klaus on June 17, 2016, 03:00:59 am
I am pretty sure the tube is inside the housing of the mic. (Why else would it need a massive power supply with all that filtering?)
Don't be shy, open it up!
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: panman on June 17, 2016, 07:44:02 am
I second what Klaus said. A Telefunken AC 701, Hiller MSC 1.2 and MSC 4 would well fit in. Without knowing the exact measures it is impossible to say if Hiller MSC 2 or MSC 1.4 would fit as well, because those tubes are of bigger diameter. Hiller M60,59,58 mics have only the capsule and the tube in the mic-housing like AKG 60 as well, but the trafo is yet included, but either as a separate unit(Zwischenverstärker for Hillers) between the mic and the PSU or inside the PSU(AKG 60). What really puzzles me is the extra long mic-cable. High impedance coming from the mic. That really would not work? Must be a cathode follower, but then the trafo should yet be inside the PSU? The pocket clip suggest a lavalier-type meant for speach, but then again the music/voice switch indicates a normal bass-responce. Just my thoughts.
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: Michael O. on June 18, 2016, 12:54:39 am
Well, if Klaus Heyne says it's okay, I'm feeling confident enough to open it up, hah. I left the studio (and the mic) early tonight, so I'll get on it tomorrow. In the meantime, I'm attaching a photo from yesterday of the mic semi-disassembled. it unscrews near the middle dividing the housing into a capsule half and a cord half with the typical pin-type connector between. The capsule half is the only obviously openable bit, requiring (what I think is called) a spanner wrench to undo that slotted ring that's half visible in the photo.

I'll measure it as well if I don't find the tube, but I'd estimate the housing's circumference is less than 3/5 that of a typical sdc (e.g. KM84 or C451E). It seems like it might be too tiny even for a nuvistor (which wasn't around in '56). If the tube isn't in the mic itself, I too am very curious how it would work impedance wise with the overlong cable going to the ps. Hopefully I can unravel some of the mystery tomorrow.
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: klaus on June 18, 2016, 04:59:24 am
Looking at the patent narrative, it's notable that the inventor proposed to use Mylar® as diaphragm material in 1956, roughly a five years after polyethylene terephthalate was invented. Neumann's first usage of that material was 1958/59 (K47/49), and none of their small diaphragm mics received Mylar diaphragms until early 1966 (KM64/U64).

It's unclear from the patent filing how the core of the invention-a "constant backplate-to-diaphragm gap", regardless of mechanical or thermal conditions- was achieved, or whether that was simply a function of the new (more stable) diaphragm material. The backplate waffle pattern of this 1956 invention looks remarkably similar to that used in Neumann's 1964 KM6x and later backplates.

The Hartsfield mic amp, from what I can glean, was probably not exceptional, because it's not mentioned in the narrative.
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: panman on June 18, 2016, 05:08:24 am
I would advice you not to try and open the capsule part yet! I see now, that the tube is not likely to be there. If i see right, there is only one connection in the middle and the housing thread for the capsule terminations. Not enough for a tube. A triode would need at least four. The tube may yet be inside the longer rear-part. There must be a way to open it.
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: klaus on June 18, 2016, 05:58:05 am
It should not be that hard to determine which of he two housing sections contains the tube by measuring the capacitance at the juncture: there seem to be two electric contacts inside of the part you are holding in your hands- if the capacitance across is 100pf or lower, you know, it's the capsule side you were measuring, hence the other section must contain the tube...
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: Michael O. on June 20, 2016, 05:33:30 pm
It's interesting that the Mylar diaphragm and the now typical waffled backplate prefigured the Neumann designs that I always assumed were totally novel and original. I suppose it makes a certain sense that it would take Dupont's then new material a few years to catch on outside the states.

Klaus and Panman, thank you for the sage advice; measuring the capacitance should be a foolproof and assuredly nondestructive indicator. I had time to take another look at the power supply, and was able to remove the socketed mystery component (photo attached). It has an octal base, and doesn't have a separate metal shield over it, but is apparently painted a metallic copper. The way it felt in my hand and my intuition are leading me to believe it is a potted transformer rather than a tube.
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: boz6906 on June 21, 2016, 09:24:53 am
Intertesting mystery component... Most of the plug-in xfmrs of that era used n 11-pin coonector.
An Ohmmeter should confirm if it's a xfmr.  Another possibility would be a solid-state rectifier replacement, is there a filament supply connection on the socket?
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: Michael O. on July 22, 2016, 06:19:21 pm
I finally got a multimeter for the studio, so I'm back with measurements. The capsule side (using the pin and spring as contacts) has a capacitance measuring .1 nf, which, if I remember my metric conversions well enough, is 100pf.

As for the mystery component: there's continuity between pins 1+3,4, and 5, 2+3,4, and 5, 6+7, and finally 8 has no continuity. This leads me to believe that it may be a transformer where 1-5 are different taps for the primary, 6+7 is the secondary, and 8 is the ground, but I'm a novice and don't know what readings to expect from e.g. a tube or ss rectifier. Additionally, the casing is definitely metallic.

Anyone know a tech who specializes in obscure 1950's American condenser mics?  :)
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: soapfoot on July 24, 2016, 08:27:20 am
Question for the panel--

is it possible that the mic is a cathode follower design, with the tube in the mic, but the transformer in the power supply?

If so, that plug-in component might be the transformer.

If you trace the PSU circuit, is the music-voice switch connected to that (transformer?)'s socket, possibly switching between different taps for music and voice?
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: boz6906 on July 26, 2016, 01:48:10 pm
Good idea... But most of the M / V switches I've seen in US mics use a separate inductor for the high-pass circuit (like RCA 77 series).  The inductor will be connected to the output xfmr.
Title: Re: Hartsfield C-2 Tube Condenser Mic
Post by: bseabrook on March 31, 2017, 06:23:45 pm
I was a recording technician with the USAF Band from 1963-67 in Washington,DC, and I remember using the Hartsfield microphones for studio recording as well as sound reinforcement at outdoor concerts. Our recording supervisor, MSgt Leslie R Ticknor had a personal connection with William Hartsfield, the inventor of the microphone. The band also owned several portable Hartsfield sound dispersion lenses which we used with portable loudspeakers to improve the coverage pattern. They resembled the Hartsfield lens which was part of the JBL Paragon speaker system.