Wednesday, January 31, 2007

RCA 77DX Computer Modeling

Creating a complete SolidWorks package describing an entire RCA 77DX ribbon microphone was quite a project.

Here is the cover shot, in case it got by you the first time it was posted, nearly a year ago. It is missing the circumferential band with the RCA logo, but has the recess like the early 77D and 77DX mics that was designed to accept an engraved brass ring. This was probably a very difficult part to produce. Later on the recess was made shallower and a simple "foil" sticker was used instead.

Sunday, January 28, 2007


A song about Larry Killip's dad, who had lots of valves, by Larry Killip. mp3

KULA Radio Honolulu

Click on this image to see it up close and in detail

Traipsing through a Hanalei antique store a little while back, in search of Hawaiiana, esoterica, or even microphonia, I stumbled across this interesting inscribed glass brick with "KULA" on it. The item was marked "KULA school - $22" so I bought it.

Look closely and see what appears to be an RCA 44ish rendering with lightning bolts radiating from it. Nice! The bamboo fronds add to the island atmosphere of this piece, which might have been made in the 40s or 50s. KULA was an AM station in Honolulu but ownership, location and frequency has changed over the years, so I don't know which "KULA" this is from. Hey that reminds me - I know over at KUAI in Hanapepe that they had and used a 77DX in addition to their boom-mounted BK5. I think I will contact them and see if I can get a shot of that one.

I kept rectifier tubes in this brick for a while, and now I am thinking of filling it with glass marbles instead, for a better look.

About those tubes

Few electronic devices have developed a lore about them greater than the mysterious vacuum tube. Since the first DeForest Audions, vacuum tubes (or valves as the British call them) have had a certain appeal, an observable life to them that was and still is fascinating. After all, how boring it is to gaze upon stone cold FETs.?

It is Winter here in the Northern Hemisphere, and so there is time to test tubes. Shown here is a store-type tube checker, not really a tester, that I keep filled with tubes. Actually, it isn't big enough. Not nearly. I have a lot of tubes here. Octals, loctals, minis, Nuvistors, ceramic tetrodes, power pentodes, glass, metal, ceramic, transmitting, recieving, regulators, ballast, gas discharge, thyratrons, common, obscure, NOS, rectifiers, and more. Names like Sylvania, Tung-Sol, RCA, Mullard, Philips, Telefunken, Raytheon, GE, Western Electric, and even a couple of Svetlanas, Gyorgy, Cunningham and many others reside in boxes, bins and bags in my basement, over by the furnace. I get them in bulk these days. People call and say "Come get these tubes", so I do. And I ask no questions, even if the numbers don't look very promising, those beginning with oddball filament voltages like 18 for instance, which mean old TV tubes that are generally not wanted. I take them all and horde them like acorns.

Real tube testers measure transconductance and can check for leakage, shorts, and are able to quantify emission. If you know what you are doing you can "run the curves" and attempt to match up sets of tubes, devices that rarely match, for a number of manufacturing reasons. I like my Weston portable (Navy type) tester and the Hickok. Both have extensive numbers of rotary switches which must be set to the exact position according to the roll chart or book of tube settings that also contains the good vs bad limits, the numbers that say when to pitch or keep.

Like anything vintage, tubes have their myths that I won't get into here, but I will say that names should not be confused with quality and performance, at least not initially, when evaluating tubes. All but the most persnickety makers relabeled tubes and made various changes throughout the production lifetime, for instance. Two seemingly identical 12AX7s will have different noise, gain and perceptible sound differences, if you have the ear. JAN (joint Army-Navy) tubes were produced by so many different manufacturers that it is often impossible to identify any specific maker, or brand. I sometimes see "JAN" referred to like it was one maker or specification. Often JAN and other mil tubes from WW2 were made in a big hurry and have wide performance variations.

It is always nice to have enough tubes, as you never know when that next Mesa MKII will come through the door, or that Collins 75A4 will show up.

Thursday, January 25, 2007

Scoping out the noise

Here's a shot of the rack mounted scope at my home where I have all kinds of ham radio equipment including many devices that are sensitive to electromagnetic interference, static, powerline noise, and cable TV leakage. Today the noise level at 1.9 Mhz is displayed: The receiver is a Hammarlund SP-600 connected to an inverted vee and the IF output goes to the scope. Yes, that is a Feedback Destroyer in the rack that I use as a notch filter to cut out heterodynes from adjacent stations. It works, up to a point, and makes some of its own interference! What do you want for $40 at a hamfest?

The Microphonium Blog has a new layout and a new template that allows you to look through the titles of the nearly 200 articles that precede this one. Click on the little label to the left of the month to expand the menu, just like you would in a file manager. Then you can select individual posts. Note that the html template I am using has no "home" function on it yet, so try not to get lost. I've been having trouble getting java and other handy features, such as site tracking, to work with this new google product, so far with little luck.

Saturday, January 20, 2007

Early days of loudspeaker audio

The horn arrangement is a familiar one, taken from nature and also from the shape and construction of various wind instruments, like the tuba. Edison used a conical horn for his very first cylinder phonographs, followed later by more esthetically pleasing morning glory shaped horns and finally internally fitted horns placed inside the gramaphone cabinet. The external horn got its reprieve in the late teens and early twenties when radio first started making its way to the well-to-do and to experimenters. Manufacturers fitted bell shaped horns to simple electromagnetic reproducers that were little more than large headphone elements in order to produce loudspeaker volumes.

The radio shown here is a battery operated tuned RF type nicely enclosed in a sloping front cabinet. The excellent lighting and photographic composition, and the appearance of the gent in the photo suggest that this is a well posed shot of someone of note - perhaps a radio pioneer - enjoying the fruits of his labor. Please comment if you know who this is.

The horn is a directional impedance matching device which can greatly improve the efficiency of a small driving membrane, but it has its limitations. Today any audio through a horn like this is considered to be "telephonic" or worse, and it has been difficult to produce horn like loudspeaker systems with adequate bandwidth in the past, though the horn is still very much a part of the auditorium and public address technology mix used today, and is still being improved.

I think he needs a sub.

Wednesday, January 17, 2007

New Search Function Added

Now you can look up every post about the RCA 77DX, or all posts about Hugh Tripp. Type into the "search blog" window above to go directly to a screen with everything at once. Not a list, but more of a compilation according to subject. Give it a try. Type in "transformer" and see what you get.
link to volcano

Sunday, January 14, 2007

Lenin's Lomo On Display at Coral Sound

Zoom in on this incredible Lomo microphone straight from the Sputnik Museum. Well, not really Sputnik, but from Coral Sound that has this great site of Lomo images here.

This mic is marked "LENIN" in cyrillic.
Click on the image to see it up close

All Hail Marx and Lennon was the apparent subtitle of Firesign Theatre's album "How can you be in two places at once when you're not anywhere at all". It showed Groucho Marx and John Lennon with cyrillic lettering. The upside down V is pronounced like an L. The H is like an I and the reversed N is like a "nEEE" Got it? P is like R and C sounds like S. On every city street in Russia you see signs saying "Pectopah". Hint: PECTOPAH is a place to eat.

Coral Sound is at and apparently took this image and all the others. They have an incredible microphone collection and a lot of vintage gear which you should look at on their very interesting website.

Tuesday, January 09, 2007

What is this?

You are looking at a greyscale inverted image of a Naked Eye ribbon mic on a prototype low diffraction suspension mount placed in front of the bikini flag. The idea is to keep the business end of the transducer away from the ring and any other obstructions that might diffract and reflect incoming wavefronts. This mount uses the original Naked Eye ring retainer which allows for easy rotary positioning.

Suspension mounts, shock mounts etc. are often ineffective and transmit low freqency vibrations into a mic body all too efficiently. This one is intended to be a little better than the classic rubber band spider thing seen all over the place. We supply a no charge StickyLipz tm spider suspension mount with Proscenium, Soundstage Image and Studio Vocalist microphones. It is a decent and secure way to hold and position these mics, but like all mounts of its type, cannot attenuate all vibrations conducted through the mic stand. One way to mitigate this common problem is to place the mic stand base on a rubber pad or cushion, or even a section of plush carpet, being careful that the stand is still stable enough so it won't tip and crash.

One did just that recently in the lab. I walked away from a Naked Eye on a long boom arm and ignored the laws of physics. Just as I reached the other end of the lab, I heard a loud crash as the Naked Eye and its low diffraction arm mount hit the hard concrete floor. The mic still worked and tested OK. These are the experiments that we don't plan, but they do happen from time to time! Avoid this kind of experimentation in your studio.

Friday, January 05, 2007

Music Break: Faces On Film

Hey check out Boston-based Faces On Film, featured here in and also here. They have an interesting flash website with a test pattern motif here.