Wednesday, July 31, 2013

ELECTRO-VOICE V-1 RIBBON MICROPHONE

     Is there any mellower microphone than a good ribbon?  These velocity mics are strikingly different to most other transducers -- for one thing, the ribbon is operating above its resonant frequency, rather than below it.   No other microphone works that way and it's part of their warmth and lack of "proximity effect." "off mic" sound even at fairly large distances, as long as the speaker is on-axis, facing one of the wider sides. (I was wrong earlier, ribbon mics actually have a lot of up-close bass-boosting proximity effect. I never worked one very closely, they're very sensitive to plosive sounds.)

     A long time ago, I owned an RCA 44BX, the iconic broadcast microphone.  I fell on hard times and sold it (hoping to buy it back some day) and the mic subsequently had a hard life (damaged ribbon, some sort of plating job, paperweight) until I lost track of it.  So when a "baby brother" (about half-size) made by Indiana's own Electro-Voice* turned up for just over $100 at an antique mall, I bought it on sight.
"After."  The base is not original.
     Of course it was dead.  Responded slightly to a fingernail tapped on the frame but not at all to sound, which is a sure sign the ribbon is against a pole piece, or jammed up with lint.  Close examination showed the ribbon well out of alignment and crumpled.  I wrapped it up and set to one side, thinking some day I'd nerve up to try rebuilding it--

     Fast-forward a couple of years; browsing for a microphone for work, I stumbled across AEA, who still make classic RCA microphones.  They're quite expensive and worth every cent.


     They're also beautiful.

     If you look carefully at their home page, you'll see a "repair form" link.  And not just the RCA types; they sell and support the STC/Coles mics made in the UK, too.   So I called and asked their ace service guy (fixing ribbon mics is the darkest of Dark Arts, combining watchmaking, ship-in-a-bottle skills and fine-arts-grade audio talent, and that's just for a start) if he'd consider fixing an Electro-Voice V-1?

     "There's a V-2 on my bench right now, so that'd be a yes."

     I filled out the form and dithered for several weeks.  This could be costly!  On the other hand, while there are several firms making ribbon mics these days (with varying degrees of success. AEA is the gold standard they aspire towards), repairing them is a much scarcer skill and very nearly died out once already.  Shipped the mic and crossed my fingers.

     Several weeks later, at the Dayton Hamvention of all places, my celphone rang.  AEA:  "Hey, what's that weird connector on your V-1?  We haven't got anything that will mate with it!"

     I had to laugh.  "Sorry, I forgot it's only 1937 in my basement.  It's an early hi-Z connector, used on mics and test gear through the 1960s.  You've called me at the one place where I can be sure of finding one; I'll make up an adapter and send it to you."

     The V-1 uses a "spot" connector.  All later E-V ribbon mics were selectable-impedance and had either an unterminated cable or one of the early multi-pin connectors.

     About a month later, they called with my bill.  New ribbon, new magnets, new rubber shockmount, alignment and test, bench time--  I braced for the bottom line.

     About $150.   Well under what I was anticipating.

     Mind you, by their standards, the little E-V is a fairly undemanding repair and the cast-zinc base prevents much cosmetic refinishing (zamak is unpredictable stuff and only gets more so the more you mess with it).  The end result looks better than it did when I bought it, with the grill all clean and shiny, and sounds great.  Output's not very high -- that's a small ribbon, after all -- but well within what a decent mic preamp can deal with.

     I'm tempted to ask them if they'll look at my Amperite ribbon mic:
     These were, shall we say, built to a price.  This one hasn't suffered a lot of zinc-alloy creep, but it's seen entirely too much knocking around and the guy I bought it from, despite his assurances to take exceeding great care, packed it for shipment like you'd pack a brickbat: no padding at all.  ...I think maybe I'll wait a bit before sending this in.  And maybe bake some cookies to send along with it by way of apology, too.

     Electro-Voice V-1: good mic, back in service.
     AEA: They've made me even more of a fan than I already was!
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* E-V started out in South Bend, eventually jumping the border for Buchanan, Michigan.  They've built good mics since Day One, if you ask me.

Tuesday, July 23, 2013

JOHNSON VIKING CHALLENGER: INDY HAMFEST SWAG, PART FIVE

     E.F. Johnson built a wide range of amateur radio transmitters (among other things), starting in the 1920s.  They hit their stride with the postwar ham radio boom and continued through the early days of SSB.  Every last one of the postwar rigs was a "Viking" something-or-other, which I suppose isn't a huge surprise for a company based in Waseca MN, up there where the descendants of Vikings farmed the land.  The first two, delightful behemoths, were the Viking I and Viking II, but after that, there were Adventurers, Rangers, Navigators, Invaders(!) and the Challenger:
     There was one on a table at the Indy Hamfest, priced over $100 (a bit high, I thought) but the guy was starting to pack up and when he noticed my interest, suggested the price was highly negotiable.  I named a rather low one and he was okay with it -- even added a microphone.

     It's a CW/AM rig, the latter provided by screen-modulating a couple of TV sweep tubes (6DQ6) with a miniature beam tetrode audio tube (6AQ5).  One does not expect high fidelity from this kind of arrangement but given a decent load, it should be adequate.

     The knobs for bandswitch, "excitation" (grid drive) and mode switch are notably missing pointers to tell you where they're set.  The originals were sort of subtle, white plastic pieces glued in at the bottom of the knobs, and I'll have to work out a replacement.

     Does it work?  The seller claimed it did -- it's got Johnson's poorly-documented "push to talk" modification (no schematic, just a step-by-step and partial pictorial) and a connections-unknown mic socket, so I'll want to trace all that out first, clean it up and then, perhaps, power it up via The Gadget. 

     What's The Gadget?  That's a post for another time!

Monday, July 22, 2013

INDY HAMFEST SWAG, PART FOUR

     It's a Weston D.C. Voltmeter, 125 Ohms per Volt:
     Dual-range, which is about par for bench instruments of this age (1920s through '30s) and a handy pair of ranges, too, 0-7 and 0-140 V.  Just right for checking out the 6 V "A" and 135 V (and lower) "B" batteries of a battery radio.

     I paid less than $5.00.

     Sorry about the glare, had to use the flash.

     The March '32 QST used as background is from my collection and features a nice one-tube Hartley oscillator-transmitter using a type '45 tube for the beginning ham.  You thought low-power "QRP" operation was a new thing? Good old George Grammer tells us this transmitter "...will deliver 5 or 6 Watts to the antenna," right in the QRP range.

Sunday, July 21, 2013

INDY HAMFEST SWAG, PART THREE

A "what-is-it?" box.  I was pretty sure I knew...
...Inside, a power transformer, top-hat diode, a lot of R and C and a relay (not visible in this photo) suggested what it might be.  Two pots and a three-circuit jack on the front were a clue.  A pair of insulated screw terminals on the back, marked "S" and "T" were the final giveaway: it's an all-passive-component automatic code keyer!  The two pots are for speed and "weight," the timing ratio between dits (dots) and dahs (dashes).
     For $3.00, it's a good steel minibox and a handful of probably-useful components.

     Elsewhere -- and I should have bought more from the guy, he had a bunch of knobs and dials -- this gadget:
     Just a calibrated knob, you say?  Have a look at the other side!
     It's a nice old vernier dial, made by Crowe up in Chicago.  (You can just read "CRO..." behind the drive at the lower center-left.)  They've been gone for decades but their knobs and dials, etc. still show up.  It's good stuff.  Paid a couple of bucks.  (The one mounting leg that looks broken was made that way.  I don't know why.)  It needs a little fixed scale to be a true "vernier" dial and I may have one the right size.

Monday, July 15, 2013

INDY HAMFEST SWAG, PART TWO

      A low-impedance/high-impedance microphone matching transformer:
     This example arrived with some barbaric ham-radio mic connector on the low-impedance side, a three-pin, threaded-shell item.  ETA: I found the mating connector in my collection!  This is why you save 'em.

       The transformer is a Shure Brothers A86A.
      "Model A86A is a  high-quality cable-type transformer which offers additional versatility when used in conjunction with Shure Models 55,556, and 51 Dynamic Microphones, which employ the impedance matching switch. It solves the frequent problem of installations requiring long  lengths of microphone cables  without the loss of high-frequency response. Model A86A matches 35 to 50 and 150 to 250 ohm microphones to high impedance amplifier input.  Compact, sturdy.
Case diameter 1-5/8", length 2-7/8",  7 foot cable.  Shipping weight 1-1/2 pounds.
Model A86A. Code: RUDEB.  List Price $13.10"

    (Link to a Shure catalog listing it at Preservation Sound.)

     That was pretty a high price, back when this was new.  Frequency response is cited as "within 1 dB, 20 to 20,000 cps," which is darned good.  The various low impedances are selected by removing three screws on the low-Z side, opening the case up and moving a jumper.

     Does this one work?  I don't know yet.  It was priced to move at $3.00 and for that kind of money, worth finding out.

Sunday, July 14, 2013

INDY HAMFEST SWAG, PART ONE

     I collect glass antenna insulators.  I'd love to use 'em but I worry about breakage.  In the old days, they were attractive targets for vandals....

     Anyway, I picked up four at the Indy hamfest, including a second "screw thread" pattern (center left) and an usual large-center one with very rounded curves (lower left).  Here's the entire collection:
     There are at least three different hues of glass: clear, yellow and pale blue-green.  The three large insulators at upper right are supposedly from electric fence service!  (Of course, there's a website.)

A SIMPLE AUDIO SPLITTING/COMBINING PAD

     This is a neat trick for professional balanced audio applications, where a stereo signal needs to be "summed" to mono or a mono one split to two destinations.
     It depends on all the sources and loads being the same impedance, which the resistors match, 600 Ohms in this (typical) example.  Isolation between the two inputs (or outputs) is very good and insertion loss is low.

     Modern practice tends more to quite low-Z sources[1] and very high-impedance loads.  With that, "splitting" one source to several destinations can be a simple (if sloppy) process of paralleling but look out if any one of them gets shorted!  This "Bridge" (as in Wheatstone) pad can be adapted to the low-Z source/High-Z load system by "building out" the pad inputs with pairs of series resistors and adding a fourth 604 Ohm[2] resistor on the load side, but insertion loss will go up as will complexity; a "branching" pad is probably easier--
     --But the isolation between ports is worse.  Still (and especially with a modern very-low-impedance source), a short on any one branch won't silence the others.  (Expanding this to more ports results in increased loss and different resistor values but simple algebra will tell you what they are.)

     This is a little off-topic but analog audio, especially balanced audio, has become more and more "black magic" as the digital age advances.  Perhaps it will be of use to some of the ham operators running AM, who sometimes use older audio mixers and feed multiple transmitters, some of which are former AM broadcast rigs.

     Most sound starts out analog and ends up that way, too.
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1. Some "zero-Ohm" sources are more zero than others.  Back when Purdue Univeristy's WBAA still used Ralph W. Townsely's amazing homebrew (tube-type!) audio console, the line outputs were very high-fidelity audio power amps set for extremely low output impedance and technically-savvy visitors might be handed a paperclip by the (irreverent) student operators and invited to "short out the Line audio."  All that happened was the paperclip got hot; the audio level never changed.  Please don't try that with the "0 Ohm" analog outputs of most pro audio gear -- your results will be quite different.

2. Why 604 Ohms?  It's the nearest 1% metal-film resistor value to 600.  1% resistors cost pennies these days but you could use the closest from the (even cheaper) 5/10/20% carbon resistor bins, either 560 or 620 Ohms.  If you're feeding something critical like a passive equalizer, you may be able to tell the difference.