Look what’s on the bench today….
 |
| RCA PB90 ribbon microphone from 1934 |
This is a beautiful RCA PB90, in complete original condition, but need of a little loving care to bring it back to its former glory. Here’s the output transformer….
 |
| RCA PB90 Ribbon mic transformer, 1934 |
It’s date stamped 21st September 1934! 78 years old. The mic shares a birthday with Leonard Cohen!
That’s amazing.
Here are links to some manuals and documents from Grampian Reproducers Ltd.
Grampian DP4 dynamic microphone manual.
Grampian price list from June 1976.
Here are some more ribbon microphone related patents, both US and British, from the 1930s, 40s & 50s.
The diagram (above) is from one of the patents, and may be the first tube ribbon mic!
Xaudia customer services!
 |
| Xaudia ‘Trashcaster’ guitar |
A couple of coats of clear lacquer helps the slide-on decal blend in. But perhaps I should have called it the ‘Nashmaster’. Too late now! The last job was to file the nut to take the narrower gauge Nashville strings, and to do this you really need a proper set of nut files. They are not cheap but I found these ones on ebay for around £40, and they did the job quickly and accurately.
Here’s a broken EF40 pentode tube. I dropped it, so took the opportunity to take a peek inside. The construction is intricate and quite beautiful.
 |
| Inside the EF40 tube, glass envelope removed |
 |
| EF40 pentode with screen removed |
 |
| EF40 with getter, mica insulation and plate removed |
 |
| EF40 showing grids heaters and cathode |
Vacuumtubes.net have a good description of how tubes work, and this diagram from shows all the parts:
I have no idea who made October’s microphone of the month. But it is quite a beast, dwarfing the SM57 that I have used as a geologist’s hammer*.
 |
| Italian ribbon microphone by an unknown maker |
This one came from Italian ebay, and has a whiff of DIY about it. Except that if it is, the work quality is very high. It could well have been a project for an engineering degree. Or perhaps a prototype from a professional workshop? Some parts of the mic are well thought out, but ultimately it is let down by the transformer and low magnetic field.
The microphone body is based around brass tubing with an imposing chromed grill and chromed bells at each end. The base is fitted with a (horrible) 4 pin CB-radio style plug. The yoke is nicely bent aluminium with a turned base and wingnuts.
 |
| Italian ribbon mic – motor and magnets |
Inside, four large cylinder magnets are clamped between two heavy blocks of mild steel, and the ribbon motor sits at the centre. There is a hint of the RCA 44BX in this approach, and the ribbon is of similar dimensions to that mic. The ribbon itself is clamped with sturdy brass blocks. These magnets here are not really strong enough for the job and the measured field in the ribbon gap is around 1000 gauss – a bit low really. Perhaps they were once stronger than this.
The transformer has its own internal can for extra screening, and the connections to the tranny primary are made with heavy copper for very low resistance. Good thinking!
 |
| Ribbon mic transformer |
The transformer itself looks home-made from recycled laminations. Although the thick copper wire for the primary is a sensible choice, the inductance is a rather low 24 microHenries and there is no chance of reproducing a full frequency range. These lams look familiar – I have seen similar ones in Thiele microphones for the power transformer, and also, I think, in Geloso amplifiers
 |
| Domed end cap of the mic, with striped output leads |
The maker, whoever he was, has used brass, mild and stainless steels, copper and aluminium for the construction. Parts are turned, milled, brazed, folded and domed. It looks more and more like an engineering workshop project designed to showcase the maximum number of skills, and in that respect it does a very good job.
Better laminations or a new transformer would show the true potential of this mic, as would some stronger magnets to boost the field. I will try that!
A few weeks ago we received this rather forlorn looking box of Melodium bits for service!
 |
| Melodium 42B stripped down to parts |
The good news was that all the important parts were there, and despite some corrosion, the magnets and the transformer were on good shape, which meant that this vintage gem could be restored to some of its previous beauty.
 |
| Melodium 42B repaired and re-ribboned |
The mic was stripped down and all the parts cleaned up in the ultrasonic bath. Then the magnets and some other bits were painted to stop the rust returning, and the motor reassembled. Dino (the owner) wanted to retain the vintage look of the mic, so the grills were straightened, de-rusted and then given some clear lacquer rather than being refinished. New grill cloth gives some protection against pops and wind blasts.
Then it was put back together and a new cable fitted with XLR output. And of course a new ribbon. The mic looks pretty damn cool!
Thanks to Dino Jakobsen of The Why Project.
The paint is just transparent enough to let a little of the grain show through. For a high gloss finish it should be given a few layers of clear lacquer, but I’m aiming for the less shiny look of budget 60s guitars so will probably leave it. The scratchplate is less than mint, and so a perfect body may look a bit incongruous. The replacement chrome control plate came from a Fender Jazz bass, adjusted slightly with a file to take off some pointy edges.
The extant screw holes in the body didn’t fit a standard neck plate, so I’ve used these ferrules and screws from StewMac. They work very well and solve the problem. Also, the original tremolo bridge was missing. I have replaced it with a used hardtail bridge from ebay (£6!), and machined a brass block to fill the hole where the vibrato spring must once have been. Perhaps that will give a little more sustain too.
The Oktava MK18 is the less well known, but more sophisticated older brother of the ubiquitous MK219, which is used and abused by hoards of mic-modders who love to chop and swap grills, capacitors and resistors.
 |
| Oktava MK18, front view with pattern switch |
In contrast to the cardioid-only 219, the MK18 is a dual diaphragm, multipattern condenser mic, with bass roll off (HPF), a -10dB pad, and pattern selector switches.
 |
| Oktava MK18 rear view with pad, HPF and rear capsule switch |
It also has a fourth switch with large and small cardioid symbols. This controls the ‘mix’ of the rear capsule, allowing the in-between patterns to be selected, to give hypercardioid and supercardioid response. Clever!
 |
| Oktava MK18 inside, front view showing components |
The switches are actuated by sprung bits of bent metal, and they make the mic very tricky to reassemble.
 |
| Oktava MK18 inside, rear view showing PCB traces |
The mic has that typical Oktava quirky build style, with etched PCBs and a small square can output transformer that looks different in size style from the ones in the MK219 and 319. The connector is a push-fit din plug!
Unfortunately this mic is designed to run with 60V rather than 48V phantom power. It will work with a normal phantom supply, but is currently giving erratically output, and may need some small modification to make it behave itself!
