Month: April 2011

Categories
Guitar Equipment Technical

Magnasync Moviola URS as a guitar amp – UPDATE

Magnasync/Moviola was a Los-Angeles based manufacturer of film editing equipment.  They did make a few audio products designed to support the large upright and flatbed Moviola film-editing machines which were their main products.   The most common of these audio-products is the URS 5-watt tube amplifier with built in 4″ AlNiCo speaker.  The URS apparently debuted sometime around 1955.

I recently purchased a pair of these things for a few bucks.  Aside from needing new pilot bulbs and some contact cleaner in the volume pot, they seemed to be working alright.

The rear of the unit (not shown) has a 1/4″ speaker jack (labeled ‘Headphones” which mutes the built in speaker when a cable is inserted.  Other than that, though, there was no obvious input jack.  There is a 4-pin amphenol jack with DC present on 3 of the pins.  the 4th pin is the audio input. Ahh Ok.    There is a also a single-pin DC power connector/takeoff?  Not sure.

Anyhow, kinda irrelevant.  Since no one is going to be using these things with an actual Moviola sound-head, figured I would just make em into lil guitar amps. OK it’s gonna get a little technical here, so pls skip the next paragraph if you wanna just get to the sounds…

Since the tube compliment is 7025 (aka 12AX7) – into 6AQ5 power tube (aka basically lower-voltage rated, small-bottle 6V6) with a single volume pot between the two 7025 stages, this thing is basically…. a tweed fender champ.  Yes, there are some important differences – different plate resistor values, no cathode bypass caps on the preamp stages, and i think some frequency compensation in a the feedback loop?  But basically a tweed champ.  ANYhow…  Following the advice offered here, i put a 1/4″ guitar jack where the single-pin power socket had been.    Then i simply added the other basic components of a Fender guitar-input-stage:  a 1m resistor to ground, and a 68k resistor between the input and the grid of the input tube stage.  Replaced the 2-wire AC cable with a 3-wire grounded cable, and done.

There is plenty of talk on the web about these things…  lotsa folks have converted these to guitar amps in the same manner that i discussed… and people seem to be very happy with them.    Check out this fellow’s work. He did an especially thorough job.  I was personally kinda shocked with the sound that it makes.  Never in my life have i heard so much distortion and fuzz out of an amp.  It really is, pardon my language, fucking insane. Here’s 3 sound clips.  (Gibson Firebird gtr.   SM57 3″ in front of the speaker, into MBox.  ‘Clean’ and ‘Overdrive’ examples have analog echo pedal between the guitar and the amp.  Fuzz is just the amp.  No other eq or processing applied).  Check it…

Clean: Magnasync_URS_clean

Overdrive, fingerpicked: Magnasync_URS_overdriven

Fuzzed out (max volume): Magnasync_URS_fuzz

*************

*******

***

UPDATE:  One of the two URS conversions that I built was purchased by producer P.K.  for use on an album project with a new band out of London.  When the record comes out (assuming the URS tracks make the final mix), I’ll post links to the cuts here so that y’all can hear the sound of these fantastic little amps in-context.

Oh and about that other URS conversion that I built: as of today, it is still on-sale at Main Drag Music in Brooklyn NY for a mere $250.  Call or email them if yr interested.

 

 

Categories
Custom Fabrication Technical

Recent Custom-Build Stereo Amplifier (Home Use)

Above: a very minimal single-ended stereo amplifier with very low gain, designed to accept headphone-level input from an iPod or Laptop and drive a pair of loudspeakers.

The circuit design is extremely simple – Each 1/2 of a 6SN7 feeds the grid of a 6L6 through a .1uf capacitor.  There is a 3-stage power supply with a choke filter before the B+ hits the output transformer.   I have built a couple dozen of these, and the design works well – the sound is very clean and direct; the small 8w output transformers do roll off the very low end, but I have never found this to be an issue with the music that I listen to.   Easy to add a powered sub to the system if one was into rap or heavy orchestral music. BTW, I use this very same circuit, with the same components, for music listening at home; I have mine connected to the output of an Airport Express, powering a pair of Bose 201 speakers. For instances when customers have wanted to hook the unit up to a line-level (rather than headphone-level) output, I substitute a 6SL7 for the 6SN7.  This requires simply changing the cathode resistors on the input tube socket and gives 3x the voltage gain.

What interests me in this design is not the mundane circuit – it is the overall appearance/sculptural aspect of the unit.  I arrived at this particular form through consideration of the appearance of the vacuum tube; as much as possible, I have tried to make the overall complete unit an amplified echo of the tube itself.  The circuit is laid out extremely carefully and the components/wires color-coded (red for B+, orange for audio), green for grounds); it’s my perhaps naive hope that someone unfamiliar with audio circuits could look at/into this piece and maybe gain some understanding of the way that a tube audio amplifier works.

Categories
Uncategorized

Lafayette Microphones of the 1960s

Do any of the Top-Quality mics in the above scan look familiar?

I seem to see… an RCA BK-1A?

….a shure 777 crystal mic?

an Electrovoice 664?

…and what is this thing exactly?

*************

*******

***

I picked up this Lafayette PA-46 for a few dollars at the Flea Market yesterday.  It appeared to be unused; the odd 3-pin socket was unsoldered.  I hooked it up and it works well.  I wired it ‘hi-z’ and the sound is good for a 50 year old dynamic mic. The design seems to be an ‘homage’ to the contemporary Electrovoice 636.  Here they are side-by-side:

…although it also seems to have connection to the earlier Altec 21b ‘coke bottle’ condensor mic:

(image source)

…and maybe the Dukane 95D as well.

*************

*******

***

Lafayette Electronics was an importer and retailer of consumer-electronics active from the mid 1920s through the early 1980s.  They were a competitor of Radio Shack and Allied electronics; when those two retail giants merged, Lafayette was squeezed out of the market.  So what happened to all of the Lafayette shops?  Turns out that may of them became Circuit Cities.  Crazy.  Anyhow, a lot of Lafayette’s merchandise in the 1960s was re-branded Japanese goods that bore a certain resemblance to US-made goods of the period.  These microphones are a good example.  When the Japanese designers got the ‘copy’ exactly right, it’s not that interesting…  but the odd pastiches, such as the PA-46, have a certain weird-dream charm to them.   Check out the complete history of Lafayette at this link.