Gorgeous 1967 Bruel & Kjaer All Tube Swept BPF In Action!

[HideawayStudio]

More lovely old tube gear returned to life....

I've spent the last few days resurrecting two jaw droppingly gorgeous 1967 Bruel & Kjaer type 2107 all tube swept band pass filters to life in The Lab. These units are complex and its been a h**l of an intense three days chasing a string of weird faults but having heard one of them almost running after a day of fiddling I was soon very much driven to fully breath life back into them.



Having finally returned the units to life I then set about fitting IEC sockets (B&K used a now very rare mains inlet socket on most of their gear) and making them a little safer and more appropriate for audio use in the studio as these units feature 90 volt low impedance cathode follower outputs with no DC blocking on the ext filter output (confusingly labelled as External Filter Input!) which is a big accident waiting to happen and their main outputs are very hot indeed so I bypassed the final stages, added high voltage metal film blocking capacitors and the beasts are now very much tamed.



Like so much tube gear they have tons of headroom, are surprisingly quiet for their age and distort very beautifully too.

Here is a quick test recording simply sustaining a chord whilst sweeping the filter to reveal its harmonic content. The filter is regenerative (effectively has a resonance control but in steps) and can go right up to 60dB per octave!

[ppg_wavecomputer]

Hmmm.... I came across one a while ago at a car-boot sale for musical instruments.

I guess I should have bought it. Sounds like 50 Euros well-spent...

Stephen

[monolith]

Wow.. sounds great too.

What is the function of the gears on the back?

[HideawayStudio]

Hmmm.... I came across one a while ago at a car-boot sale for musical instruments.

I guess I should have bought it. Sounds like 50 Euros well-spent...

Stephen

There are indeed bargains to be had (these are fairly rare now and were extremely expensive pieces of highly respected test equipment when new) but just bear in mind that all three examples I've encountered in the past two years have required significant work to return them to life and make them safe. For some reason this particular model seems to be less likely to work out of the box than others but at nearly 50 years old its far from surprising and really needs to be carefully checked over before applying power at the very least.

[HideawayStudio]

Wow.. sounds great too.

What is the function of the gears on the back?

The filter sweeps the full audio range but in bands. The gears on the rear are part of a clever mechanism to switch the bands automatically when you move the tuning control to either one of its extremes. You can turn the pointer off the end of the scale in either direction, the signal is momentarily muted, and the range then automatically changes to the next frequency band.

In some ways its a shame one sweep doesn't cover the entire audio frequency range but it does have the advantage you get much finer control around the frequency range of your choice.

Some of the Bruel & Kjaer filters do cover the entire audio spectrum in one hit but often they do so in 1/3 octave steps like the lovely 2112 which is packed full of a bank of tube buffered passive L/C resonator circuits. In this case the steps can actually produce some very beautiful effects but it does click/pop a little from one frequency to the next.

[Hybrid88]

Sounds lovely, I'd like to get more into tube gear.

Bruel & Kjaer made top notch lab kit right? I don't suppose they every did an audio compressor?

[HideawayStudio]

Sounds lovely, I'd like to get more into tube gear.

Bruel & Kjaer made top notch lab kit right? I don't suppose they every did an audio compressor?

They did and still do have a very strong reputation in vibration/acoustic/signal analysis gear.

No compressor that I know of but they made and still make some of the finest small diaphragm high voltage condenser microphones around. DPA continue to market Bruel & Kjaer microphones packaged/terminated for studio use.

[Mooger5]

This particular unit seems to be pretty iconic. I spotted one in the Captain America - The First Avenger movie. It's instantly recognizable among some other "ancient" equipment in the Red Skull's command center. Funny thing is the action is supposed to take place in the 1940s!

[madtheory]

Lovely looking piece of kit. It's always audio gear used for props in labs in films. Why is that? Is it always better looking than regular lab gear?

[HideawayStudio]

Lovely looking piece of kit. It's always audio gear used for props in labs in films. Why is that? Is it always better looking than regular lab gear?

There are two very famous series of vintage test equipment that have found their way into countless films and TV series over the years and still do to this day..

In many ways the 1960s were the golden years for test equipment in terms of their sheer build quality, longevity, with intuitive and well thought out front panel designs - some of which are now industrial design classics...

1. The 1960s Tektronix 500 Series

The finest scope money could buy back then - they were were used in well kitted out labs and control rooms of all kinds including IBM and even NASA where they can be clearly seen in historic photos (in fact several 500 Series scopes and plugin modules still donning their NASA asset number plates have come up on eBay in the past year).

I have a (huge!) 500 Series in The Lab - built in 1968, with over 60 tubes, it still works to this day with a pin sharp trace and its a work of art both inside and out - in fact I can't think of a finer example of hand built tube technology!







2. 1960s Bruel & Kjaer Acoustic/Vibration Analysis Gear

The Bruel and Kjaer units are actually very confusing - they all have a strong resemblance from model to model and oscillators look remarkably like filters with the same large dials on the front panel. At a glance it is often not easy to tell if the tuning is smooth or stepped.

Probably the most famous picture of a Bruel and Kjaer unit in a studio setting is the infamous Wobbulator used at the Radiophonics Workshop during the Delia Derbyshire / Daphne Oram era:



This is actually a Beat Frequency Oscillator of which I have the model that followed it in my studio..

How many pieces of test equipment are housed in polished dovetailed hardwood cabinets!!?

[Mooger5]

Yes it's the large dial that makes them so distinctive looking. For movie props I guess the crew go for anything unfamiliar to the general public that looks complex, functional and futuristic. So there's a 50s sci-fi movie where three large oscilloscopes stand in a row inside an alien spaceship. And in Space 1999 they're staring at a lissajous figure that's supposed to be a message from outer space. Well this one makes some sense in a way.

[c-level]

forget what i said about you lot only talking about filters all f**k day. this ones well worth the discussion. giving me flashbacks to spacemen 3/spectrum albums. a fine piece i will look out for this brand. top choice of gear and restoration and a great story once again Hideaway!

[ppg_wavecomputer]

Hmmm.... I came across one a while ago at a car-boot sale for musical instruments.

I guess I should have bought it. Sounds like 50 Euros well-spent...

Stephen

There are indeed bargains to be had (these are fairly rare now and were extremely expensive pieces of highly respected test equipment when new) but just bear in mind that all three examples I've encountered in the past two years have required significant work to return them to life and make them safe. [...]

I thought so, and I wasn't sure about the technical state it was in. That's why I decided to leave my wallet in my pocket and be a reasonable boy.

[...] There are two very famous series of vintage test equipment that have found their way into countless films and TV series over the years and still do to this day..

In many ways the 1960s were the golden years for test equipment in terms of their sheer build quality, longevity, with intuitive and well thought out front panel designs - some of which are now industrial design classics...

1. The 1960s Tektronix 500 Series

The finest scope money could buy back then - they were were used in well kitted out labs and control rooms of all kinds [...]

About ten years ago, I found one of these standing outside by the wayside in the pouring rain (if that isn't the stuff romances are made of I don't know what is), to be taken to the landfill. I loaded it into my car (I have a soft spot for all things electronic and neglected) and took it home, gave it a thorough cleaning, and it didn't even electrocute me when powering up. It didn't work properly (of course) and, being such a huge and heavy beast, I put it up for auction on eBay since it was only serving decorative purposes in my music room.

A very happy buyer came to my place to collect it in person. He didn't mind a 700-kilometre drive at all.

The missus wasn't all too happy, telling by the look on her face.

Stephen

[Cybercardinal]

Wow..cool units. My sister used to live next door to B&K, I have a 10 min drive to them from where I live. Will be looking out for those units..

[meatballfulton]

Missed this thread the first time around. Love that Tek 545. Mine is not as pristine as yours but for $25 with three probes, I am not complaining I also have an old RCA audio oscillator I paid $20 for...not B&K quality, that's for sure!