More pictures of OA-6 Type I
HISTORY
In 1953 Stig Carlsson designed the predecessor to OA-6, The coalscuttle.
Thanks to its natural and un-strained sound the Coalscuttle got a lot of attention and became a reference for many Swedish audio maniacs in spite of the small quantities manufactured. The great success with this speaker lead to the development of the speaker resulting in the new design first called “Lund OA-25” or “McProud”.
In 1966 Sonab started to produce the speaker in its finalized design, now named Sonab OA-6.
This was the first model that was given the now classic Carlsson design, like a standing matchbox, simple and elegant.
The OA stands for “Ortho Acoustical” and the figure six for the number of elements used, one woofer, one midrange and four tweeters. This was the same elements as used in the Coalscuttle except for the Peerless woofer laced by a new Sinus woofer.
The OA-6 was mainly intended for professional use and for audiophiles with large rooms. The critics at the time were baffled. Quote from R&T: “a level of clarity that is remarkable… … an astounding sense of being present”. The Sonab OA-6 became the reference for several musicians and composers like Liegti and Stockhausen. For sound technicians and also for the Manager of Stockholm Concert Hall.
The design is now more than 35 years but even today it is difficult to find a speaker giving the same natural, musical and airy sound reproduction.
Sonab OA-6 Type 1 (Mk I) was on the market from 1966 to 1970.
There was also a special preamplifier for the OA-6 Type 1 (MK I) the CC-1, a mono design. It was for its time an advanced design with unusual functions like a tape monitor with level adjustment, all inputs were doubled, a four way loudspeaker switch and with tone controls that could be engaged in incoming or outgoing signal.
Sonab brochure from 66
RESTORATION
The OA-6 is partly very difficult to restore. Many of the easier parts can be made according to Restoration and upgrading.
There are however some areas that requires special attention.
The Tube amplifiers need seeing to after perhaps 35 years of service. Capacitors, tubes and perhaps some of the resistors need to be replaced. Changing the signal connectors and wiring as well as power cables and power inlets is also a good idea.
Consider to rebuild the speakers to grounded operation.
Removing the tube amplifiers is rather easy, preferably do your work on a soft carpet to protect the surface of the veneer. Start with undoing the two screws holding the lower protective grill, remove the grill and pull out the four output tubes and put these away. Put a sock or similar in the bass reflex tube to prevent screws and other things to fall into the tube. Disconnect the signal cable leading to a large grey coupling with many pins. Remove the power intake. After this find the two screws fixing the amplifiers. They are large and hexagonal placed one on each side of the big transformer between the two bass reflex tubes. Unscrew the screws and remove the amplifiers, this might take some coaxing as glue/damping material is probably stuck to the wood. Have the amplifiers restored.
Gently de-solder and unscrew the 10” woofer. Be very careful as this unit is almost impossible replace. Restore the woofer acc to same principles as for the 9710 in Restoration and upgrading. Change all the internal wiring, connectors etc in the bottom plate.
Turn over the speaker and de-solder and unscrew all elements.
The crossover for midrange and tweeters is located in the midrange cavity. Remove and inspect, preferably re-solder all joints and replace all wiring. Check the midrange and restore according to Restoration and upgrading Don’t forget that these are 800 ohms elements, they shall read approx 700 ohms on the meter.
Remove the brass borders and polish them.
Most other things according to Restoration and upgrading
UPGRADING
Damp the midrange cavity with asphalt damping felt, preferably a thick one.
Change the capacitor in the filter but use the double value for better sound.
Change the tweeters to Peerless CT62, 16 ohms, preferably in “Y configuration”, connect these in series parallel (resulting in 16 ohms in total).
Setting the tweeters in a “Y configuration” is highly recommended by CarlssonPlanet and the Stig Carlsson Trust. A brief instruction is given in our Swedish newsletter no 5.
If you want detailed information how this is done with instructions and pictures this can be purchased prom CarlssonPlanet, click here.
If you still have problems contact CarlssonPlanet.
TECHNICAL DATA Sonab OA-6 Type 1
According to manual and technical magazines
Box volume: 45 litres, estimated
Measures: B x W x D: 29 x 77 x 51 cm
Wall thickness of box: 10mm heavily braced.
Weight: 22 Kg
Principle: Omni directional, three way bass reflex type. Two separate tube amplifiers, one for low frequencies up to 250 Hz and one for higher frequencies above 250Hz.
Frequency range: 24-18.000 Hz (Estimated)
Frequency response: 25-14.000 Hz ± 4dB
Crossover frequency: 250 and 2.500 Hz
Woofer: 10” 800 ohms downwards firing.
Mid-range: 1pc 22 cm 800 ohms Philips 9710AM full range speaker
Tweeters: 4 pcs 5 cm 16 ohms Peerless MT20HFC
Distorsion: 0,2 % at 12W and 30-12.000 Hz.
Power consumption: 100VA.
Tubes for both amplifiers:
Driver & phase splitter: 3 x ECC83
Output tubes: 4 x EL86
Two built in tube OTL amplifiers.
Finishes: Rosewood, walnut, teak, oak, natural pine and white lacquer.
Etc.
Prices
Price 1966
2.400 SEK excl VAT
(One of the cheapest speakers ever, that is, it gives good value for money.)
Second hand
Nonrestored pairs can be found for 5-6.000 SEK.
Restored and upgraded can cost something like 12 – 25.000 SEK per pair depending on how much is done and how well it is done.
Martin Dellstig - Co-Editor
This page was last edited 2007-09-26 21:01:14 by Per