1950's Parmeko Co-axial 15" BBC speakers

[tom1356]

Hi,
I have recently acquired a pair of Parmeko LSU/10 Broadcast monitors used in the BBC studios in the 1950's and 1960's.
I am looking for any information about them.
They contain a LS-1 15" co-axial driver with an (added by the BBC) Lorenz LPH 65 super tweeter. The co-axial tweeter has a six segment cellular horn. The Lorenz tweeter has a clear poly cone.
There is a space in the cabinets for a tube amplifier.
Any and all help/information would be very much appreciated.

[tom1356]

front

[EC8010]

My word! How on earth did a pair of those wardrobes make their way across the Atlantic?

The amplifier was an LSM/8 which was a slightly modified Leak TL12 using triode-strapped KT66 to produce about 15W. I've never seen an LSU/10 with those tweeters - the one we had (when I was at BBC Broadcasting House) had a pair of Celestion HF1300 mounted in front of the 15" driver.

[tom1356]

EC8010

They made thier way across the pond with the help of a whole lot of money and Virgin Atlantic airways.
In this photo you can see the tweeter clearly.

According to Edward Pawley's book All the LSU/10's were augmented with the Lorenz tweeters when the 405 line television standard was introduced.
Thanks
Tom

[EC8010]

They flew?! Ours definitely had Celestion HF1300 - perhaps a later modification. (By the way, 405 line television predated the LSU10 - the Marconi/EMI 405 line and the Baird system were in use in parallel before WWII.)

Here's something that may be of interest.


Instruction S.8

Cabinet LB/8 (Fig. 1.3)

The cabinet is constructed in the form of a vented enclosure which gives a smoother and wider-range bass response than the box baffles used hitherto. The rear of the cabinet is completely boxed in and an opening or vent in the form of a rectangular hole is provided in the base. This vent opens into a cavity between the base and the floor and is bounded by the plinth which supports the cabinet. The plinth is arranged as three sides of a square and directs sound waves transmitted through the vent outwards from the front of the cabinet. The vent and associated cavity have the nature of an acoustic inductance whilst the air enclosed inside the cabinet behaves as a capacitance and the dimensions are chosen to give resonance at the lowest frequency it is desired to reproduce. Low-frequency sound waves generated at the rear of the loudspeaker diaphragm are transmitted through the vent and, over a limited frequency range, are in phase with the radiation from the front of the loudspeaker. The design is such that the useful range of the loudspeaker is extended downwards by about half an octave.

To prevent high-frequency sound waves being transmitted through the vent the upper half of the cabinet is lined with about 1¼ inch thick slag wool. This is obtained in pre-formed slabs which are stuck to the sides of the cabinet and are then covered with a layer of carpet loom felt. Such treatment is effective in suppressing high-frequency sound waves but does not sufficiently eliminate low-frequency vertical air-column resonance which occurs at about 120 c/s. This is suppressed by three ¼ inch layers of carpet loom felt stretched horizontally across the centre of the cabinet where the standing waves have maximum particle velocity. Very little high-frequency radiation reaches the lower half of the cabinet and the only acoustic treatment necessary is provided by a single layer of carpet loom felt. If a thin layer of absorbing material were attached directly to the walls the absorption would be low because particle velocity is a minimum close to a reflecting surface. For this reason the felt in the lower half of the cabinet is attached to battens to space it about 1 ¼ inch from the walls.

The loudspeaker amplifier LSM/8 is situated on a shelf within a small compartment in the lower half of the loudspeaker cabinet and at the right-hand side (viewed from the front) and the gain control is accessible through a hole in the lower right-hand panel. This panel and the rear panel adjacent to the amplifier have windows of expanded metal to provide ventilation and a baffle is fitted to the amplifier chassis to direct hot air out of the amplifier compartment. Before the amplifier can be removed the side and rear panels of the amplifier compartment must be detached. These are each held in place by a number of screws and the amplifier itself is secured to the shelf by four bolts. There is, of course, no need for acoustic treatment of the interior of the amplifier compartment but the outside is lagged with three layers of carpet felt.

[tomtt]

http://www.german-vintage-loudspeake...hp2_nav_id=109
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could you post a pic of the backs of the drivers?

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http://www.oldsms.co.uk/gear/typeb.php

[tom1356]

EC8010
Thank you so much for that information. It really helps a lot.

tomtt
Here is the back of the driver. They have been in cloth bags since new.

PS: Thank you very much for the links.

[tom1356]

spider

[tom1356]

Dividing network.

[tom1356]

I forgot to mention these are the speakers used at Abbey Road studios to record The Beatles.
Pretty cool huh?