The Mil-Spec LS-3 Loudspeaker (An obvious candidate for use on 1950s 'boatanchors')
 
 

LS-3 loudspdeaker, a bombproof speaker cabinet with some slits to pass the sound. Made to use with a variety of Signal Corps equipment during the WWII, including BC-189, BC-312, BC-314, BC-324, BC-342, BC-348 and BC-652 radios. These speakers were made by several mfg's; units shown below were manufactured in the 50's by Best Mfg Co Inc. See the image on the left showing LS-3s connected to Collins 51J receivers.

R-390s have 600 ohm audio outputs only, so some sort of adaptation must be made to be used with standard speaker impedances of 4/8 ohms. Although the R-390A operator's manual says the speaker should be the LS-166/U or equivalent, it's unclear this mini speaker is the best option, an LS-3 will provide better audio and will also match the rugged construction of an R-390. For an LS-3 to work with an R-390A, the original LS-3 output transformer must be replaced with a line-to-voice transformer (500/600 ohm input). See examples below.

CASE:
Weight: About 14 lbs (6,3 kg), heavy, uncomfortable to handle.
Dimensions: 8" Width x 8" Height x 4-1/2" Deep (20,8 x 20,8 x 11 cm).
Characteristics: Iron cabinet, satin black finish painting, fully covered inside with about 1 cm thick layer of felt.
INPUT spec:
- JK-33 type jack (Switchcraft S12B Telecom Relay Service Panel Mount connector, 5.23 mm), mates with plug PJ-068, M642/5-1.
- Transformer: 8,000 ohms impedance primary.
- Frequency range: 270 to 3,000 cps (measured at 1 watt out).
SPEAKER:
- Permanent Magnet Loudspeaker.
- Cone Diameter: 6-inch speaker (15 cm).
- Coil impedance: 3.2 ohms.
- Handling power: 5 watts (RMS, not peak).
Original LS-3 parts: Front panel and input jack, iron case padded inside by felt, output transformer and speaker.
 
Matching LS-3 Speakers to Receivers Audio Output Impedances

Original schematic and a basic design to adapt the usual speaker impedances of 500/600 Ω line (e.g. 75A-4 / R-390) or 4/8 Ω audio outputs (mouseOver pic).

* Original transformer replaced by a line-to-voice transformer (right).
* Now LS-3 input impedances will be 500-600 Ω (sleeve) and 6-8 Ω (ring).

Impedance matching for vintage receivers that require or have the option of a 3.2/4 ohm low impedance speaker output (e.g. 75A-4) can be easily accomplished by removing the LS-3 original output transformer and then connecting the speaker terminals to the input jack (see pic below).

• The protective cover on the input jack is removed for easy connection.
• A hand grip is added on top for easy handling.
• Using the original mounting screws, two hooks are added to the upper rear part to hang the speaker on the wall, two large rubber bumpers are added to the lower rear to dampen vibrations.
• 4 adhesive-type bumper feets were added to the base for tabletop use.
 Original and
new transformer
 
Adapting LS-3 speakers to 500/600 Ω line ℤ, 8/4 Ω moving coil ℤ, and planning for wall/tabletop use.