The speakers:
The larger of the two, a huge SABA 1675U10 with TIGGES Permadyn magnet in Alnico measures 35 x 24cm.
It is a loudspeaker built starting from 1953, with a magnet with a flux density of 8000 Gauss, an impedance of 4.1 Ohm and a voice coil of about 25 mm.
It has a frequency response that extends from 55 to 10,500Hz within +/-3dB.
The entire frequency range is sent to this loudspeaker and the high part is cut mechanically.
The cone, despite being very large is relatively light, this property allows you to have bass that are very ready, really powerful and without tails.

In our opinion, the original 5330F15 electrostatic tweeter is too inefficient compared to the large oval mid-woofer and above all it has a too high cutoff frequency. It begins to feel starting from 9600 Hz, and at a level that is too low anyway. It works therefore as a supertweeter rather than a tweeter.
This loudspeaker is placed in front of the full-range, almost in the centre. This arrangement (if it were placed right in the centre) would in our opinion be the ideal arrangement, see Goodmans Triaxiom, Altec Lansing Duplex and so on, because it allows very precise focusing of the sound image. However, since the mid-high frequencies emitted by the full range are concentrated right in the central part of the cone, the treble is mechanically hindered by the electrostatic loudspeaker placed in front.
The end result is a sound that tends towards gloom and with some unpleasant reflections as the electrostatic tweeter is not placed exactly coaxially with the mid-woofer.

We fixed these issues with two essential changes:
1 - Replacement and correct placement of the original tweeter with a Saba 5238U8 cone tweeter
2 - Made the frame with the front canvas removable

1)  The Saba 5238U8 cone tweeter has a diameter of 112 mm, is equipped with a 7500 Gauss magnet, a very light 12 mm voice coil and an impedance of 5.1 Ohm.
The frequency response starts already regular from 1500 Hz and rises up to about 13 kHz, and then returns to decrease towards 20kHz.
Remember that all acoustic musical instruments that reach higher are the cymbals (Cymbals) that reach just over 10 kHz.
The efficiency of the tweeter and its low cut-off frequency compensate for the mids and trebles generated by the central part of the rear mid-woofer which are absorbed by the shadow cone of the tweeter itself.
The tweeter is positioned exactly in the center of the mid-woofer, in a perfectly coaxial position, thus avoiding the annoying reflections generated by the misalignment of the original electrostatic tweeter, maintaining the invaluable advantages of coaxial speakers.
Finally, the crescendo response curve up to 13 kHz means that the treble is really very bright.

2) The front canvas covering the speakers is never completely acoustically transparent.
For this reason, the only way to obtain maximum performance from the appliance is to eliminate it.
We built a dummy frame into which the original canvas was installed. This frame attaches to the speaker panel with magnets.
In this way it is possible to detach it when you want to listen to music with maximum transparency and put it back in its place when the radio is at rest to protect the loudspeakers from dust and to restore the original aesthetic appearance of the unit.

All this translates into exciting dynamics, brilliance of sound, lack of oscillation of the canvas in the presence of low frequencies, making listening to this beautiful device very realistic and pleasant. With a pinch of eccentricity.