Philips BD753A Capella 1955
Circuit: 10-tube superheterodyne, AM/FM,
Boradcast Band, Short and Long Waves, AC, automatic tuning, 3D Sound. FM
to 100 Mhz. This is likely to be the largest table model by Philips
after WWII. The only one that I know that is larger is in the B9X
series, and I am keeping an eye open for one of those, as are many other
collectors, unfortunately. This is, of course, a superb radio.
The FM tuner has a typical ECC85, followed by an ECH81 as local
oscillator/mixer in the AM bands, also working as the first IF amplifier
in FM. There is nothing out of the ordinary in the circuit,
apparently, except that it works with great sensitivity in all
bands. The IF amplifier consists of an EF89 and the pentode section
of an EBF80 working in all bands. One of the diodes of the EBF80
does the demodulation for the AM bands, while the other diode provides the
AVC voltage for those bands, acting on all the RF and IF tubes in
AM. For FM, the double-diode of an EABC80 is the ratio detector for
the FM band, with the third diode providing AVC for that band. The
triode section of the EABC80 is the first audio preamplifier. Following
this tube are the various tone controls, eventually driving an EC92 triode
which acts as a second preamplifier tube. The output of the EC92 is
split into two channels, low and high frequencies. Each channel is
amplified by a series-pair of UL84/EL84 tubes (the four output tubes can
be seen on the right side of the second picture). The output of the lower
frequency channel goes directly to the large speaker (about 10 inches)
without output transformer. The voice coils of all the speakers in
this radio have an impedance of approximately 600 ohms which is matched by
the output impedance of the series-pair of output tubes. The higher
audio frequency channel output drives either all the smaller speakers (two
lateral and one frontal) or, in a 3D mode, only the two lateral speakers,
thus creating a 3-dimensional effect. Again, there is no output
transformer. An EM80 is the tuning indicator. Rectification is
by solid state diodes.
Automatic tuning is of an electro-mechanical type, a
modern version of that in the Philips D59AU in my collection. The
first left button in the "keyboard" is the off-on switch, then
the next three buttons are presets for the AM band, while the three
right-most buttons are presets for the FM bands. In order to preset
one station, one pushes the desired button and lets the dial go wherever
it was initially preset to. Then, one pulls the tuning knob and rotates it
until the desired new station is tuned, then released. On any
subsequent pushing of that same keyboard knob, the last station assigned
to it will be tuned quickly and very accurately.
It must be mentioned that the "upper" tube on
each series-pair is an UL84 for this radio, as it was the only tube at the
time that could stand a high filament-to-cathode voltage. That
cathode is working in the vicinity of 120 volts and leakage would result
in hum. The UL84's necessitated a special winding in the power
transformer to supply the high voltage filaments. Soon after 1955 the EL86
tube was developed that also had high filament isolation, but worked at
6.3 volts. The series-pair tubes work very well, with extremely
clean sound, provided that the coupling capacitors in all the grids and in
the feedback circuits are free of any substantial leakage currents.
Also, the quality of the UL84 tubes must be excellent. The pairs
work a bit like the later output transistorized circuits, in which the
upper tube works as a current source for the lower tube. I have not
studied the circuit well, but it certainly provides very clean sound,
although not as powerful as a push-pull of EL84s, for example.
Tubes: ECC85, ECH81, EF89, EBF80, EABC80, EC92, 2 x UL84,
2 x EL84, EM80.
Source: Helmut Bremen, from Aachen, Germany, through
eBay Germany. Was initially shipped to by friend Felix Valencia, in
Spain, as it was not possible to ship it directly to the US in any
reasonable way. Felix took it apart, sending me the cabinet and
speakers in one box and the chassis in another box. Felix sent the
two boxes by Air Parcel Post as a Christmas present.
Date acquired: October 2003.
Initial Price: Radio 116 Euros, shipment to Spain 40
Schematic: Obtained from RadioMuseum.
Condition: Cabinet is in very good condition, with
no major problems, except that the lacker is not very shiny any more.
The chassis needed substantial attention, particularly with filter and
coupling capacitors. A few tubes needed to be replaced, particularly
critical was to obtain high quality UL84s. I have not touched the
mechanical station tuning, as it works quite well, with 2 FM stations and
4 AM stations being tunable. I learned earlier
with a D59AU that I had that you do not lubricate the mechanism indiscriminately,
one has to know well what one is doing. Since the one in this set
works well, I have not touched it. The set is functioning very well now in
all respects. I believe that this is the most satisfying FM monaural
tube sound that I have in any of the radios that I have or have had,
including a Saba 300, the Freudenstadt 18 and Telefunkens Opus 6 and 7.