On this site:

Philips B7X63A radio (1957)

The B7X63A was one of Philips' top models. As a “Bi-Ampli” radio with OTL output amplifiers, it delivers a high sound quality. It has tuning preset buttons operated with an electric motor. I wanted to own this radio after I saw and heard one at a fellow collector's. After a while, I found one and bought it. But it needed some work.

The B7X63A is a so-called "Bi-Ampli" radio, having separate amplifiers for bass and treble. In its time, it was the top of Philips' product line. Moreover, the two amplifiers are transformerless single-ended push-pull amplifiers, that deliver low distortion and no saturation on the bass amplifier. As a result, the radio gives magnificent bass and a beautiful and clear sound.

It covers the FM band from 88 to 100 MHz, and it has 4 AM bands: LW, MW and 2 short wave bands with bandwidth control.

It is using no less than 14 valves and has push-button pre-set tuning driven by an electric motor. The valve line up is:

In late 2007, I saw a B7X63A for the first time at a fellow radio collector's home. First I thought it was just one of those 1950-ies piano-key band switch radios. But he explained this was a special type of radio and let me hear its tremendous sound and demonstrated the motor tuning. I developed a desire to have one of these. But this is a bit of a rare model.

front view when bought

What the B7X63A looked like. Not bad, but damaged speaker fabric.

In march 2008, I saw this radio on an Internet auction site. I decided to try and make a bid. When I collected it, its condition was not as good as I had hoped. But it still was an interesting radio. So I took it.

inside view

A view from the back. This radio contains a lot of stuff.

The radio was playing reasonably well. The sound was quite good, although the tone control potmeter was a bit unreliable, and sometimes low frequency oscillations occurred. The tuning mechanism wasn't working well.

There were serious cosmetic problems. The speaker cloth was worn and torn at the bottom. At middle left, there was a stain where a previous owner had smeared some glue over the cloth in an attempt to fix it. The high gloss finish of the case was cracked. So I decided I needed to replace the cloth and fix the varnish.

Case finished

Polished case and new speaker fabric.

In 2009, I started the job. I unmounted the speaker baffle and took it to Corrien Maas, a weaver who makes very nice facsimile speaker fabric. After I received the replacement fabric, I removed the old fabric and replaced it. I decided to use French polish to fix the varnish. Now of course this is an anachronism, but with shellack you can fill up the gaps in a cellulose laquer finish where litle chips have fallen off. After tens of layers of French polish and some careful sanding in between, the little gaps are almost invisible. The result was a dark brown high gloss finish, not perfect, but satisfactory. You can still see the pattern of cracks beneath the shellack, but the small (few mm) spots where chips had detached, had become invisible.

Detail of the output amplifiers

A view of the 4 UL84 valves of the output amplifiers.

After I finished the outside, I tackled the inside. First, I used an external DC power supply to reform the power electrolytics. The total leakage current went under 0.5 mA quickly, so the condition of the electrolytics and by-pass capacitors seemed to be quite good. Then I checked all the electrolytic capacitors and was pleasantly surprised that most of them had ample capacity and acceptable ESR (effective series resistance).

Chassis outside the case, tuning dial removed for better accessibility.

Only the cathode bypass electrolytic in the tone control amplifier was bad. I also checked the paper capacitors. Most of them are not in places where they would cause trouble if they would leak, but two coupling capacitors in the tone control amplifier offset the bias of the ECC83, so I replaced them.

Then I connected two 820 Ω resistors at the outputs and connected an extra loudspeaker to the loudspeaker output. The extra loudspeaker is connected to the amplifiers in an odd way: a 800 to 8 ohms transformer is connected in bridge, to both outputs. I guess this causes an ugly phase shift between bass and treble, while in the transition range the output of the two amplifiers will annihilate each other.


Tuning dial mounted again after cleaning and repairs.

I switched on the power and found that the radio was working well again. Low frequency oscillations had gone and the sound seemed to be better than it used to be. I checked all voltages and found they were all in range. This was great.

Then I cleaned the inside, replaced a broken string that makes the ferroceptor turn, lubricated the tuning mechanism and tried to make sense of the preset mechanism. Everything except for the motor tuning and pre-set mechanism was working again.

speakers on baffle

The two 800 Ω speakers back on the baffle.

I was curious how the radio would sound with its original (800 Ω) speakers so I decided to leave the pre-sets as they were. Putting together again the B7X63A after a year went OK. When I switched it on, I was not disappointed: very sensitive on FM and AM, fine sound and really good bass. This radio is an nice one to have.

Copyright © 2008,2010 by Onno's E-page         published 2008-09-02, last updated 2010-04-18