Item nr.

Philips B3X81U/01 Table top

Mini four band set

Data for Philips B3X81U/01
ProductionBelgium, 1958.
Price was 198 guilders.
BandsLW (750-2000m), MW (185-580m), SW (25-50m), FM (87.5-100MHz); IF 452kHz and 10.7MHz.
TubesUCC85 (FM tuner), UCH81 (osc/mix), UF89 (IF), UABC80 (det/premp), UL84 (power), UY85 (rect.).
CabinetPlastic. Size 28x19x17cm. Weight 3.6kg.
PowerACDC 127/220V, 52W (measured 54W at 230V).
DocumentsService Docs.

The Design

Now that many AM stations have closed down, and tubed DAB radio's don't exist, FM radio's are the way to go if you want to listen to tubed radio sets. Early 2016 I bought a B1X42A and later that year, this earlier mini set with FM.

The chassis is built very compactly! It is quite a stress to work on the chassis, especially underneath where the parts are packed tightly together.

The sound quality is very good, certainly when taking into account the small size of the set. Unfortunately, some hum is heard during operation. When filling a living room with sound, this hum is unnoticable, but when playing at really low volumes, it can be a bit annoying. After several attempts to reduce the noise, I have drawn the conclusion that this level of hum is normal for this type of radio.
It is well known that tube radios can become significantly warm, especially in the area around the rectifier and output tube, and the dropper resistor. The high surface temperature is visible in this IR mage.

Obtained11/2016 from Rob Veldhoen via Marktplaats; sn=PL29899, dial code A392714 NE-BE.
Sound samplePLAY SOUND   Enjoy Georg Philipp Telemann and, at the end of this fragment, decide for yourself if you find this hum acceptable for an fl.198 radio.

This Object

The radio was advertised as "needs maintenance" so I didn't expect a working set, and indeed found the main fuse missing. The knobs, dial, and back panel were there and in good condition. The cabinet had been repainted and this paint was heavily worn, but I could redo this in the colors of my living. It seems that on the top edge of the dial there should be some decorative strip but this is missing. Also the Philips decal at the bottom is not there.

In December 2016, I took the set apart and replaced all paper capacitors. C4, connected directly over the mains, was shorted out (measured 30 Ohms), probably causing the fuse to blow. I found the dial lights to be OK. I measured the six tubes and found five of them fine, but the UL84, the only one I didn't have in stock, defective. So I had to wait for a swap meet and asked Bert to buy an UL84 for me. Meanwhile, I did the cabinet with primer and RAL F6.04.86.

On December 19, I got the new UL84, placed it (and a new fuse) and first put 110V on the set, which went fine. Then I applied 220V and the set worked immediately. I measured the voltage on all pins I could reach, and found the screen grid of the MF tube at 60V (should be 76). The resistor R33 was 49k instead of 22k, and replacing R33 increased the screen voltage. For the rest everything was OK, except the documentation (see left). With a plate current of 45mA and a screen current of 7mA, the cathode current of the output tube is 52mA. Over the cathode resistor of 250 Ohms, this places the cathode at 12 to 13V, like found in the UL84 data sheets and the tube manuals, not 8V as indicated in the diagram.

After the finishing job, the radio already looked nicer that when I got it. When taken in use, the radio produced a low, but clearly audible level of hum. Electrolytes C1/C2 appeared alright, but I replaced them without success. I checked the grounding of the anode lead of B5 and this was OK. I measured R1 (which is part of a noise cancelling system including S45 and S46, and may cause hum if changed in value) but this was within 10% of tolerance, 860 instead of 900 Ohms. Derated UL84 may cause the same disbalance, but the problem hum is present with two different UL84, including a NOS one.

If I ground the grid of B5 (through a capacitor) the noise disappears, but if I ground the anode of B4, or the joint C82/R56, the noise remains. This made me act on this hypothesis: the grid of B5 picks up some hum from unknown source, and because it is connected in a quite high impedance, the hum remains there. Coupling the grid stronger to B4 should then decrease hum. I reduced R56 from 330k to 4k7, and increased C82 from 4n7 to 100nF. This had some effect, but the hum is still clearly audible. It would be better, of course, to kill the source of the hum getting into the grid. The tube socket was a bit dirty, and after I cleansed it with ammonia, the hum got as loud as a space rocket taking off. This makes the tube heater a more plausible source of the hum, and when the socket had dried properly, the hum was almost gone again. I followed the suggestion to introduce feedback in the output tube, by removing the cathode decoupling capacitor. This hardly made any difference. I heard that heater-cathode leaks in the UCC85 can cause hum, and placed a new one, also to improve sensitivity. No effect on the hum, however. So, finally, with everything I could measure in order, and everything I could change without effect, I had to conclude that this level of hum is just part of the game in this nice little set.

Part of Gerard's Radio Corner.
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