KB 666 "Standard"

[crackle]

I have been restoring one of these radios over the last week or so. It has been rather poorly got at in the years gone by. The capacitors have been replaced with a variety of different types, but a handful of them were "mustards" so they should be OK.
3 wiring faults were found where components had been replaced onto the wrong tags. The special multi tapped variable capacitor used for the tone control which connects from the control grid of the OP valve to ground had been removed and replaced with a 1M pot connected to the same points, so all that would do is lower the volume as the pot was turned to zero. I connected a 1nF cap in series with the pot and it works well now as a tone control.
Luckily the original volume control and on off switch was still present and working, it has a loudness tap about 80k from the lower end of the track.
There were still some old black paper wax caps so these were replaced with known good new ones, together with the assortment of other different style caps which had been used as replacements over the years.
The large capacitor block had been re-stuffed with blue Philips 450v electrolytics but I re-fitted these so they were less likely to short against each other.

V1 and V2 both had 7 pin valves which had been adapted with 5 pin bases to fit into the 5 valve sockets. V1 should be a VP4, but turned out to be a AC/VP1 which I only managed to decipher after scanning the valve and messing with the colour correction to enable the faint green on grey background to stand out more.. V2 should have been a SP4 but had a MVS Pen in place. This turned out to be rather problematic as when stations were tuned the radio suddenly went silent, rather than fade of tune, and then came back as the tuning was moved back slightly.

The most difficult issue is the speaker energising coil being open circuit, as usual.
I have ordered 500g of .2mm enamelled copper wire and fitted cheeks to the old tube former ready to rewind a new coil.

Anyway the radio is now working on the workshop speaker, and working very well indeed following a trip to Westcliff 's place to get some replacement valves for V1 & V2.
I have never noticed MW so crowded as it has been today, this radio can certainly pull in some stations. I counted 7 distinct stations on LW as well, that is the most I have ever heard.

This radio benefits from a RF amplifier stage so I suppose that helps a lot.


Here is a link to a video demonstrating the crowded MW.
https://youtu.be/E2sinop8a08

Chassis after restoration

Under chassis after restoration

Components cut out

Wrong valves

Former ready to wind on the new coil, new paxolin cheeks were made and fitted to the original tube.


Mike

[turretslug]

It's both heartening and a little depressing to hear that an 85 year old radio design is so much better at its job than many others! Well done on getting it back in shape.

[crackle]

I thought someone on here may be able to help identify this output valve which came with the KB 666.
It has 7 pins, pentode type and the filament draws 2 amps at 4 volts.
The specified OP valve for the KB 666 is an AC/Pen type which has a heater current of 1 amp, so I thought it best to not use the valve which came with the radio to save overloading the mains transformer.


Is there a useful website, or tool where you can input known factors and come up with possible valve numbers?

Thanks
Mike

[Herald1360]

PENA4?

If you have a picture with the remaining writing clearly in focus, playing with it in a picture editing package can often make faint markings clearer.

[crackle]

It could easily be a PenA4 the heater current is rated as 1.95A http://www.r-type.org/exhib/aaq0042.htm and it fits in with what remains of the lettering.
Its a bit too modern for this radio then and too much current, best stick with the AC/Pen I found spare.

Thanks
Mike

[G0HFCFrank]

Crackle,
I don't know if this is common knowledge but if you have partially lost lettering on a glass (as opposed to metalised) valve envelope don't rub or brush but just breathe heavily on it and the missing figures will often show up in the condensation on the glass.
Frank C.

[crackle]

Hi Frank
I didnt know that, I will give it a try.
thanks
Mike

[crackle]

The new wire for the energising coil arrived today and the new coil is now finished. I wound on the full 500 grams of wire, it just about filled the former. The final resistance was rather lower than expected.
The original coil was rated as 1.7k ohms and the wire was .19mm. The wire I bought was .2mm and the final resistance is 1050 ohms. I didn't think .01mm would make that much difference.
I am wondering if, as the original coil was close wound on every layer how many more turns you could fit into the coil. My new coil is slightly more random wound, but the overall weight of copper is about the same as the original coil.
I put a 1k 5 watt resistor in series with the coil as the HT on the OP valve was a little on the high side. Any way the original speaker is now working very well, and is still sounding very good and loud.

Mike

[Boater Sam]

0.19 to 0.2mm DIAMETER is a fair change in cross section terms, over 3%, not 1%.
The insulation on the old wire could well have been thicker, not modern self stripping. It is also possible that the purity of the new wire is better than the original hence lower resistance.

[Herald1360]

Modern enamel may also be thinner = even more copper......

[crackle]

Oh well, the next coil I wind I will go for exactly the same size copper, but the result is there is just a bit of energy wasted in the extra resistor I fitted in series with the energising coil.
But now the speaker is fitted back in the cabinet, I dont think it could sound better, there is loads of volume and a very good pleasing range of lovely rich tones and remarkably clear treble.
KB went to a lot of trouble with this model in shaping the tone, attached is the circuit showing the original OP stages and the mod I made to compensate for the missing original variable capacitor tone control.

Mike