KB 429

[Geordie McBoyne]

Except it would appear, the suppressor grid.

Geordie McBoyne.

[Mike Watterson]

They would have to be to make it be a pure rectifier with a low voltage drop. the G1 to Cathode spacing limits the PIV rating, so it must be a poor pentode, as better ones have a close g1 / cathode spacing.

Any valve can work as "power" rectifier, not just signal diode, though PIV would be limited on some. The current ratings of g1, g2 and anode would limit power.

[Hartley118]

ISTR that those early Western Electric movie sound amplifiers from the 1920s used power triodes both for audio output and as HT rectifiers.

Martin

[Geordie McBoyne]

Yesterday I took a look at my KB radio. For starters it needs a replacement main dropper resistor. The total heater chain voltage is 114V and it works out for 240 volt mains the total value of the heater dropper will be 630 ohms. Dissipation is 25watts!
It's possible that the HT and heaters share the volts adjustable part of the dropper so the resistance values will have to altered to accommodates this.
About the set. It is a short superhet, that is the output across the detector load is supplied directly to the grid of a high slope output pentode, a Mazda PEN3520 in this set. I'd guess the intermediate frequency is low, say 128Kc/s, my reasoning being that a three gang tuning capacitor is used, indicates bandpass RF tuning.
Waveband changeover is accomplished by pushing in the tuning knob and turning it, neat job. Also, variable IF selectivity is a nice feature.
Much of the rubber wiring in the set is perished and needs replacing, chassis is a bit rusty but so what?
The pentagrid frequency changer valve is a Brimar 15D1 which heater volts apart resembles the American 6A8G.
The IF amplifier valve in my set is an ex-services VR106 = Brimar 9D2, apart from the heater volts the 9D2 = 6K7G.

Tomorrow I will make up some sort of mains dropper and tidy up the wiring.

Geordie McBoyne.

[Crackle]

Hi David
In case you have not already got it, I have recently acquired a whole batch of 1930's KB service manuals. You can find the one for the KB 426 on this page.
http://kbmuseum.org.uk/service_manuals.htm
The IF is listed as 130kHz.

I am slowly  working my way through uploading the rest of the ones I have, I am up to the 1960 models, so I am well over half way now, thank goodness it is slow repetitive work cleaning up all the scans.

I seem to remember I had to rewind one of the sections on the mains dropper on my KB 426.
   


Mike

[Geordie McBoyne]

The AC/DC mains KB radio.  Among the useful items donated to me by Wilfrid wiwior in France is this magnificent vitreous enamelled wirewound high wattage resistor. This will be an ideal replacement for the burnt out main dropper resistor.
The resistive value is 560 ohms, 40 ohms short of the correct value for 240 volt mains but I'm sure I can get away with that slight reduction of value. If there is any evidence of over running of the valve heaters an additional series dropper resistor can be hidden under the chassis.
There is a lot more of perished wiring to replace before this set gets a taste of electricity.

Geordie McBoyne.

[Crackle]

Well that wont look too much out of place, a lucky find.

Mike

[Crackle]

Here is the circuit and component list with details of the rest of the dropper.
       
There was a separate 35 ohm winding on the main dropper for the HT, which by the look of it also provided for the dial lamps.

I think it may be a good idea to change the title of this thread now to KB 426.

cheers
Mike

[Crackle]

I have finished the cabinet for my KB 429 chassis.     what do you think?

No seriously, I was lucky to get a complete KB 429 at the BVWS RWB auction on Sunday.    
I have just given it a clean to see how the veneers would come up, I think it looks pretty good for 82 years old, I am pretty pleased with this purchase. (and the other 5 radios I got)

It comes complete with an EKCO mains battery eliminator, here is a photo of the insides.    
So far I have only cleaned up the cabinet, I have started work on another radio, also from the same auction, a lovely 1931 KB 279 which strangely enough I also have a spare "very rusty" chassis for.

Mike

[Nowhere-Man]

I had to learn some basic woodworking for boatbuilding.
I live next door to the joiners workshop. The joiners told me you can get a lot done with screws and glue. For the door of my home-made kitchen cupboard I just took one of their rejects and fitted it. However, you would laugh at my first attempts to build a frame for the cupboard. The first leaned. It took 5 attempts. With screws, glue and a drill you should get the hang of it.