KB 429

[Geordie McBoyne]

Many pre-war Kolster-Brandes AC/DC mains radios employ the Brimar 10D1 or Mullard 2D13C double-diode as the detector and AVC rectifier.  These valves are quite scarce so today I made up an adaptor so the more common 12H6 octal base double-diode can be employed as a replacement.  The 12H6 has a 12.6volt 0.15amp heater so if the valve is to be used in 0.2amp heater chains a 250 ohm resistor has to be shunted across the heater pins 2 and 7.  An old B5 valve base and an octal socket are used to make up the adaptor. A 2ohm resistor is connected in series with the 12H6 heater just in case I decide to use the valve in the MRG radiogram which employs 13 volt valves.

Geordie McBoyne.  

[Refugee]

I did the same as a replacement for an expensive ECC33 with a 6/30L2 and then I put the original ECC33 back again declaring the adapter a success.

[Geordie McBoyne]

Just removed the card cased HT smoothing electrolytic capacitor from the KB radio.  It's useless so I must decide whether it is a viable proposition to rebuild the capacitor or simply assemble individual capacitors on a group board.

Geordie McBoyne.

[Crackle]

With a heat gun on low you should be able to get it warm enough to peel it open.

Mike

[Crackle]

Geordie did we establish that your radio was a KB 427? I have just uploaded a set of user instructions for this set.
see http://kbmuseum.org.uk/kb_images/kb427/KB_427_inst.pdf
I have managed to get hold of a massive amount of KB related stuff, it looks like a whole collection from the KB archives and includes dozens of user instructions, catalogues and schematics dating back to 1929. I have also managed to find the user instructions, schematic, and decent photo of the fabled "KB Kitten"
It is going to be a slow process but the plan is to eventually add them all to the KB Museum website.

Mike

[Geordie McBoyne]

Hi Mike,
I'm not sure of the precise model number of my KB radio. I'd say it is a model 426. The only difference I can detect is the position of the mains dropper resistor, the KB 426 that appears in Radiomuseum website has a horizontal mains dropper whereas it is vertical in my set.
Only two of the original valves still exist, the 15D1 frequency changer and the PEN 3520 output valve. The IF valve is an ex-services 9D2 and the detector double-diode a Mullard 2D13C. I've fitted a Mazda U4020 rectifier as the replacement for the Brimar 1D5.

From the Radiomuseum: http://www.radiomuseum.org/r/kolsterbr_426.html

Geordie McBoyne.

[Crackle]

Ah yes, if it is the AC/DC version then it will be the 426, the same as my one.
Got a set of instructions for that as well I think.
Mike

[Crackle]

Last weekend I found a KB 427 at the boot sale. It is rotten with woodworm and the front and left sides are rather badly damaged. I did actually find a live grub asleep under a little plug of silk in one hole.
   
   
   
But it has some good parts in it.
The mains transformer windings seem OK which is good as it is rather unique having, 300 + 300v,  13v, plus 4v + 4v secondaries.
Also the speaker transformer primary and the energising coil measure OK.
All the valves including the 13v valve heaters also measure OK so it looks like I have some useful spares in the chassis.
The tuning dial escutcheon is good and the 2 remaining knobs are good.
These are all parts I have previously had to repair or make new for other sets.
As the cabinet is identical to my KB 426 I am only keeping the front panel for the veneers and dumping the rest of the worm eaten cabinet.

Mike

[Terry]

The use of 13V valves in that set is interesting - where did the 13V come from in the first place?

Because of the use of lead acid accumulators, early valves ran off 2V, ignoring that the nominal voltage of a single cell is actually 2.1V. Similarly this increased to 4V but by the time that 6V ranges were introduced, manufacturers obviously decided it was time to account for the previous discrepancies and the standard became 6.3V. Then 12.6V for 12V operation.

I think 6.3V valves started off in the US but they were certainly standard in the UK towards the end of the 30s.

So why 13V? And weren't there 6.3V or 4V versions of the valves used in this AC set to avoid the non-standard secondary winding on the mains transformer?

[Refugee]

I would be inclined to burn the infested wood to kill any more live worms that are in it.
These days it may be more economic to just buy solid examples of the original wood and just make up the sides.
The harder wood may also help to deter the bugs.