KB 429

[Crackle]

Is it this one David, http://www.radiomuseum.org/r/kolsterbr_r...r_550.html

Mike

[Geordie McBoyne]

Hi Mike,
             two rather poor quality pictures of the unrestored KB radio.  The mains dropper resistor is positioned at the front of the chassis.

Next week I might bring it down from upstairs for servicing.  

Geordie McBoyne.

[Crackle]

I think your KB has to be a 426 David, I don't know of any other radios of that style front except the 427 which is the AC only version and has a shelf added for the power supply.
Very attractive front panel, with beautiful veneers .

[Geordie McBoyne]

The KB is now in the workshop but before a start is made a Brimar 1D5 rectifier valve must be found. The Mazda U4020 is similar so I might borrow one from my Murphy D30. The 1D5 has a 40volt 0.2amp heater. The Mullard CY1C is not a direct replacement because it has a 20 volt heater.

The Brimar 1D5: http://www.radiomuseum.org/tubes/tube_1d5.html

Geordie McBoyne.

[Geordie McBoyne]

Unable to find locally a Brimar 1D5 or an equivalent I've bought a Mazda U4020 from an Ebay seller.  The U4020 is a near equivalent to the 1D5.
A close up picture of the U4020 reveals that this HT rectifier is in fact a tetrode or pentode connected internally as a diode.

Geordie McBoyne.

[Crackle]

Hi David
How strange that they should adapt a pentode. I suppose it is the heater volts/current that is the issue.

Your Pentode version does not look anything like the one in the valve museum. http://www.r-type.org/exhib/aaa0670.htm

The KB service manual lists a 40SUA as an alternative to the 1D5.

Mike

[westcliff]

Geordie, are you sure that is a picture of a U4020?  The valve museum picture is very different: http://www.r-type.org/exhib/aaa0670.htm

Edit - sorry, I should have read Mike's post first.

Gus.

[Mike Watterson]

I'm sure I had a U4020 that looked the same as any other rectifier of the period, not a Pentode wired as a diode. No grids in it.

The URC1 is often listed, but you need and extra 20V of dropping somewhere as it's 20V rather than 40V.

[Geordie McBoyne]

The Mazda U4020 arrived today from the Ebay vendor.
I've taking a close up picture of the lower part of the electrode structure.  This confirms that this rectifier valve is a diode connected pentode.   The links between the grids and anode can clearly be seen.

Geordie McBoyne.

[Crackle]

Hi
So are you saying that all the grids are connected to the anode.

Mike