KB FB10 receiver. - UK Vintage Radio Repair and Restoration Discussion Forum

Johnbarn · 25th Oct 2018, 12:01 am

This radio was practally unsalvageable, chassis rusted, speaker seized, and waxies exploded - but good case and tuning scale. After much work it's now going surprisingly well but at some time in the distant past someone has replaced the 6X5GT rectifier valve with three OA202 diodes in series, each one paralleled by a 1/2 watt 1 meg resistor. All these components are wired on the unused terminals of the 6X5GT valve base. I assume three diodes were used to reduce the voltage drop across each to less than 150v piv. This "interesting" arrangement works surprisingly well - for now, but these days a single 1N4007 and 220 ohm 5watt series resistor might be better. Has anyone seen this unusual arrangement before? I know KB were renowned for making changes as production continued but this surely can't be one of theirs?

TrevorG3VLF · 25th Oct 2018, 12:48 am

The three diodes are used to provide sufficient voltage capability.How are three diodes used with two anodes in the 6X5? Have they abandoned full wave rectification?

The resistors are used to ensure fair sharing of voltage and capacitors were often used also. I have just replaced a rectifier in a scope using several 1N4007 in series with no added components. The diodes came from the same band so should be similar.

Johnbarn · 25th Oct 2018, 7:35 am

Yes, full wave rectification has been abandoned in this arrangement and I would have thought that it would have caused more mains hum but there is actually very little. I'm still tempted to remove the present circuitry and replace it with either a 1N4007 and 220 ohm resistor or a full bridge. But "if it ain't broke....!!"

boxdoctor · 25th Oct 2018, 8:35 am

Quote:

Originally Posted by Johnbarn

Yes, full wave rectification has been abandoned in this arrangement and I would have thought that it would have caused more mains hum but there is actually very little. !"

The FB 10 has, as far as I know, always used half wave rectification, as the H.T. is derived from rectifying the incoming mains supply.
The 6X5 was used with both anodes strapped together i.e. as a single diode. Tony

crackle · 25th Oct 2018, 9:45 am

There were many variations on the KB FB10, I believe it was the most popular of all the radios they made, it seems the most plentiful, and they turn up in their dozens every year on Ebay. But to my knowledge they never used a solid state rectifier in any of the variations.

However the FB10FM did use a solid state half wave rectifier (metal) see circuit diagrams from Nov 1955 onwards. Prior to that they used an EZ80 wired as half wave.

Mike

Mike. Watterson · 25th Oct 2018, 10:21 am

A full bridge isn't needed. Also it puts chassis at about 1/2 supply voltage no matter which way round the mains cable is wired.

Johnbarn · 25th Oct 2018, 10:42 pm

Thank you to everyone, I will leave the arrangement of the 3 diodes "as is" for the moment. The set is performing amazingly well. I hadn't realised that the coupling together of the two anodes of the rectifier valve in the original circuit did not mean it was acting as a full wave rectifier - obvious when you think about it!! Thanks for putting me right.
On to the next project.

25th Oct 2018, 12:01 am	#1
Johnbarn Pentode Join Date: Feb 2015 Location: Belfast, Northern Ireland, UK. Posts: 130	KB FB10 receiver. This radio was practally unsalvageable, chassis rusted, speaker seized, and waxies exploded - but good case and tuning scale. After much work it's now going surprisingly well but at some time in the distant past someone has replaced the 6X5GT rectifier valve with three OA202 diodes in series, each one paralleled by a 1/2 watt 1 meg resistor. All these components are wired on the unused terminals of the 6X5GT valve base. I assume three diodes were used to reduce the voltage drop across each to less than 150v piv. This "interesting" arrangement works surprisingly well - for now, but these days a single 1N4007 and 220 ohm 5watt series resistor might be better. Has anyone seen this unusual arrangement before? I know KB were renowned for making changes as production continued but this surely can't be one of theirs?

25th Oct 2018, 12:48 am	#2
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Kb fb10 The three diodes are used to provide sufficient voltage capability.How are three diodes used with two anodes in the 6X5? Have they abandoned full wave rectification? The resistors are used to ensure fair sharing of voltage and capacitors were often used also. I have just replaced a rectifier in a scope using several 1N4007 in series with no added components. The diodes came from the same band so should be similar.

25th Oct 2018, 7:35 am	#3
Johnbarn Pentode Join Date: Feb 2015 Location: Belfast, Northern Ireland, UK. Posts: 130	Re: Kb fb10 Yes, full wave rectification has been abandoned in this arrangement and I would have thought that it would have caused more mains hum but there is actually very little. I'm still tempted to remove the present circuitry and replace it with either a 1N4007 and 220 ohm resistor or a full bridge. But "if it ain't broke....!!"

25th Oct 2018, 9:45 am	#5
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: KB FB10 receiver. There were many variations on the KB FB10, I believe it was the most popular of all the radios they made, it seems the most plentiful, and they turn up in their dozens every year on Ebay. But to my knowledge they never used a solid state rectifier in any of the variations. However the FB10FM did use a solid state half wave rectifier (metal) see circuit diagrams from Nov 1955 onwards. Prior to that they used an EZ80 wired as half wave. Mike Last edited by crackle; 25th Oct 2018 at 9:50 am.

25th Oct 2018, 10:21 am	#6
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: KB FB10 receiver. A full bridge isn't needed. Also it puts chassis at about 1/2 supply voltage no matter which way round the mains cable is wired.

25th Oct 2018, 10:42 pm	#7
Johnbarn Pentode Join Date: Feb 2015 Location: Belfast, Northern Ireland, UK. Posts: 130	Re: KB FB10 receiver. Thank you to everyone, I will leave the arrangement of the 3 diodes "as is" for the moment. The set is performing amazingly well. I hadn't realised that the coupling together of the two anodes of the rectifier valve in the original circuit did not mean it was acting as a full wave rectifier - obvious when you think about it!! Thanks for putting me right. On to the next project.