Help needed with Smiths/Radiomobile Type B Amp - UK Vintage Radio Repair and Restoration Discussion Forum

Murphy Maniac · 30th Jun 2021, 5:32 pm

I am in the process of re-building a Smiths/Radiomobile Type E amplifier unit from a P4 Rover. This amp is the same as Type B with the exception of a couple of extra components not related to my query.
I want to convert the unit to Negative Earth operation, which appears simple enough except for the instruction to remove R123 and C121.

My questions are:
a) What are R123 and C121 doing there in the first place?
b) What happens to R116 and R117, are they still needed?
c) If R116 and R117 are left in place, do I just remove the wire which leads to the junction of R123 and C121?

R118 and R119 (both 47k) lead to the grids of a pair of N78's in push pull.

Apologies for being dim, but push/pull always leaves me confused.

Thanks.

Murphy Maniac · 30th Jun 2021, 5:33 pm

Sorry the pictures are on their side again, this always seems to happen to me!

Herald1360 · 30th Jun 2021, 10:56 pm

R123 and C121 provide~ -6V grid bias for the output pair from the potential divider provided by their series heaters across the -12V car supply when the set is used in a +ve earth car. When -ve earth supply is used, the added RC in the chassis connection from the output transformer provides cathode bias instead. (R123 and C121 would have provided positive bias otherwise).

For this to work I believe the junction of R116 and R117 should be taken to chassis, not left floating when R123 and C121 are removed. This snippet seems to be missing from the datasheet!

It's an interesting circuit using cathode coupled feedback from the output transformer a la QUAD II.

Murphy Maniac · 1st Jul 2021, 12:26 pm

Hi Chris,
Thank you for that explanation, it is becoming clearer to me now, I had wondered why the extra components in the tag 2 connections of socket 102 were necessary. I had not even noticed the cathode coupled feedback from the output transformer, this is something completely new to me.
By connecting the centre of R116 and R117 to chassis, are they effectively acting as grid leaks for V104 and V105?

Herald1360 · 1st Jul 2021, 7:41 pm

Yes, they are the grid leaks for V104 and V105 in cathode bias mode. In the fixed bias (+ve earth) mode they couple the negative bias from R123/C121 to the grids.

R118 and R119 are grid stoppers for the output valves to stop them from taking off at RF, they'll be needed either way (+ or - earth).

Murphy Maniac · 1st Jul 2021, 8:29 pm

Many thanks for the explanation, things are looking less intimidating now. I will probably be back as the job progresses.

30th Jun 2021, 5:32 pm	#1
Murphy Maniac Hexode Join Date: Aug 2009 Location: Liverpool, Merseyside, UK. Posts: 254	Help needed with Smiths/Radiomobile Type B Amp I am in the process of re-building a Smiths/Radiomobile Type E amplifier unit from a P4 Rover. This amp is the same as Type B with the exception of a couple of extra components not related to my query. I want to convert the unit to Negative Earth operation, which appears simple enough except for the instruction to remove R123 and C121. My questions are: a) What are R123 and C121 doing there in the first place? b) What happens to R116 and R117, are they still needed? c) If R116 and R117 are left in place, do I just remove the wire which leads to the junction of R123 and C121? R118 and R119 (both 47k) lead to the grids of a pair of N78's in push pull. Apologies for being dim, but push/pull always leaves me confused. Thanks. Attached Thumbnails __________________ Geoff. (BVWS Member)

30th Jun 2021, 5:33 pm	#2
Murphy Maniac Hexode Join Date: Aug 2009 Location: Liverpool, Merseyside, UK. Posts: 254	Re: Help needed with Smiths/Radiomobile Type B Amp Sorry the pictures are on their side again, this always seems to happen to me! __________________ Geoff. (BVWS Member)

30th Jun 2021, 10:56 pm	#3
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Help needed with Smiths/Radiomobile Type B Amp R123 and C121 provide~ -6V grid bias for the output pair from the potential divider provided by their series heaters across the -12V car supply when the set is used in a +ve earth car. When -ve earth supply is used, the added RC in the chassis connection from the output transformer provides cathode bias instead. (R123 and C121 would have provided positive bias otherwise). For this to work I believe the junction of R116 and R117 should be taken to chassis, not left floating when R123 and C121 are removed. This snippet seems to be missing from the datasheet! It's an interesting circuit using cathode coupled feedback from the output transformer a la QUAD II. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

1st Jul 2021, 12:26 pm	#4
Murphy Maniac Hexode Join Date: Aug 2009 Location: Liverpool, Merseyside, UK. Posts: 254	Re: Help needed with Smiths/Radiomobile Type B Amp Hi Chris, Thank you for that explanation, it is becoming clearer to me now, I had wondered why the extra components in the tag 2 connections of socket 102 were necessary. I had not even noticed the cathode coupled feedback from the output transformer, this is something completely new to me. By connecting the centre of R116 and R117 to chassis, are they effectively acting as grid leaks for V104 and V105? __________________ Geoff. (BVWS Member)

1st Jul 2021, 7:41 pm	#5
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Help needed with Smiths/Radiomobile Type B Amp Yes, they are the grid leaks for V104 and V105 in cathode bias mode. In the fixed bias (+ve earth) mode they couple the negative bias from R123/C121 to the grids. R118 and R119 are grid stoppers for the output valves to stop them from taking off at RF, they'll be needed either way (+ or - earth). __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

1st Jul 2021, 8:29 pm	#6
Murphy Maniac Hexode Join Date: Aug 2009 Location: Liverpool, Merseyside, UK. Posts: 254	Re: Help needed with Smiths/Radiomobile Type B Amp Many thanks for the explanation, things are looking less intimidating now. I will probably be back as the job progresses. __________________ Geoff. (BVWS Member)