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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 5th Aug 2021, 8:36 am   #1
Michael Maurice
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Default Live chassis on Decca radiograms and record players.

Does anyone know the reasoning behind Decca’s use of a mains transformer, effectively 4 rectifiers in parallel giving half wave rectification and a live chassis?

Look at the diagrams for the diagrams for the SRG500, 600, 650 and early 700’s also the SG188.

To me it doesn’t make sense when considering their use of a mains transformer they could have had an isolated chassis and an EZ81 (or 2 EZ80’s) giving full rectification.

Can anyone enlighten me?
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Old 5th Aug 2021, 9:04 am   #2
ms660
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Default Re: Live chassis on Decca radiograms and record players.

Load current.

Lawrence.
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Old 5th Aug 2021, 9:20 am   #3
Robert Gribnau
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Default Re: Live chassis on Decca radiograms and record players.

Hello Michael,

I just downloaded the schematics for the SRG500 and the SG188 from Radiomuseum.org. I did not look at schematics of the other types.

In the schematic for the SRG500 there is full-wave rectification with an EZ81, and no live chassis. But perhaps there were different versions of the SRG500?

In the schematic for the SG188 there is half-wave rectification and a live chassis like you wrote. The mains transformer is only there for the filament supply and for choosing lower mains voltages than 240 Vac (than the primary acts as an auto-transformer). My guess is that such a mains transformer is a bit cheaper than one with an extra winding for isolated (meaning: no live chassis) full-wave rectification.
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Old 5th Aug 2021, 1:05 pm   #4
vidjoman
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Default Re: Live chassis on Decca radiograms and record players.

I think you have to consider what was the common factor for the day. TVs had live chassis and so did many radios and record players.
Of course Decca and others could have used U series valves, put them in series and do away with the transformer, but I think they opted for 6.5 volt valves as they would have a lower hum level. Apart from the connections to the P U cartridge which is easy to isolate with a couple of capacitors, same as many record players, the extension speaker terminals were isolated by the output transformers.
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