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

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Old 16th May 2020, 11:16 am   #1
John10b
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Default Decca SG188 Record Player

I was taking a look at the circuit of above RP. With such a decent amp I wondered why they used an auto transformer configuration and not a fully mains isolated design, was it just cost?
John
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Old 16th May 2020, 1:08 pm   #2
vidjoman
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Default Re: Decca SG188 Record Player

They used an auto transformer in the SRG300 radiogram as well. Possibly the same transformer as a very similar amp. As with most players at the time cost was an issue but at least the amp is decent.
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Old 17th May 2020, 8:42 pm   #3
Valvepower
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Default Re: Decca SG188 Record Player

Hi,

Curious this post popping up at this time as in the last few days was only looking at a Decca SG188 amplifier lurking in the workshop thinking the same thing about the transformer.

I did think about replacing the mains transformer with one with isolated wingdings so it could be used as a standalone amplifier. To give me a rough idea I did look at the ECL82 stereo amplifier in the Mullard Circuits for Audio Amplifiers to get a rough idea of the required HT winding if you were to use a full wave rectifier and you'd need HT winding of 250-0-250V at 150mA.

Alternatively, it could just run it off an isolation transformer or use an isolation transformer to feed the HT.

Regards
Terry
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Old 18th May 2020, 7:06 am   #4
Chris55000
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Default Re: Decca SG188 Record Player

Hi!

You might find one of the "R Core" style of transformer (these are available with h.t. & 6.3V secondaries for valved equipment) are compact enough to fit the chassis in place of the old heater transformer.

For example:–

https://www.ebay.co.uk/itm/332070928843

This one, with a bridge h.t. rectifier, or the original rectifier valves plus two 1N4007 diodes added for the negative half, would give almost exactly the same h.t. as the original direct half–wave rectified mains supply Decca originally used did!

Chris Williams
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Last edited by Chris55000; 18th May 2020 at 7:21 am.
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Old 19th May 2020, 8:38 pm   #5
Valvepower
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Default Re: Decca SG188 Record Player

Hi,

Chris’s comments had me thinking and it reminded me I have a mains transformer salvaged from a real basket case Armstrong 222 stereo amplifier using two push pull pairs of ECL86 and not dissimilar to the Decca push pull ECL82’s. This transformer is quite usable with decent LT and HT windings for the ECL86’s. The Armstrong uses a half wave voltage doubler power supply so there is some similarity between the Armstrong and half wave Decca power supply.

It’s obviously bigger than the Decca transformer, however I feel, with a prevailing wind, it can be shoe horned in

Regards
Terry.
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Old 26th May 2020, 8:32 pm   #6
Valvepower
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Default Re: Decca SG188 Record Player

Hi,

First stage of the gaffing of the Armstrong transformer and power supply onto the Decca has been completed with a very rough bench hook-up.

The HT needed a bit of taming to reduce the higher HT used in the Armstrong to that required of the ECL82 based Decca. The transformer appears to be up to the job and runs at an acceptable temperature. Personally I feel it’s being slightly under run.

I’ve attached a picture of the bench hook-up.

Regards
Terry
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