90 Volt feed. - UK Vintage Radio Repair and Restoration Discussion Forum

stumted2 · 27th Mar 2017, 5:17 pm

HI all,

When using a rescued record player valve amp, (UCL82 and UY85) which was originally powered by a BSR motor via 90v tap, could any kind member advise me on a suitable value dropper resistor (and wattage) in order to power the amp (at 90v) from the 240v mains supply. (maths is not my strongest point.)

I fully understand it would be much preferable to use a transformer, but if possible to use a suitable value and wattage rated resistor, that would be great.

Keeping my fingers crossed that a kind member of the forum can help.

Cheers, Ted.

ms660 · 27th Mar 2017, 5:24 pm

1,520 ohms, power dissipated 15.2 watts.

Lawrence.

julie_m · 27th Mar 2017, 5:42 pm

The UY85 needs 38 V.
The UCL82 needs 50 V.
The resistor therefore has to drop 230 - ( 38 + 50 )
= 230 - 88
= 142 V.
The current is 0.1 A
Therefore the resistance = 142 / 0.1
= 1420 ohms
And the power = 142 * 0.1
= 14.2 W.

Try a 1K5, 25 watt wirewound resistor. Bear in mind, it will get hot; and only make connections to the input via a transformer, because the chassis is connected to mains.

GMB · 27th Mar 2017, 5:44 pm

Or about 1.5uF and get less heat.

Edward Huggins · 27th Mar 2017, 6:59 pm

Quote:

Originally Posted by julie_m

Try a 1K5, 25 watt wirewound resistor. Bear in mind, it will get hot; and only make connections to the input via a transformer, because the chassis is connected to mains.

This will need the addition of a 250V primary transformer as the original amplifier will not have one.

Nuvistor · 27th Mar 2017, 7:34 pm

I always thought the motor was used as an auto transformer, the chassis would be live with that type of transformer anyway, why will it require another transformer?

Frank

Clydeuk · 27th Mar 2017, 8:08 pm

I think the reason a transformer is being recommended is that this is going to be used as a stand alone amplifier and therefore anything connected to it has the potential to become live. In it's original job there wouldn't have been any means of connecting an external source.

Nuvistor · 27th Mar 2017, 8:16 pm

OK, hadn't realised that, then yes an isolating transformer is required.

Any thoughts on using 6.3v heater valves with one transformer supplying heaters and HT feed to a rectifier?

Frank

stumted2 · 27th Mar 2017, 9:40 pm

Many, many thanks to everyone for all your very much appreciated kind help and advice.
My plan was to add XI/Y2 isolating caps to the amp for safety reasons, but have decided not to press ahead with this venture, as it seems the best way forward, would have been to fit an isolating transformer.

Many, many thanks once again to all, for your very much appreciated kind help.

Cheers, Ted.

27th Mar 2017, 5:17 pm	#1
stumted2 Retired Dormant Member Join Date: Aug 2012 Location: Holmfirth, West Yorkshire, UK Posts: 292	90 Volt feed. HI all, When using a rescued record player valve amp, (UCL82 and UY85) which was originally powered by a BSR motor via 90v tap, could any kind member advise me on a suitable value dropper resistor (and wattage) in order to power the amp (at 90v) from the 240v mains supply. (maths is not my strongest point.) I fully understand it would be much preferable to use a transformer, but if possible to use a suitable value and wattage rated resistor, that would be great. Keeping my fingers crossed that a kind member of the forum can help. Cheers, Ted.

27th Mar 2017, 5:24 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: 90 Volt feed. 1,520 ohms, power dissipated 15.2 watts. Lawrence.

27th Mar 2017, 5:42 pm	#3
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: 90 Volt feed. The UY85 needs 38 V. The UCL82 needs 50 V. The resistor therefore has to drop 230 - ( 38 + 50 ) = 230 - 88 = 142 V. The current is 0.1 A Therefore the resistance = 142 / 0.1 = 1420 ohms And the power = 142 * 0.1 = 14.2 W. Try a 1K5, 25 watt wirewound resistor. Bear in mind, it will get hot; and only make connections to the input via a transformer, because the chassis is connected to mains. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

27th Mar 2017, 5:44 pm	#4
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,094	Re: 90 Volt feed. Or about 1.5uF and get less heat.

27th Mar 2017, 7:34 pm	#6
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: 90 Volt feed. I always thought the motor was used as an auto transformer, the chassis would be live with that type of transformer anyway, why will it require another transformer? Frank

27th Mar 2017, 8:08 pm	#7
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: 90 Volt feed. I think the reason a transformer is being recommended is that this is going to be used as a stand alone amplifier and therefore anything connected to it has the potential to become live. In it's original job there wouldn't have been any means of connecting an external source.

27th Mar 2017, 8:16 pm	#8
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: 90 Volt feed. OK, hadn't realised that, then yes an isolating transformer is required. Any thoughts on using 6.3v heater valves with one transformer supplying heaters and HT feed to a rectifier? Frank

27th Mar 2017, 9:40 pm	#9
stumted2 Retired Dormant Member Join Date: Aug 2012 Location: Holmfirth, West Yorkshire, UK Posts: 292	Re: 90 Volt feed. Many, many thanks to everyone for all your very much appreciated kind help and advice. My plan was to add XI/Y2 isolating caps to the amp for safety reasons, but have decided not to press ahead with this venture, as it seems the best way forward, would have been to fit an isolating transformer. Many, many thanks once again to all, for your very much appreciated kind help. Cheers, Ted.