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27th Mar 2017, 5:17 pm | #1 |
Retired Dormant Member
Join Date: Aug 2012
Location: Holmfirth, West Yorkshire, UK
Posts: 292
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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 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: 90 Volt feed.
1,520 ohms, power dissipated 15.2 watts.
Lawrence. |
27th Mar 2017, 5:42 pm | #3 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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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.
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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 |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,094
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Re: 90 Volt feed.
Or about 1.5uF and get less heat.
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27th Mar 2017, 6:59 pm | #5 |
Dekatron
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
Posts: 8,327
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Re: 90 Volt feed.
This will need the addition of a 250V primary transformer as the original amplifier will not have one.
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Edward. |
27th Mar 2017, 7:34 pm | #6 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,433
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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 |
Hexode
Join Date: Mar 2012
Location: Liverpool, Merseyside, UK.
Posts: 453
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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.
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27th Mar 2017, 8:16 pm | #8 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,433
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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 |
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. |