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 Post subject: Newcomb R-16
PostPosted: Nov Thu 14, 2019 5:42 am 
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Joined: Sep Tue 22, 2009 12:34 am
Posts: 157
I picked up a Newcomb R-16 record player today. It's a 1950s tube suitcase "classroom" type record player with an extra long tonearm to play transcription discs. A couple quick questions before I proceed.

The schematic says all capacitors are rated at 600v. However, the can (which looks to be original) has the three higher-rated caps at 475v. I have 450V caps in my box. A link to the schematic is below... am I close enough here? I've heard of some tube radios needing 450 but anything higher is new to me.

Also, the two-prong power cord was rotted and bad. If I chose to go to a 3 prong cord, would I just attach the 3rd ground wire to the same place where the neutral connects?

Schematic:
https://flic.kr/p/2hKxexm


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 Post subject: Re: Newcomb R-16
PostPosted: Nov Thu 14, 2019 4:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4278
Location: Boston, MA USA
The B+ on this is high -- indicated at 325V. 450V filter capacitors would be marginal. You won't be able to find 475V but you can get 500V which is what you need to use. Note that one of the capacitors in the can is rated at only 50V (the 6V6 cathode bypass capacitor) and that rating is fine.

On a 120V appliance you do not connect a ground lead to line neutral. Ground would be connected to chassis.

-David


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 Post subject: Re: Newcomb R-16
PostPosted: Nov Fri 15, 2019 2:02 am 
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Joined: Sep Mon 16, 2013 2:42 am
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Location: Tucson, Arizona U.S.A.
Electrolytic capacitors rated at more then 450 V are, and always have been, more expensive. The manufacturer wouldn't have used them if they weren't necessary.

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Jim Mueller


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 Post subject: Re: Newcomb R-16
PostPosted: Nov Fri 15, 2019 4:06 am 
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Joined: Sep Tue 22, 2009 12:34 am
Posts: 157
Thanks for the responses. I'll look into some 500v caps. As someone trying to learn, is there a general rule of how much over the normal working voltage the capacitors in an amplifier should be? I take it the concern is in-rush current here.


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 Post subject: Re: Newcomb R-16
PostPosted: Nov Fri 15, 2019 4:15 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4278
Location: Boston, MA USA
The issue is at power-on, before the power stage has warmed-up and is starting to draw plate current, the unloaded B+ can rise very high, almost to 1.414 * Secondary Voltage. Even though this phonograph uses a cathode-type rectifier which itself has to warm up, the 6V6 might take longer, or might fail, allowing the B+ to rise close to the unloaded level.

So the only way to figure this out is by the secondary voltage rating of the power transformer. For example, if it is 300-0-300 (a common value), then assuming zero rectifier voltage drop without load the electrolytic capacitor voltage rating would need to be 300V * 1.414 * 1.15 (say 15% allowance for power line variation) = 488V.

-David


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 Post subject: Re: Newcomb R-16
PostPosted: Nov Sun 17, 2019 12:41 am 
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Joined: Sep Mon 16, 2013 2:42 am
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Location: Tucson, Arizona U.S.A.
Also, the transformer voltage is specified with full load. Unloaded, the voltage will be higher.

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