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 Post subject: what wattage for resistors where nothing is specified
PostPosted: Jun Wed 05, 2019 1:36 am 
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Hi. to everyone. I am in the process of restoring a stromberg carlson 535m. I am starting in the power supply/amplifier section. I have a parts list from the made in Rochester website. My thanks to the author.
The information lists the value of the resistors, but does not mention the wattage.There is only one place where it says that a particular resistor should be 3 watts. Nothing mentioned on any others in the information.
What should I use for all the others , 1 watt,1/2 watt?

my thanks in advance


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 Post subject: Re: what wattage for resistors where nothing is specified
PostPosted: Jun Wed 05, 2019 2:18 am 
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Chances are they were originally rated at 1/2 watt. It just makes sense to replace them with modern 1 watt units as their physical size is closer to the old 1/2 watt units plus they are inexpensive, usually about 20 cents each.


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 Post subject: Re: what wattage for resistors where nothing is specified
PostPosted: Jun Wed 05, 2019 2:31 am 
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Scott wrote:
Chances are they were originally rated at 1/2 watt. It just makes sense to replace them with modern 1 watt units

+1.

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 Post subject: Re: what wattage for resistors where nothing is specified
PostPosted: Jun Wed 05, 2019 12:18 pm 
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A quick examination of the chassis should answer that. I usually use half watts for half watts because that is what I have on hand. However I look at resistors and see if they appear overheated or drastically changed value and may benefit from a higher rating. Or perhaps replacement with a different type of resistor in the case of power supply related resistors. Say a 2 watt carbon comp is used in a voltage divider and runs hot, I might replace with a 5 watt wirewound or ceramic type that is more durable under heat. IF it reasonably fits. I guess they assumed half watt when there is no wattage listed. In the case of many high value resistors used in bias for tubes that is sufficient. Sometimes they use resistors for stuff like cathodes in output tubes that seem a bit underrated, to me.

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 Post subject: Re: what wattage for resistors where nothing is specified
PostPosted: Jun Wed 05, 2019 1:32 pm 
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For completeness, you can always get the power dissipation by noting the voltage across the resistor and applying P = E^2 / R
Use a resistor rated for at least twice the actual power dissipated

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: what wattage for resistors where nothing is specified
PostPosted: Jun Wed 05, 2019 5:04 pm 
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Location: Maryland 20709, USA
I use 2-watt 2% resistors to replace all non-power resistors in sets including large consoles and military equipment. They're closer to the size of the earlier resistors, particularly lead length.

They have the highest power and voltage* ratings of any non-power resistors, so I know I can use them anywhere in any set without worrying whether they meet spec or not.
-----
*NB - Yes, resistors have voltage ratings, usually the following:
1/4 watt = 250 volts - May be too low for some places in transformer sets.
1/2 watt = 350 volts
1&2 watt = 450 volts

- Leigh

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