ST-70 Dropping Resistor Question

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ST-70 Dropping Resistor Question

Postby skidave » Mon Oct 12, 2009 11:32 am

OK, so I messed up one resistor on a huge Mouser order and now I need some help. On the quad cap, the 6.8K dropping resistor I ordered was a two watt resistor instead of a three watt. I messed up the BOM & I did not realize it until I got my order. The part I ordered was 282-6.8K-RC instead of a 283-6.8K-RC. The 22K is a three watt.

I am trying to do the math to see if I can use the two watt safely. Otherwise I have to reorder and pay 16 times the price of the resistor for shipping ($.49 x 16 = $8.00 shipping). This math I can do!

Any thoughts?

Thanks,

Dave
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Postby TomMcNally » Mon Oct 12, 2009 11:59 am

Dave -

The original Dynaco ST-70 manual doesn't mention resistor wattage,
but I am pretty sure they used a 6.8K 2 watt part. Put yours in,
if it gets really really hot, or smokes, get a bigger one.

I think it will work just fine.
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Postby Ty_Bower » Mon Oct 12, 2009 12:01 pm

Is this on a stock ST70, between cap lugs B & C? The manual shows only a 40 volt drop across that part (415V -> 375V).

V=I*R and P=I*V. Rearranging, I get I=V/R, so P=(V/R)*V. Therefore, P=(40/6800)*40=0.24, or less than a quarter watt. I guess it really boils down to the initial inrush current (charging the empty cap) but I'd imagine you'll be fine with the two watt part.
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Postby skidave » Mon Oct 12, 2009 1:09 pm

Thanks Ty. Yes, you have the resistor location correct. I thought I was way off in my math. This high wattage must be for inrush???

Tom, The original part is a two or three watt carbon. I did check the manual and you are correct--no specification on wattage.

I'll check the temp after some use (when it is off).

Thanks,

Dave
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Re: ST-70 Dropping Resistor Question

Postby kheper » Mon Oct 12, 2009 2:21 pm

skidave wrote:OK, so I messed up one resistor on a huge Mouser order and now I need some help. On the quad cap, the 6.8K dropping resistor I ordered was a two watt resistor instead of a three watt. I messed up the BOM & I did not realize it until I got my order. The part I ordered was 282-6.8K-RC instead of a 283-6.8K-RC. The 22K is a three watt.

I am trying to do the math to see if I can use the two watt safely. Otherwise I have to reorder and pay 16 times the price of the resistor for shipping ($.49 x 16 = $8.00 shipping). This math I can do!

Any thoughts?

Thanks,

Dave


I am using a 2W as a dropping resistor on the can cap in my st70, but I do not know which one it is.

The 2W should be ok. Carefully check it for signs of heat after it has been on for an hour or so.
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Postby Geek » Mon Oct 12, 2009 4:23 pm

I use 1 or 2W in power supplies weather they need them or not for voltage rating safety factor.

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