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Old Yesterday, 04:42 PM   #1
markusA is offline markusA  Sweden
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Default Resistor ratings for a small tube circuit?

I'm somewhat embarrassed to be asking some truly trivial things here but please bear with me.
It'll be a tube splitter.
The circuit is nice and tidy but I need some help in picking the right resistors for the job. I have a 10kΩ cathode resistor with a idle current of 10mA. Maximum current Swing will naturally be 0-20mA.
What kind of resistor is suitable and what power rating should I aim for?
The power supply will be very basic with a CRC filter. So, the power supply will be driving the circuit mentioned above. (10mA idle current draw.) The "R" will be 1kΩ , what power rating should I use and what kind of resistor would be suitable?
I was originally thinking of using 220μF caps but I might consider going bigger if the cost increase isn't too steep. Maybe 470μF?
It doesn't have to be special or fancy I just want functional and avoid letting the magic smoke out.
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Old Yesterday, 05:14 PM   #2
DF96 is offline DF96  England
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You can calculate (or estimate) the power dissipation in each resistor. Then pick a bigger one, bearing in mind that the resistors will be under a hot chassis.

However, in many cases the real issue is voltage rating. Resistors may only be dissipating a few 100's of mW so 1/2W might look OK, but you may need to use 1W to get enough voltage rating. Read resistor datasheets.

10k at 10mA is 100V which means 1W, so this one needs to be 2W or greater.
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Old Yesterday, 05:18 PM   #3
markusA is offline markusA  Sweden
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Does it matter what kind of resistor I use? It seems wirewounds are the most common for higher than W.
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Old Today, 11:08 AM   #4
DF96 is offline DF96  England
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Wirewounds are expensive. You can get 1W film resistors. In some cases 0.6W film will be fine too.
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Old Today, 11:22 AM   #5
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It also depends in the resistor position. If it is mounted vertically, say the axis of it vertically, the disipation of the heat is better and the capacity to ground, lower; than if it is horizontaly positioned.
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Old Today, 12:39 PM   #6
markusA is offline markusA  Sweden
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Since I'll be doing the PCB myself I can choose the orientation freely.
I'm tempted to opt for 1W and 0.6W film resistors and just keep my fingers crossed. Maybe standing the borderline resistors up would be the final touch that keep them from letting the smoke out?
PCB is probably redundant but I like to keep things neat if possible. Would mounting the PCB upside down mess with heat dissipation? It's a handy way of having the tube peaking through the top of the case...
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Old Today, 12:58 PM   #7
DF96 is offline DF96  England
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Try to avoid 'borderline' resistors. Don't cross your fingers; instead, read datasheets and use a calculator.

Poking the valves out of the top of the case will help keep the rest of the circuit cool, but it can be dangerous for the valves - especially if there are children in the house - and dangerous for the children too.
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