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Old 7th February 2011, 04:27 AM   #1
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Join Date: Nov 2010
Location: Wiltshire UK
Default Calculating Power requirements

Being an enthusiastic amateur in the art of hi fi diy I am largely following other peoples work. I am reluctant to do this blindly and also want to understand and check their workings. What is the best way to calculate power requirements for a circuit? Particularly I want to make sure resistors selected are capable of handling the right load. Can I simply calculate this from the load requirement of the valves in the circuit and work back to identify what the requirements for any given component will be? For very complex circuits should I be looking at some sort of simulation software? It would be nice to be able to do this all with paper and pencil. Somehow building a valve amp using software to design seems wrong.....
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Old 7th February 2011, 04:55 AM   #2
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Use the plate currents as a basis for calculating dissipation, etc.
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Old 7th February 2011, 02:17 PM   #3
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
In my opinion paper an pencil gives a clearer understanding of what the circuit does, why each component need to have certain parameters and how the overall circuit operates.

Voltages and resistances allow calculation of currents.
From voltage or current one can find dissipation. Keep in Factor of Safety
eg.
If you have a 400mW transistor, then it will survive longer if Ppk <=50% of rated Pmax.
And Pq <=50% of Ppk. So aiming for Pq<100mW and Ppk <200mW should give a long life if you can keep the device cool. If you can't keep the device cool then it must be temperature de-rated and the de-rated Pmax is what is used to guide you to Ppk and Pq.

Valve dissipations are often quite close to Pmax 80% to 95% being quite common. This is a low Factor of Safety and is probably one of the contributors to an expected short valve life. Whereas some valves are deliberately run with a large FoS and can keep going for decades.
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regards Andrew T.

Last edited by AndrewT; 7th February 2011 at 02:20 PM.
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