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12/1/2004 1:46 PM
another question
Where can a novice find out how to determine what the appropriate capacitor voltage is. I can't get specific enough information concerning etermination of these values based on the circuit.  
For example, the cathode bypass cap voltage, the coupling cap voltage, etcetera.  
Also, if it is important to know, why is it not on the schematic?
12/1/2004 2:27 PM
Well it usually is. It might only appear in the Bill Of Material though.  
Coupling caps should be rated no lower than the next highest standard value as compared to the plate supply voltage. So if the plate supply voltage is 400VDC, use a 500VDC/630VDC cap.  
I personally use a 1.5 factor of safety approach, so 630VDC would be my choice.  
Bypass caps are less critical and 50 - 100 volt caps are suitable for almost any application in the preamp stage. You can use bigger if you have the room.
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12/1/2004 5:07 PM

It really is on many schematics. Some right by the part, others have a note somewhere that says all caps 630 volt unless otherwise noted or something similar.  
But part of the reason it isn't pointed out is that a certain level of knowledge is assumed when the drawings are made.  
If you connect a cap to the plate of a tube, the potential is there for the other end to be grounded, which means the full plate voltage is across the cap. Also, if the plate of a tube sits at 200v, the signal adds to that. So a signal of 30v peak adds that 30v to the 200v. That is why I would use a 400v cap on my 200v plate. You alway must look at a cap and ask yourself what is the most voltage that could possibly be across this part?  
The cathode bypass cap on a 12AX7 can be small. The cathode voltage is typically a volt or two. Signal will never exceed that, so a 25v cap is plenty. I have seen many circuits where the cathode cap was only rated 6v or 10v.  
Cathode biased power tubes are a bit different. You might find 20v or more on a cathode there, and signals can be all of that at times. So there I would use a 100v cap to feel safe.
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