greiswig wrote:So you're saying that more power output requires more capacitance at this stage?
Well if you will notice on schematics, generally higher power amps meaning more than two output tubes, have more or higher capacity filters on the plate supply.
For the technical reasons somebody else will have to explain better than I.
Or consult Merlin's power supply book.
In between charging pulses from the rectifier, the amp is running off energy stored in the caps. The caps get charged at 120Hz (and only on the portion of the charging pulse where its voltage exceeds the voltage currently on the cap). Most of your signal is at a much higher frequency, so with higher current output tubes, the cap's energy is depleted quicker than with lower current tubes. With sustained high signal, at some point -- and I have no idea where that point is -- the output would lose power and become increasingly modulated by the 120Hz ripple.
The energy stored in a cap is J=V^2*C/2, where C is in Farads and J is Joules or Watt-Seconds. So a 50uF cap at 450VDC stores 50.625 Joules, while a 100uF cap at 450V stores 101.25 Joules. So obviously, the second case is better able to support tubes with higher current demands.
Excellent! I think I understand the why, as well as the difference between sag and having a signal inadvertently modulated by ripple. Thank you!
Next question: how close to the rated voltage can you safely run? My plate voltage ends up being right around 520V, and I can easily get a pair of F&T's that are rated at 300V in there...but is the 80V margin of error enough, or am I pushing my luck and need to find some 350V caps?
80v margin should be ok. Although the voltage is initially higher than where the plates will settle, the voltage rating of the cap is based on the running voltage and not the initial startup voltage. Don't forget the balancing resistors to keep the voltage split between them.
