Jim,
>> How do you conclude the proper value (both the voltage and capacitance)
>> for a filter capacitor for a 1500 watt linear amplifier?
I will go into a little more detail than Carl did! ;-)
You need to decide how much ripple is acceptable, and then simply go by
the definition of the units. One farad is the amount of capacitance that
will gain or loose one volt in one second, if a current of one ampere
charges or discharges it. As simple as that.
Let me calculate it with an example: Suppose you have an amp that takes
3000V and 1A, and that you decide that 100V ripple is OK. Suppose too
that you live in a country that has 60Hz line frequency, and that the
power supply uses a bridge rectifier and a simple capacitive filter,
without a choke.
In that case, a half cycle lasts for 0.0083 seconds, and the filter cap
will have to power the amp for ALMOST that entire half cycle, before
being charged by the next AC peak. We can simplify that "almost" and say
that your filter cap needs to power the amp for 0.008 seconds only, and
in that time it's allowed to drop by 100V, when delivering 1A. So this
turns to simple math:
1A * 0.008s / 100V = 0.00008F, or 80uF.
Note that this assumes that the transformer and the power line can
actually deliver the intense charge pulse, which would be several
amperes! In a practical case, the pulse will hardly be more than 3 or
4A, and so it will be somewhat longer than the 0.0003s I assumed, and
this results in the capacitor having to power the amp for less time, and
thus less ripple. How much less, depends on the combined soruce
resistance of the transformer, power line, and anything else in series
with them!
Carl's roughly 30uF would result in about 200V ripple, and a somewhat
softer and longer charge pulse in each half cycle. It's probably quite
workable.
It should also be good to understand how the supply voltage ripple
affects amps. Pentodes and tetrodes have a gain that's basically
independent from the plate voltage (but the screen voltage must be well
filtered to keep the gain from mecoming hum-modulated!). So a pentode or
tetrode can do with quite a lot of ripple on the plate, and you would
notice it in the output only if you drive the tube to peak power, as the
peak capability would be modulated by the ripple. On the other hand, a
triode's gain depends on the plate voltage, so any ripple in the supply
will modulate the gain and thus the entire output, at all power levels.
For that reason I would say that with a triode amp you should use a
pretty well filtered HV supply, while a pentode or tetrode amp can be OK
with less good filtering at the plate, but good filtering at the screen.
About the voltage rating, well, it has to be high enough to make the cap
survive! You need to consider the highest line voltage to be expected,
calculate how much peak voltage the transformer will put out with that,
and then select a capacitor voltage rating that has a reasonable safety
margin on top of that. How much safety margin, well, I would say at
least 20%. If you are more generous, you can enjoy a longer capacitor
life expectancy, but usually at much higher cost and bulk.
If you have to use electrolytics, you will need several in series, and
then you need to put in some higher safety margin, to account for uneven
voltage distribution between them, according to the tolerances of the
equalizing resistors, and possible uneven leakage in the caps.
Manfred
The 4CX1000 is dead. Long live the MRFE6VP61K25H!
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